Phil's Pretty Good Software


		       Pretty Good(tm) Privacy
		 Public Key Encryption for the Masses

                         PGP(tm) User's Guide
                      Volume II: Special Topics
                         by Philip Zimmermann
                         Revised 31 August 94

                    PGP Version 2.6.1 - 30 Aug 94
                             Software by
                 Philip Zimmermann, and many others.

Synopsis:  PGP(tm) uses public-key encryption to protect E-mail and
data files.  Communicate securely with people you've never met, with
no secure channels needed for prior exchange of keys.  PGP is well
featured and fast, with sophisticated key management, digital
signatures, data compression, and good ergonomic design.

Software and documentation (c) Copyright 1990-1994 Philip Zimmermann.
All rights reserved.  For information on PGP licensing, distribution,
copyrights, patents, trademarks, liability limitations, and export
controls, see the "Legal Issues" section.  Distributed by the
Massachusetts Institute of Technology.


Quick Overview
Special Topics
  Selecting Keys via Key ID
  Separating Signatures from Messages
  Decrypting the Message and Leaving the Signature on it
  Sending ASCII Text Files Across Different Machine Environments
  Leaving No Traces of Plaintext on the Disk
  Displaying Decrypted Plaintext on Your Screen
  Making a Message For Her Eyes Only
  Preserving the Original Plaintext Filename
  Editing Your User ID or Pass Phrase
  Editing the Trust Parameters for a Public Key
  Checking If Everything is OK on Your Public Key Ring
  Verifying a Public Key Over the Phone
  Handling Large Public Keyrings
  Using PGP as a Unix-style Filter
  Suppressing Unnecessary Questions:  BATCHMODE
  Force "Yes" Answer to Confirmation Questions:  FORCE
  PGP Returns Exit Status to the Shell
  Environmental Variable for Pass Phrase
  Setting Configuration Parameters: CONFIG.TXT
    TMP - Directory Pathname for Temporary Files
    LANGUAGE - Foreign Language Selector
    MYNAME - Default User ID for Making Signatures
    TEXTMODE - Assuming Plaintext is a Text File
    CHARSET - Specifies Local Character Set for Text Files
    ARMOR - Enable ASCII Armor Output
    ARMORLINES - Size of ASCII Armor Multipart Files
    KEEPBINARY - Keep Binary Ciphertext Files After Decrypting
    COMPRESS - Enable Compression
    COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed
    MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed
    CERT_DEPTH - How Deep May Introducers Be Nested
    BAKRING - Filename for Backup Secret Keyring
    PUBRING - Filename for Your Public Keyring
    SECRING - Filename for Your Secret Keyring
    RANDSEED - Filename for Random Number Seed
    PAGER - Selects Shell Command to Display Plaintext Output
    SHOWPASS - Echo Pass Phrase to User
    TZFIX - Timezone Adjustment
    CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text
    VERBOSE - Quiet, Normal, or Verbose Messages
    INTERACTIVE - Ask for Confirmation for Key Adds
    NOMANUAL - Let PGP Generate Keys Without the Manual
  A Peek Under the Hood
    Random Numbers
    PGP's Conventional Encryption Algorithm
    Data Compression
    Message Digests and Digital Signatures
  Compatibility with Previous and Future Versions of PGP
  Compromised Pass Phrase and Secret Key
  Public Key Tampering
  "Not Quite Deleted" Files
  Viruses and Trojan Horses
  Physical Security Breach
  Tempest Attacks
  Exposure on Multi-user Systems
  Traffic Analysis
  Protecting Against Bogus Timestamps
Legal Issues
  Trademarks, Copyrights, and Warranties
  Patent Rights on the Algorithms
  Freeware Status and Restrictions
  Restrictions on Commercial Use of PGP
  Other Licensing Restrictions
  Export Controls
  Philip Zimmermann's Legal Situation
Other Sources of Information on PGP
  Where to Get a Commercial Version of PGP
  Reporting PGP Bugs
  Fan Mail, Updates, and News
  Computer-Related Political Groups
  Recommended Readings
  To Contact the Author
Appendix A:  Where to Get PGP

Quick Overview

Pretty Good(tm) Privacy (PGP), from Phil's Pretty Good Software, is a
high security cryptographic software application for MSDOS, Unix,
VAX/VMS, and other computers.  PGP combines the convenience of the
Rivest-Shamir-Adleman (RSA) public key cryptosystem with the speed of
conventional cryptography, message digests for digital signatures,
data compression before encryption, good ergonomic design, and
sophisticated key management. 

This volume II of the PGP User's Guide covers advanced topics about
PGP that were not covered in the "PGP User's Guide, Volume I:
Essential Topics".  You should first read the Essential Topics
volume, or this manual won't make much sense to you.  Reading this
Special Topics volume is optional, except for the legal issues
section, which everyone should read.

Special Topics

Selecting Keys via Key ID

In all commands that let the user type a user ID or fragment of a
user ID to select a key, the hexadecimal key ID may be used instead. 
Just use the key ID, with a prefix of "0x", in place of the user ID. 
For example:

    pgp -kv 0x67F7

This would display all keys that had 67F7 as part of their key IDs.

This feature is particularly useful if you have two different keys
from the same person, with the same user ID.  You can unambiguously
pick which key you want by specifying the key ID.

Separating Signatures from Messages

Normally, signature certificates are physically attached to the text
they sign.  This makes it convenient in simple cases to check
signatures.  It is desirable in some circumstances to have signature
certificates stored separately from the messages they sign.  It is
possible to generate signature certificates that are detached from
the text they sign.  To do this, combine the 'b' (break) option with
the 's' (sign) option.  For example:

    pgp -sb letter.txt

This example produces an isolated signature certificate in a file
called "letter.sig".  The contents of letter.txt are not appended to
the signature certificate.

After creating the signature certificate file (letter.sig in the
above example), send it along with the original text file to the
recipient.  The recipient must have both files to check the signature
integrity.  When the recipient attempts to process the signature
file, PGP notices that there is no text in the same file with the
signature and prompts the user for the filename of the text. Only
then can PGP properly check the signature integrity.  If the
recipient knows in advance that the signature is detached from the
text file, she can specify both filenames on the command line:

    pgp letter.sig letter.txt
or: pgp letter letter.txt

PGP will not have to prompt for the text file name in this case.

A detached signature certificate is useful if you want to keep the
signature certificate in a separate certificate log.  A detached
signature of an executable program is also useful for detecting a
subsequent virus infection.  It is also useful if more than one party
must sign a document such as a legal contract, without nesting
signatures.  Each person's signature is independent.

If you receive a ciphertext file that has the signature certificate
glued to the message, you can still pry the signature certificate
away from the message during the decryption.  You can do this with
the -b option during decrypt, like so:

    pgp -b letter

This decrypts the letter.pgp file and if there is a signature in it,
PGP checks the signature and detaches it from the rest of the
message, storing it in the file letter.sig.

Decrypting the Message and Leaving the Signature on it

Usually, you want PGP to completely unravel a ciphertext file,
decrypting it and checking the nested signature if there is one,
peeling away the layers until you are left with only the original
plaintext file.

But sometimes you want to decrypt an encrypted file, and leave the
inner signature still attached, so that you are left with a decrypted
signed message.  This may be useful if you want to send a copy of a
signed document to a third party, perhaps re-enciphering it.  For
example, suppose you get a message signed by Charlie, encrypted to
you.  You want to decrypt it, and, leaving Charlie's signature on it,
you want to send it to Alice, perhaps re-enciphering it with Alice's
public key.  No problem.  PGP can handle that.

To simply decrypt a message and leave the signature on it intact,

    pgp -d letter

This decrypts letter.pgp, and if there is an inner signature, it is
left intact with the decrypted plaintext in the output file.

Now you can archive it, or maybe re-encrypt it and send it to someone

Sending ASCII Text Files Across Different Machine Environments

You may use PGP to encrypt any kind of plaintext file, binary 8-bit
data or ASCII text.  Probably the most common usage of PGP will be for
E-mail, when the plaintext is ASCII text.  

ASCII text is sometimes represented differently on different
machines.  For example, on an MSDOS system, all lines of ASCII text
are terminated with a carriage return followed by a linefeed.  On a
Unix system, all lines end with just a linefeed.  On a Macintosh, all
lines end with just a carriage return.  This is a sad fact of life.

Normal unencrypted ASCII text messages are often automatically
translated to some common "canonical" form when they are transmitted
from one machine to another.  Canonical text has a carriage return
and a linefeed at the end of each line of text.  For example, the
popular KERMIT communication protocol can convert text to canonical
form when transmitting it to another system.  This gets converted
back to local text line terminators by the receiving KERMIT.  This
makes it easy to share text files across different systems.

But encrypted text cannot be automatically converted by a
communication protocol, because the plaintext is hidden by
encipherment.  To remedy this inconvenience, PGP lets you specify
that the plaintext should be treated as ASCII text (not binary data)
and should be converted to canonical text form before it gets
encrypted.  At the receiving end, the decrypted plaintext is
automatically converted back to whatever text form is appropriate for
the local environment.

To make PGP assume the plaintext is text that should be converted to
canonical text before encryption, just add the "t" option when
encrypting or signing a message, like so:

   pgp -et message.txt her_userid

This mode is automatically turned off if PGP detects that the
plaintext file contains what it thinks is non-text binary data.

For PGP users that use non-English 8-bit character sets, when PGP 
converts text to canonical form, it may convert data from the local
character set into the LATIN1 (ISO 8859-1 Latin Alphabet 1) character
set, depending on the setting of the CHARSET parameter in the PGP
configuration file.  LATIN1 is a superset of ASCII, with extra
characters added for many European languages.

Leaving No Traces of Plaintext on the Disk

After PGP makes a ciphertext file for you, you can have PGP
automatically overwrite the plaintext file and delete it, leaving no
trace of plaintext on the disk so that no one can recover it later
using a disk block scanning utility.  This is useful if the plaintext
file contains sensitive information that you don't want to keep

To wipe out the plaintext file after producing the ciphertext file,
just add the "w" (wipe) option when encrypting or signing a message,
like so:

    pgp -esw message.txt her_userid

This example creates the ciphertext file "message.pgp", and the 
plaintext file "message.txt" is destroyed beyond recovery.

Obviously, you should be careful with this option.  Also note that
this will not wipe out any fragments of plaintext that your word
processor might have created on the disk while you were editing the
message before running PGP.  Most word processors create backup
files, scratch files, or both.  Also, it overwrites the file only
once, which is enough to thwart conventional disk recovery efforts,
but not enough to withstand a determined and sophisticated effort to
recover the faint magnetic traces of the data using special disk
recovery hardware.

Displaying Decrypted Plaintext on Your Screen

To view the decrypted plaintext output on your screen (like the
Unix-style "more" command), without writing it to a file, use the -m
(more) option while decrypting:

     pgp -m ciphertextfile

This displays the decrypted plaintext display on your screen one
screenful at a time.

Making a Message For Her Eyes Only

To specify that the recipient's decrypted plaintext will be shown
ONLY on her screen and will not be saved to disk, add the -m option:

     pgp -sem message.txt her_userid

Later, when the recipient decrypts the ciphertext with her secret key
and pass phrase, the plaintext will be displayed on her screen but
will not be saved to disk.  The text will be displayed as it would if
she used the Unix "more" command, one screenful at a time.  If she
wants to read the message again, she will have to decrypt the
ciphertext again.

This feature is the safest way for you to prevent your sensitive
message from being inadvertently left on the recipient's disk.  This
feature was added at the request of a user who wanted to send
intimate messages to his lover, but was afraid she might accidentally
leave the decrypted messages on her husband's computer.

Note that this feature will not prevent a clever and determined
person from finding a way to save the decrypted plaintext to disk--
it's to help prevent a casual user from doing it inadvertently.

Preserving the Original Plaintext Filename

Normally, PGP names the decrypted plaintext output file with a name
similar to the input ciphertext filename, but dropping the 
extension.  Or, you can override that convention by specifying an
output plaintext filename on the command line with the -o option.
For most E-mail, this is a reasonable way to name the plaintext file,
because you get to decide its name when you decipher it, and your
typical E-mail messages often come from useless original plaintext
filenames like "to_phil.txt".  

But when PGP encrypts a plaintext file, it always saves the original
filename and attaches it to the plaintext before it compresses and
encrypts the plaintext.  Normally, this hidden original filename is
discarded by PGP when it decrypts, but you can tell PGP you want to
preserve the original plaintext filename and use it as the name of
the decrypted plaintext output file.  This is useful if PGP is used
on files whose names are important to preserve.

To recover the original plaintext filename while decrypting, add 
the -p option, like so:

     pgp -p ciphertextfile

I usually don't use this option, because if I did, about half of my
incoming E-mail would decrypt to the same plaintext filenames of
"to_phil.txt" or "prz.txt".

Editing Your User ID or Pass Phrase

Sometimes you may need to change your pass phrase, perhaps because
someone looked over your shoulder while you typed it in.  

Or you may need to change your user ID, because you got married and
changed your name, or maybe you changed your E-mail address.  Or
maybe you want to add a second or third user ID to your key, because
you may be known by more than one name or E-mail address or job
title.  PGP lets you attach more than one user ID to your key, any
one of which may be used to look up your key on the key ring.

To edit your own userid or pass phrase for your secret key:

     pgp -ke userid [keyring]

PGP prompts you for a new user ID or a new pass phrase.

If you edit your user ID, PGP actually adds a new user ID, without
deleting the old one.  If you want to delete an old user ID, you will
have to do that in a separate operation.

The optional [keyring] parameter, if specified, must be a public
keyring, not a secret keyring.  The userid field must be your own
userid, which PGP knows is yours because it appears on both your
public keyring and your secret keyring.  Both keyrings will be
updated, even though you only specified the public keyring.

The -ke command works differently depending on whether you use it on
a public or secret key.  It can also be used to edit the trust
parameters for a public key.

Editing the Trust Parameters for a Public Key

Sometimes you need to alter the trust parameters for a public key on
your public key ring.  For a discussion on what these trust
parameters mean, see the section "How Does PGP Keep Track of Which
Keys are Valid?" in the Essential Topics volume of the PGP User's

To edit the trust parameters for a public key:

     pgp -ke userid [keyring]

The optional [keyring] parameter, if specified, must be a public
keyring, not a secret keyring.

Checking If Everything is OK on Your Public Key Ring

Normally, PGP automatically checks any new keys or signatures on your
public key ring and updates all the trust parameters and validity
scores.  In theory, it keeps all the key validity status information
up to date as material is added to or deleted from your public key
ring.  But perhaps you may want to explicitly force PGP to perform a
comprehensive analysis of your public key ring, checking all the
certifying signatures, checking the trust parameters, updating all
the validity scores, and checking your own ultimately-trusted key
against a backup copy on a write-protected floppy disk.  It may be a
good idea to do this hygienic maintenance periodically to make sure
nothing is wrong with your public key ring.  To force PGP to perform
a full analysis of your public key ring, use the -kc (key ring check)

     pgp -kc

You can also make PGP check all the signatures for just a single
selected public key by:

     pgp -kc userid [keyring]

For further information on how the backup copy of your own key is
checked, see the description of the BAKRING parameter in the
configuration file section of this manual.

Verifying a Public Key Over the Phone

If you get a public key from someone that is not certified by anyone
you trust, how can you tell if it's really their key?  The best way
to verify an uncertified key is to verify it over some independent
channel other than the one you received the key through.  One
convenient way to tell, if you know this person and would recognize
them on the phone, is to call them and verify their key over the
telephone.  Rather than reading their whole tiresome (ASCII-armored)
key to them over the phone, you can just read their key's
"fingerprint" to them.  To see this fingerprint, use the -kvc

     pgp -kvc userid [keyring]

This will display the key with the 16-byte digest of the public key
components.  Read this 16-byte fingerprint to the key's owner on the
phone, while she checks it against her own, using the same -kvc
command at her end.  

You can both verify each other's keys this way, and then you can sign
each other's keys with confidence.  This is a safe and convenient way
to get the key trust network started for your circle of friends.

Note that sending a key fingerprint via E-mail is not the best way to
verify the key, because E-mail can be intercepted and modified.  It's
best to use a different channel than the one that was used to send
the key itself.  A good combination is to send the key via E-mail,
and the key fingerprint via a voice telephone conversation.  Some
people distribute their key fingerprint on their business cards,
which looks really cool.

For current versions of PGP, the key fingerprint is computed using 
the MD5 hash function.  A future version of PGP will optionally use a
new and different hash function, SHA, instead of MD5.

If you don't know me, please don't call me to verify my key over the
phone-- I get too many calls like that.  Since every PGP user has a
copy of my public key, no one could tamper with all the copies that
are out there.  The discrepancy would soon be noticed by someone who
checked it from more than one source, and word would soon get out on
the Internet.

For those of you who want to verify my public key (included in the
standard PGP release package), here are the particulars:

  UserID: "Philip R. Zimmermann "
  Key Size: 1024 bits;  Creation date: 21 May 1993;  KeyID: C7A966DD
  Key fingerprint:  9E 94 45 13 39 83 5F 70  7B E7 D8 ED C4 BE 5A A6

The information printed above conceivably could still be tampered
with in the electronic distribution of the PGP User's Guide.  But if
you read this in the printed version of the manual, available in
bookstores from MIT Press, it's a safe bet that it really is my own
key's fingerprint.

Handling Large Public Keyrings

PGP was originally designed for handling small personal keyrings for
keeping all your friends on, like a personal rolodex.  A couple
hundred keys is a reasonable size for such a keyring.  But as PGP has
become more popular, people are now trying to add other large
keyrings to their own keyring.  Sometimes this involves adding
thousands of keys to your keyring.  PGP, in its present form, cannot
perform this operation in a reasonable period of time, while you wait
at your keyboard.  Not for huge keyrings.

You may want to add a huge "imported" keyring to your own keyring,
because you are only interested in a few dozen keys on the bigger
keyring you are bringing in.  If that's all you want from the other
keyring, it would be more efficient if you extract the few keys you
need from the big foreign keyring, and then add just these few keys
to your own keyring.  Use the -kx command to extract them from the
foreign keyring, specifying the keyring name on the command line. 
Then add these extracted keys to your own keyring.

The real solution is to improve PGP to use advanced database
techniques to manage large keyrings efficiently.  We are working on
this, and should have it done Real Soon Now.  Until this happens, you
will just have to use smaller keyrings, or be patient.

Using PGP as a Unix-style Filter

Unix fans are accustomed to using Unix "pipes" to make two
applications work together.  The output of one application can be
directly fed through a pipe to be read as input to another
application.  For this to work, the applications must be capable of
reading the raw material from "standard input" and writing the
finished output to "standard output".  PGP can operate in this mode.
If you don't understand what this means, then you probably don't need
this feature.

To use a Unix-style filter mode, reading from standard input and
writing to standard output, add the -f option, like so:

     pgp -feast her_userid outputfile

This feature makes it easier to make PGP work with electronic mail

When using PGP in filter mode to decrypt a ciphertext file, you may
find it useful to use the PGPPASS environmental variable to hold the
pass phrase, so that you won't be prompted for it.  The PGPPASS
feature is explained below.

Suppressing Unnecessary Questions:  BATCHMODE

With the BATCHMODE flag enabled on the command line, PGP will not ask
any unnecessary questions or prompt for alternate filenames.  Here
is an example of how to set this flag:

    pgp +batchmode cipherfile 

This is useful for running PGP non-interactively from Unix shell
scripts or MSDOS batch files.  Some key management commands still
need user interaction even when BATCHMODE is on, so shell scripts may
need to avoid them.  

BATCHMODE may also be enabled to check the validity of a signature on
a file.  If there was no signature on the file, the exit code is 1. 
If it had a signature that was good, the exit code is 0.

Force "Yes" Answer to Confirmation Questions:  FORCE

This command-line flag makes PGP assume "yes" for the user response
to the confirmation request to overwrite an existing file, or when
removing a key from the keyring via the -kr command.  Here is an
example of how to set this flag:

    pgp +force cipherfile 
    pgp -kr +force Smith

This feature is useful for running PGP non-interactively from a Unix
shell script or MSDOS batch file.

PGP Returns Exit Status to the Shell

To facilitate running PGP in "batch" mode, such as from an MSDOS
".bat" file or from a Unix shell script, PGP returns an error exit
status to the shell.  An exit status code of zero means normal exit,
while a nonzero exit status indicates some kind of error occurred.
Different error exit conditions return different exit status codes to
the shell.

Environmental Variable for Pass Phrase

Normally, PGP prompts the user to type a pass phrase whenever PGP 
needs a pass phrase to unlock a secret key.  But it is possible to
store the pass phrase in an environmental variable from your
operating system's command shell.  The environmental variable PGPPASS
can be used to hold the pass phrase that PGP will attempt to use
first.  If the pass phrase stored in PGPPASS is incorrect, PGP 
recovers by prompting the user for the correct pass phrase.

For example, on MSDOS, the shell command:

    SET PGPPASS=zaphod beeblebrox for president

would eliminate the prompt for the pass phrase if the pass phrase
were indeed "zaphod beeblebrox for president". 

This dangerous feature makes your life more convenient if you have to
regularly deal with a large number of incoming messages addressed to
your secret key, by eliminating the need for you to repeatedly type
in your pass phrase every time you run PGP.

I added this feature because of popular demand.  However, this is a
somewhat dangerous feature, because it keeps your precious pass
phrase stored somewhere other than just in your brain.  Even worse,
if you are particularly reckless, it may even be stored on a disk on
the same computer as your secret key.  It would be particularly
dangerous and stupid if you were to install this command in a batch
or script file, such as the MSDOS AUTOEXEC.BAT file.  Someone could
come along on your lunch hour and steal both your secret key ring and
the file containing your pass phrase.  

I can't emphasize the importance of this risk enough.  If you are
contemplating using this feature, be sure to read the sections
"Exposure on Multi-user Systems" and "How to Protect Secret Keys from
Disclosure" in this volume and in the Essential Topics volume of the 
PGP User's Guide.

If you must use this feature, the safest way to do it would be to
just manually type in the shell command to set PGPPASS every time you
boot your machine to start using PGP, and then erase it or turn off
your machine when you are done.  And you should definitely never do
it in an environment where someone else may have access to your
machine.  Someone could come along and simply ask your computer to
display the contents of PGPPASS.

Sometimes you want to pass the pass phrase into PGP from another
application, such as an E-mail package.  In some cases, it may not
always be desirable to use the PGPPASS variable for that purpose. 
There is another way to pass your pass phrase into PGP from another
application.  Use the "-z" command line option.  This option is
designed primarily for invoking PGP from inside an E-mail package. 
The pass phrase follows the -z option on the command line.  There are
risks associated with using this approach, similar to those risks
described above for using the PGPPASS variable.

Setting Configuration Parameters: CONFIG.TXT

PGP has a number of user-settable parameters that can be defined in a
special configuration text file called "config.txt", in the directory
pointed to by the shell environmental variable PGPPATH.  Having a
configuration file enables the user to define various flags and
parameters for PGP without the burden of having to always define
these parameters in the PGP command line.

Configuration parameters may be assigned integer values, character
string values, or on/off values, depending on what kind of
configuration parameter it is.  A sample configuration file is
provided with PGP, so you can see some examples.

In the configuration file, blank lines are ignored, as is anything
following the '#' comment character.  Keywords are not

Here is a short sample fragment of a typical configuration file:

   # TMP is the directory for PGP scratch files, such as a RAM disk.
   TMP = "e:\"    # Can be overridden by environment variable TMP.
   Armor = on     # Use -a flag for ASCII armor whenever applicable.
   # CERT_DEPTH is how deeply introducers may introduce introducers.
   cert_depth = 3

If some configuration parameters are not defined in the configuration
file, or if there is no configuration file, or if PGP can't find the
configuration file, the values for the configuration parameters
default to some reasonable value.

Note that it is also possible to set these same configuration
parameters directly from the PGP command line, by preceding the
parameter setting with a "+" character.  For example, the following
two PGP commands produce the same effect:

     pgp -e +armor=on message.txt smith
or:  pgp -ea message.txt smith

The following is a summary of the various parameters than may be
defined in the configuration file.

TMP - Directory Pathname for Temporary Files

Default setting:  TMP = ""

The configuration parameter TMP specifies what directory to use for
PGP's temporary scratch files.  The best place to put them is on a
RAM disk, if you have one.  That speeds things up quite a bit, and
increases security somewhat.  If TMP is undefined, the temporary
files go in the current directory.  If the shell environmental
variable TMP is defined, PGP instead uses that to specify where the
temporary files should go.

LANGUAGE - Foreign Language Selector

Default setting:  LANGUAGE = "en"

PGP displays various prompts, warning messages, and advisories to the
user on the screen.  For example, messages such as "File not found.",
or "Please enter your pass phrase:".  These messages are normally in
English.  But it is possible to get PGP to display its messages to
the user in other languages, without having to modify the PGP
executable program.

A number of people in various countries have translated all of PGP's
display messages, warnings, and prompts into their native languages. 
These hundreds of translated message strings have been placed in a
special text file called "language.txt", distributed with the PGP
release.  The messages are stored in this file in English, Spanish,
Dutch, German, French, Italian, Russian, Latvian, and Lithuanian. 
Other languages may be added later.  

The configuration parameter LANGUAGE specifies what language to
display these messages in.  LANGUAGE may be set to "en" for English,
"es" for Spanish, "de" for German, "nl" for Dutch, "fr" for French,
"it" for Italian, "ru" for Russian, "lt3" for Lithuanian, "lv" for
Latvian, "esp" for Esperanto.  For example, if this line appeared in
the configuration file:

   LANGUAGE = "fr"

PGP would select French as the language for its display messages.
The default setting is English.

When PGP needs to display a message to the user, it looks in the
"language.txt" file for the equivalent message string in the selected
foreign language and displays that translated message to the user.
If PGP can't find the language string file, or if the selected
language is not in the file, or if that one phrase is not translated
into the selected language in the file, or if that phrase is missing
entirely from the file, PGP displays the message in English.

To conserve disk space, most foreign translations are not included 
in the standard PGP release package, but are available separately.

MYNAME - Default User ID for Making Signatures

Default setting:  MYNAME = ""

The configuration parameter MYNAME specifies the default user ID to
use to select the secret key for making signatures.  If MYNAME is not
defined, the most recent secret key you installed on your secret key
ring will be used.  The user may also override this setting by
specifying a user ID on the PGP command line with the -u option.

TEXTMODE - Assuming Plaintext is a Text File

Default setting:  TEXTMODE = off

The configuration parameter TEXTMODE is equivalent to the -t command
line option.  If enabled, it causes PGP to assume the plaintext is a
text file, not a binary file, and converts it to "canonical text"
before encrypting it.  Canonical text has a carriage return and a
linefeed at the end of each line of text.

This mode will be automatically turned off if PGP detects that the
plaintext file contains what it thinks is non-text binary data.  If
you intend to use PGP primarily for E-mail purposes, you should turn

For VAX/VMS systems, the current version of PGP defaults TEXTMODE=ON.

For further details, see the section "Sending ASCII Text Files Across
Different Machine Environments".

CHARSET - Specifies Local Character Set for Text Files

Default setting:  CHARSET = NOCONV

Because PGP must process messages in many non-English languages with
non-ASCII character sets, you may have a need to tell PGP what local
character set your machine uses.  This determines what character
conversions are performed when converting plaintext files to and from
canonical text format.  This is only a concern if you are in a
non-English non-ASCII environment.

The configuration parameter CHARSET selects the local character set. 
The choices are NOCONV (no conversion), LATIN1 (ISO 8859-1 Latin
Alphabet 1), KOI8 (used by most Russian Unix systems), ALT_CODES
(used by Russian MSDOS systems), ASCII, and CP850 (used by most
western European languages on standard MSDOS PCs).

LATIN1 is the internal representation used by PGP for canonical text,
so if you select LATIN1, no conversion is done.  Note also that PGP
treats KOI8 as LATIN1, even though it is a completely different
character set (Russian), because trying to convert KOI8 to either
LATIN1 or CP850 would be futile anyway.  This means that setting
CHARSET to NOCONV, LATIN1, or KOI8 are all equivalent to PGP.

If you use MSDOS and expect to send or receive traffic in western
European languages, set CHARSET = "CP850".  This will make PGP
convert incoming canonical text messages from LATIN1 to CP850 after
decryption.  If you use the -t (textmode) option to convert to
canonical text, PGP will convert your CP850 text to LATIN1 before
encrypting it.

For further details, see the section "Sending ASCII Text Files Across
Different Machine Environments".

ARMOR - Enable ASCII Armor Output

Default setting:  ARMOR = off

The configuration parameter ARMOR is equivalent to the -a command
line option.  If enabled, it causes PGP to emit ciphertext or keys in
ASCII Radix-64 format suitable for transporting through E-mail
channels.  Output files are named with the ".asc" extension.

If you intend to use PGP primarily for E-mail purposes, you should
turn ARMOR=ON.

For further details, see the section "Sending Ciphertext Through
E-mail Channels: Radix-64 Format" in the Essential Topics volume. 

ARMORLINES - Size of ASCII Armor Multipart Files

Default setting:  ARMORLINES = 720

When PGP creates a very large ".asc" radix-64 file for sending
ciphertext or keys through the E-mail, it breaks the file up into
separate chunks small enough to send through Internet mail
utilities.  Normally, Internet mailers prohibit files larger than
about 50000 bytes, which means that if we restrict the number of
lines to about 720, we'll be well within the limit.  The file chunks
are named with suffixes ".as1", ".as2", ".as3", ... 

The configuration parameter ARMORLINES specifies the maximum number
of lines to make each chunk in a multipart ".asc" file sequence.  If
you set it to zero, PGP will not break up the file into chunks.

Fidonet E-mail files usually have an upper limit of about 32K bytes,
so 450 lines would be appropriate for Fidonet environments.

For further details, see the section "Sending Ciphertext Through
E-mail Channels: Radix-64 Format" in the Essential Topics volume.

KEEPBINARY - Keep Binary Ciphertext Files After Decrypting

Default setting:  KEEPBINARY = off

When PGP reads a ".asc" file, it recognizes that the file is in
radix-64 format and will convert it back to binary before processing
as it normally does, producing as a by-product a ".pgp" ciphertext
file in binary form.  After further processing to decrypt the ".pgp"
file, the final output file will be in normal plaintext form.

You may want to delete the binary ".pgp" intermediate file, or you
may want PGP to delete it for you automatically.  You can still rerun
PGP on the original ".asc" file.

The configuration parameter KEEPBINARY enables or disables keeping
the intermediate ".pgp" file during decryption.

For further details, see the section "Sending Ciphertext Through
E-mail Channels: Radix-64 Format" in the Essential Topics volume.

COMPRESS - Enable Compression

Default setting:  COMPRESS = on

The configuration parameter COMPRESS enables or disables data
compression before encryption.  It is used mainly for debugging PGP. 
Normally, PGP attempts to compress the plaintext before it encrypts
it.  Generally, you should leave this alone and let PGP attempt to
compress the plaintext.

COMPLETES_NEEDED - Number of Completely Trusted Introducers Needed

Default setting:  COMPLETES_NEEDED = 1

The configuration parameter COMPLETES_NEEDED specifies the minimum
number of completely trusted introducers required to fully certify a
public key on your public key ring.  This gives you a way of tuning
PGP's skepticism.

For further details, see the section "How Does PGP Keep Track of 
Which Keys are Valid?" in the Essential Topics volume.

MARGINALS_NEEDED - Number of Marginally Trusted Introducers Needed

Default setting:  MARGINALS_NEEDED = 2

The configuration parameter MARGINALS_NEEDED specifies the minimum
number of marginally trusted introducers required to fully certify a
public key on your public key ring.  This gives you a way of tuning
PGP's skepticism.

For further details, see the section "How Does PGP Keep Track of 
Which Keys are Valid?" in the Essential Topics volume.

CERT_DEPTH - How Deep May Introducers Be Nested

Default setting:  CERT_DEPTH = 4

The configuration parameter CERT_DEPTH specifies how many levels deep
you may nest introducers to certify other introducers to certify
public keys on your public key ring.  For example, If CERT_DEPTH is
set to 1, there may only be one layer of introducers below your own
ultimately-trusted key.  If that were the case, you would be required
to directly certify the public keys of all trusted introducers on
your key ring.  If you set CERT_DEPTH to 0, you could have no
introducers at all, and you would have to directly certify each and
every key on your public key ring in order to use it.  The minimum
CERT_DEPTH is 0, the maximum is 8.

For further details, see the section "How Does PGP Keep Track of 
Which Keys are Valid?" in the Essential Topics volume.

BAKRING - Filename for Backup Secret Keyring

Default setting:  BAKRING = ""

All of the key certification that PGP does on your public key ring
ultimately depends on your own ultimately-trusted public key (or
keys).  To detect any tampering of your public key ring, PGP must
check that your own key has not been tampered with.  To do this, PGP
must compare your public key against a backup copy of your secret key
on some tamper-resistant media, such as a write-protected floppy
disk.  A secret key contains all the information that your public key
has, plus some secret components.  This means PGP can check your
public key against a backup copy of your secret key.

The configuration parameter BAKRING specifies what pathname to use
for PGP's trusted backup copy of your secret key ring.  On MSDOS, you
could set it to "a:\secring.pgp" to point it at a write-protected
backup copy of your secret key ring on your floppy drive.  This check
is performed only when you execute the PGP -kc option to check your
whole public key ring.

If BAKRING is not defined, PGP will not check your own key against
any backup copy.

For further details, see the sections "How to Protect Public Keys
from Tampering" and "How Does PGP Keep Track of Which Keys are
Valid?" in the Essential Topics volume.

PUBRING - Filename for Your Public Keyring

Default setting:  PUBRING = "$PGPPATH/pubring.pgp"

You may want to keep your public keyring in a directory separate from
your config.txt file in the directory specified by your $PGPPATH
environmental variable.  You may specify the full path and filename
for your public keyring by setting the PUBRING parameter.  For
example, on an MSDOS system, you might want to keep your public
keyring on a floppy disk by:

   PUBRING = "a:pubring.pgp"

This feature is especially handy for specifying an alternative
keyring on the command line.

SECRING - Filename for Your Secret Keyring

Default setting:  SECRING = "$PGPPATH/secring.pgp"

You may want to keep your secret keyring in a directory separate from
your config.txt file in the directory specified by your $PGPPATH
environmental variable.  This comes in handy for putting your secret
keyring in a directory or device that is more protected than your
public keyring.  You may specify the full path and filename for your
secret keyring by setting the SECRING parameter.  For example, on an
MSDOS system, you might want to keep your secret keyring on a floppy
disk by:

   SECRING = "a:secring.pgp"

RANDSEED - Filename for Random Number Seed

Default setting:  RANDSEED = "$PGPPATH/randseed.bin"

You may want to keep your random number seed file (for generation of
session keys) in a directory separate from your config.txt file in
the directory specified by your $PGPPATH environmental variable. 
This comes in handy for putting your random number seed file in a
directory or device that is more protected than your public keyring. 
You may specify the full path and filename for your random seed file
by setting the RANDSEED parameter.  For example, on an MSDOS system,
you might want to keep it on a floppy disk by:

   RANDSEED = "a:randseed.bin"

PAGER - Selects Shell Command to Display Plaintext Output

Default setting:  PAGER = ""

PGP lets you view the decrypted plaintext output on your screen (like
the Unix-style "more" command), without writing it to a file, if you
use the -m (more) option while decrypting.  This displays the
decrypted plaintext display on your screen one screenful at a time.

If you prefer to use a fancier page display utility, rather than
PGP's built-in one, you can specify the name of a shell command that
PGP will invoke to display your plaintext output file.  The
configuration parameter PAGER specifies the shell command to invoke
to display the file.  For example, on MSDOS systems, you might want
to use the popular shareware program "" to display your
plaintext message.  Assuming you have a copy of "", you may 
set PAGER accordingly:

   PAGER = "list"

However, if the sender specified that this file is for your eyes
only, and may not be written to disk, PGP always uses its own
built-in display function.

For further details, see the section "Displaying Decrypted Plaintext 
on Your Screen".

SHOWPASS - Echo Pass Phrase to User

Default setting:  SHOWPASS = off

Normally, PGP does not let you see your pass phrase as you type it
in.  This makes it harder for someone to look over your shoulder
while you type and learn your pass phrase.  But some typing-impaired
people have problems typing their pass phrase without seeing what
they are typing, and they may be typing in the privacy of their own
homes.  So they asked if PGP can be configured to let them see what
they type when they type in their pass phrase.

The configuration parameter SHOWPASS enables PGP to echo your typing 
during pass phrase entry.

TZFIX - Timezone Adjustment

Default setting:  TZFIX = 0

PGP provides timestamps for keys and signature certificates in
Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC), which
means the same thing for our purposes.  When PGP asks the system for
the time of day, the system is supposed to provide it in GMT.  

But sometimes, because of improperly configured MSDOS systems, the
system time is returned in US Pacific Standard Time time plus 8
hours.  Sounds weird, doesn't it?  Perhaps because of some sort of US
west-coast jingoism, MSDOS presumes local time is US Pacific time,
and pre-corrects Pacific time to GMT.  This adversely affects the
behavior of the internal MSDOS GMT time function that PGP calls. 
However, if your MSDOS environmental variable TZ is already properly
defined for your timezone, this corrects the misconception MSDOS has
that the whole world lives on the US west coast.  

The configuration parameter TZFIX specifies the number of hours to
add to the system time function to get GMT, for GMT timestamps on
keys and signatures.  If the MSDOS environmental variable TZ is
defined properly, you can leave TZFIX=0.  Unix systems usually
shouldn't need to worry about setting TZFIX at all.  But if you are
using some other obscure operating system that doesn't know about
GMT, you may have to use TZFIX to adjust the system time to GMT. 

On MSDOS systems that do not have TZ defined in the environment, you
should make TZFIX=0 for California, -1 for Colorado, -2 for Chicago,
-3 for New York, -8 for London, -9 for Amsterdam.  In the summer,
TZFIX should be manually decremented from these values.  What a mess.

It would be much cleaner to set your MSDOS environmental variable TZ
in your AUTOEXEC.BAT file, and not use the TZFIX correction.  Then
MSDOS gives you good GMT timestamps, and will handle daylight savings
time adjustments for you.  Here are some sample lines to insert into
AUTOEXEC.BAT, depending on your time zone:

For Los Angeles:  SET TZ=PST8PDT
For Denver:       SET TZ=MST7MDT
For Arizona:      SET TZ=MST7
   (Arizona never uses daylight savings time)
For Chicago:      SET TZ=CST6CDT
For New York:     SET TZ=EST5EDT
For London:       SET TZ=GMT0BST
For Amsterdam:    SET TZ=MET-1DST
For Moscow:       SET TZ=MSK-3MSD
For Aukland:      SET TZ=NZT-13

CLEARSIG - Enable Signed Messages to be Encapsulated as Clear Text

Default setting:  CLEARSIG = on

Normally, unencrypted PGP signed messages have a signature
certificate prepended in binary form.  Also, the signed message is
compressed, rendering the message unreadable to casual human eyes,
even though the message is not actually encrypted.  To send this
binary data through a 7-bit E-mail channel, radix-64 ASCII armor is
applied (see the ARMOR parameter).  Even if PGP didn't compress the
message, the ASCII armor would still render the message unreadable to
human eyes.  The recipient must use PGP to strip the armor off and
decompress it before reading the message.

If the original plaintext message is in text (not binary) form, there
is a way to send a signed message through an E-mail channel in such a
way that the signed message is not compressed and the ASCII armor is
applied only to the binary signature certificate, but not to the
plaintext message.  The CLEARSIG flag provides this useful feature,
making it possible to generate a signed message that can be read with
human eyes, without the aid of PGP.  Of course, you still need PGP to
actually check the signature.

The CLEARSIG flag is preset to "on" beginning with PGP version 2.5. 
To enable the full CLEARSIG behavior, the ARMOR and TEXTMODE flags
must also be turned on.  Set ARMOR=ON (or use the -a option), and set
TEXTMODE=ON (or use the -t option).  If your config file has CLEARSIG
turned off, you can turn it back on again directly on the command
line, like so:

     pgp -sta +clearsig=on message.txt

This message representation is analogous to the MIC-CLEAR message type
used in Internet Privacy Enhanced Mail (PEM).  It is important to
note that since this method only applies ASCII armor to the binary
signature certificate, and not to the message text itself, there is
some risk that the unarmored message may suffer some accidental
molestation while en route.  This can happen if it passes through
some E-mail gateway that performs character set conversions, or in
some cases extra spaces may be added to or stripped from the ends of
lines.  If this occurs, the signature will fail to verify, which may
give a false indication of intentional tampering.  But since PEM
lives under a similar vulnerability, it seems worth having this
feature despite the risks.

Beginning with PGP version 2.2, trailing blanks are ignored on each
line in calculating the signature for text in CLEARSIG mode.

VERBOSE - Quiet, Normal, or Verbose Messages

Default setting:  VERBOSE = 1

VERBOSE may be set to 0, 1, or 2, depending on how much detail you
want to see from PGP diagnostic messages.  The settings are:

0 - Display messages only if there is a problem.  Unix fans wanted
this "quiet mode" setting.

1 - Normal default setting.  Displays a reasonable amount of detail
in diagnostic or advisory messages.

2 - Displays maximum information, usually to help diagnose problems
in PGP.  Not recommended for normal use.  Besides, PGP doesn't have
any problems, right?

INTERACTIVE - Ask for Confirmation for Key Adds

Default Setting:  INTERACTIVE = off

Enabling this mode will mean that if you add a key file containing
multiple keys to your key ring, PGP will ask for confirmation for
each key before adding it to your key ring.

NOMANUAL - Let PGP Generate Keys Without the Manual

Default Setting:  NOMANUAL = off

It is important that the freeware version of PGP not be distributed
without the user documentation, which normally comes with it in the
standard release package.  This manual contains important information
for using PGP, as well as important legal notices.  But some people
have distributed previous versions of PGP without the manual, causing
a lot of problems for a lot of people who get it.  To discourage the
distribution of PGP without the required documentation, PGP has been
changed to require the PGP User's Guide to be found somewhere on your
computer (like in your PGP directory) before PGP will let you generate
a key pair.  However, some users like to use PGP on tiny palmtop
computers with limited storage capacity, so they like to run PGP
without the documentation present on their systems.  To satisfy these
users, PGP can be made to relax its requirement that the manual be
present, by enabling the NOMANUAL flag on the command line during key
generation, like so:

    pgp -kg +nomanual

The NOMANUAL flag can only be set on the command line, not in the
config file.  Since you must read this manual to learn how to enable
this override feature, I hope this will still be effective in
discouraging the distribution of PGP without the manual.

A Peek Under the Hood

Let's take a look at a few internal features of PGP.

Random Numbers

PGP uses a cryptographically strong pseudorandom number generator for
creating temporary conventional session keys.  The seed file for this
is called  "randseed.bin".  It too can be kept in whatever directory
is indicated by the PGPPATH environmental variable.  If this random
seed file does not exist, it is automatically created and seeded with
truly random numbers derived from timing your keystroke latencies.  

This generator reseeds the disk file each time it is used by mixing
in new key material partially derived with the time of day and other
truly random sources.  It uses the conventional encryption algorithm
as an engine for the random number generator.  The seed file contains
both random seed material and random key material to key the
conventional encryption engine for the random generator.

This random seed file should be at least slightly protected from
disclosure, to reduce the risk of an attacker deriving your next or
previous session keys.  The attacker would have a very hard time
getting anything useful from capturing this random seed file, because
the file is cryptographically laundered before and after each use. 
Nonetheless, it seems prudent to at least try to keep it from falling
into the wrong hands.

If you feel uneasy about trusting any algorithmically derived random
number source however strong, keep in mind that you already trust the
strength of the same conventional cipher to protect your messages. 
If it's strong enough for that, then it should be strong enough to
use as a source of random numbers for temporary session keys.  Note
that PGP still uses truly random numbers from physical sources
(mainly keyboard timings) to generate long-term public/secret key

PGP's Conventional Encryption Algorithm

As described earlier, PGP "bootstraps" into a conventional single-key
encryption algorithm by using a public key algorithm to encipher the
conventional session key and then switching to fast conventional
cryptography.  So let's talk about this conventional encryption
algorithm.  It isn't the DES.

The Federal Data Encryption Standard (DES) used to be a good
algorithm for most commercial applications.  But the Government never
did trust the DES to protect its own classified data, because the DES
key length is only 56 bits, short enough for a brute force attack. 
Also, the full 16-round DES has been attacked with some success by
Biham and Shamir using differential cryptanalysis, and by Matsui
using linear cryptanalysis.

The most devastating practical attack on the DES was described at the
Crypto '93 conference, where Michael Wiener of Bell Northern Research
presented a paper on how to crack the DES with a special machine.  He
has fully designed and tested a chip that guesses 50 million DES keys
per second until it finds the right one.  Although he has refrained
from building the real chips so far, he can get these chips
manufactured for $10.50 each, and can build 57000 of them into a
special machine for $1 million that can try every DES key in 7 hours,
averaging a solution in 3.5 hours.  $1 million can be hidden in the
budget of many companies.  For $10 million, it takes 21 minutes to
crack, and for $100 million, just two minutes.  With any major
government's budget for examining DES traffic, it can be cracked in
seconds.  This means that straight 56-bit DES is now effectively dead
for purposes of serious data security applications.  

A possible successor to DES may be a variation known as "triple DES",
which uses two DES keys to encrypt three times, achieving an
effective key space of 112 bits.  But this approach is three times
slower than normal DES.  A future version of PGP may support triple
DES as an option.

PGP does not use the DES as its conventional single-key algorithm to
encrypt messages.  Instead, PGP uses a different conventional
single-key block encryption algorithm, called IDEA(tm).

For the cryptographically curious, the IDEA cipher has a 64-bit block
size for the plaintext and the ciphertext.  It uses a key size of 128
bits.  It is based on the design concept of "mixing operations from
different algebraic groups".  It runs much faster in software than
the DES.  Like the DES, it can be used in cipher feedback (CFB) and
cipher block chaining (CBC) modes.  PGP uses it in 64-bit CFB mode.

The IPES/IDEA block cipher was developed at ETH in Zurich by James L.
Massey and Xuejia Lai, and published in 1990.  This is not a 
"home-grown" algorithm.  Its designers have a distinguished
reputation in the cryptologic community.  Early published papers on
the algorithm called it IPES (Improved Proposed Encryption Standard),
but they later changed the name to IDEA (International Data
Encryption Algorithm).  So far, IDEA has resisted attack much better
than other ciphers such as FEAL, REDOC-II, LOKI, Snefru and Khafre. 
And recent evidence suggests that IDEA is more resistant than the DES
to Biham & Shamir's highly successful differential cryptanalysis
attack.  Biham and Shamir have been examining the IDEA cipher for
weaknesses, without success.  Academic cryptanalyst groups in
Belgium, England, and Germany are also attempting to attack it, as
well as the military services from several European countries.  As
this new cipher continues to attract attack efforts from the most
formidable quarters of the cryptanalytic world, confidence in IDEA is
growing with the passage of time.

Every once in a while, I get a letter from someone who has just
learned the awful truth that PGP does not use pure RSA to encrypt
bulk data.  They are concerned that the whole package is weakened if
we use a hybrid public-key and conventional scheme just to speed
things up.  After all, a chain is only as strong as its weakest
link.  They demand an explanation for this apparent "compromise" in
the strength of PGP.  This may be because they have been caught up in
the public's reverence and awe for the strength and mystique of RSA,
mistakenly believing that RSA is intrinsically stronger than any
conventional cipher.  Well, it's not.  

People who work in factoring research say that the workload to
exhaust all the possible 128-bit keys in the IDEA cipher would
roughly equal the factoring workload to crack a 3100-bit RSA key,
which is quite a bit bigger than the 1024-bit RSA key size that most
people use for high security applications.  Given this range of key
sizes, and assuming there are no hidden weaknesses in the
conventional cipher, the weak link in this hybrid approach is in the
public key algorithm, not the conventional cipher.

It is not ergonomically practical to use pure RSA with large keys to
encrypt and decrypt long messages.  A 1024-bit RSA key would decrypt
messages about 4000 times slower than the IDEA cipher.  Absolutely no
one does it that way in the real world.  Many people less experienced
in cryptography do not realize that the attraction of public key
cryptography is not because it is intrinsically stronger than a
conventional cipher-- its appeal is because it helps you manage keys
more conveniently.

Not only is RSA too slow to use on bulk data, but it even has certain
weaknesses that can be exploited in some special cases of particular
kinds of messages that are fed to the RSA cipher, even for large
keys.  These special cases can be avoided by using the hybrid
approach of using RSA to encrypt random session keys for a
conventional cipher, like PGP does.  So the bottom line is this: 
Using pure RSA on bulk data is the wrong approach, period.  It's too
slow, it's not stronger, and may even be weaker.  If you find a
software application that uses pure RSA on bulk data, it probably
means the implementor does not understand these issues, which could
imply he doesn't understand other important concepts of cryptography.

Data Compression

PGP normally compresses the plaintext before encrypting it.  It's too
late to compress it after it has been encrypted; encrypted data is
incompressible.  Data compression saves modem transmission time and
disk space and more importantly strengthens cryptographic security.  
Most cryptanalysis techniques exploit redundancies found in the
plaintext to crack the cipher.  Data compression reduces this
redundancy in the plaintext, thereby greatly enhancing resistance to 
cryptanalysis.  It takes extra time to compress the plaintext, but 
from a security point of view it seems worth it, at least in my 
cautious opinion. 

Files that are too short to compress or just don't compress well are
not compressed by PGP.  

If you prefer, you can use PKZIP to compress the plaintext before
encrypting it.  PKZIP is a widely-available and effective MSDOS
shareware compression utility from PKWare, Inc.  Or you can use ZIP,
a PKZIP-compatible freeware compression utility on Unix and other
systems, available from Jean-Loup Gailly.  There is some advantage in
using PKZIP or ZIP in certain cases, because unlike PGP's built-in
compression algorithm, PKZIP and ZIP have the nice feature of
compressing multiple files into a single compressed file, which is
reconstituted again into separate files when decompressed.  PGP will
not try to compress a plaintext file that has already been
compressed.  After decrypting, the recipient can decompress the
plaintext with PKUNZIP.  If the decrypted plaintext is a PKZIP
compressed file, PGP automatically recognizes this and advises the 
recipient that the decrypted plaintext appears to be a PKZIP file.

For the technically curious readers, the current version of PGP uses
the freeware ZIP compression routines written by Jean-loup Gailly,
Mark Adler, and Richard B. Wales.  This ZIP software uses
functionally-equivalent compression algorithms as those used by
PKWare's new PKZIP 2.0.  This ZIP compression software was selected
for PGP mainly because of its free portable C source code
availability, and because it has a really good compression ratio, and
because it's fast.  

Peter Gutmann has also written a nice compression utility called
HPACK, available for free from many Internet FTP sites.  It encrypts
the compressed archives, using PGP data formats and key rings.  He
wanted me to mention that here.

Message Digests and Digital Signatures

To create a digital signature, PGP encrypts with your secret key. 
But PGP doesn't actually encrypt your entire message with your secret
key-- that would take too long.  Instead, PGP encrypts a "message

The message digest is a compact (128 bit) "distillate" of your
message, similar in concept to a checksum.  You can also think of it
as a "fingerprint" of the message.  The message digest "represents"
your message, such that if the message were altered in any way, a
different message digest would be computed from it.  This makes it
possible to detect any changes made to the message by a forger.  A
message digest is computed using a cryptographically strong one-way
hash function of the message.  It would be computationally infeasible
for an attacker to devise a substitute message that would produce an
identical message digest.  In that respect, a message digest is much
better than a checksum, because it is easy to devise a different
message that would produce the same checksum.  But like a checksum,
you can't derive the original message from its message digest.  

A message digest alone is not enough to authenticate a message.  The
message digest algorithm is publicly known, and does not require
knowledge of any secret keys to calculate.  If all we did was attach
a message digest to a message, then a forger could alter a message
and simply attach a new message digest calculated from the new
altered message.  To provide real authentication, the sender has to
encrypt (sign) the message digest with his secret key.  

A message digest is calculated from the message by the sender.  The
sender's secret key is used to encrypt the message digest and an
electronic timestamp, forming a digital signature, or signature
certificate.  The sender sends the digital signature along with the
message.  The receiver receives the message and the digital
signature, and recovers the original message digest from the digital
signature by decrypting it with the sender's public key.  The
receiver computes a new message digest from the message, and checks
to see if it matches the one recovered from the digital signature.  If
it matches, then that proves the message was not altered, and it came
from the sender who owns the public key used to check the signature.

A potential forger would have to either produce an altered message
that produces an identical message digest (which is infeasible), or
he would have to create a new digital signature from a different
message digest (also infeasible, without knowing the true sender's
secret key).

Digital signatures prove who sent the message, and that the message
was not altered either by error or design.  It also provides
non-repudiation, which means the sender cannot easily disavow his
signature on the message.

Using message digests to form digital signatures has other advantages
besides being faster than directly signing the entire actual message
with the secret key.  Using message digests allows signatures to be
of a standard small fixed size, regardless of the size of the actual
message.  It also allows the software to check the message integrity
automatically, in a manner similar to using checksums.  And it allows
signatures to be stored separately from messages, perhaps even in a
public archive, without revealing sensitive information about the
actual messages, because no one can derive any message content from a
message digest.

The message digest algorithm used here is the MD5 Message Digest
Algorithm, placed in the public domain by RSA Data Security, Inc.
MD5's designer, Ronald Rivest, writes this about MD5:

"It is conjectured that the difficulty of coming up with two messages
having the same message digest is on the order of 2^64 operations,
and that the difficulty of coming up with any message having a given
message digest is on the order of 2^128 operations.  The MD5
algorithm has been carefully scrutinized for weaknesses.  It is,
however, a relatively new algorithm and further security analysis is
of course justified, as is the case with any new proposal of this
sort.  The level of security provided by MD5 should be sufficient for
implementing very high security hybrid digital signature schemes
based on MD5 and the RSA public-key cryptosystem."

Compatibility with Previous and Future Versions of PGP

PGP version 2.6 can read anything produced by versions 2.3 through
2.7.  However, because of a negotiated agreement between MIT and RSA
Data Security, PGP 2.6 will change its behavior slightly on 1
September 1994, triggered by a built-in software timer.  On that
date, version 2.6 will start producing a new and slightly different
data format for messages, signatures and keys.  PGP 2.6 will still be
able to read and process messages, signatures, and keys produced
under the old format, but it will generate the new format.  This
change is intended to discourage people from continuing to use the
older (2.3a and earlier) versions of PGP, which Public Key Partners
contends infringes its RSA patent (see the section on Legal Issues).
ViaCrypt PGP (see the section Where to Get a Commercial Version of
PGP), versions 2.4 and 2.7, avoids questions of infringement through
Viacrypt's license arrangement with Public Key Partners.  PGP 2.5 and
2.6 avoid questions of infringement by using the RSAREF(TM)
Cryptographic Toolkit, under license from RSA Data Security, Inc.

Outside the United States, the RSA patent is not in force, so PGP
users there are free to use implementations of PGP that do not rely
on RSAREF and its restrictions.  See the notes on foreign versions in
the Legal Issues section later in this manual.  It seems likely that
any versions of PGP prepared outside the US will follow the new
format, whose detailed description is available from MIT.  If
everyone upgrades before September 1994, or soon thereafter, there
will be little interoperability problems.

This format change beginning with 2.6 is similar to the process that
naturally happens when new features are added, causing older versions
of PGP to be unable to read stuff from the newer PGP, while the newer
version can still read the old stuff.  The only difference is that
this is a "legal upgrade", instead of a technical one.  It's a
worthwhile change, if it can achieve peace in our time.

According to ViaCrypt, which sells a commercial version of PGP,
ViaCrypt PGP will evolve to maintain interoperability with new
freeware versions of PGP.

There is a another change that effects interoperability with earlier
versions of PGP.  Unfortunately, due to data format limitations
imposed by RSAREF, PGP 2.5 and 2.6 cannot interpret any messages or
signatures made with PGP version 2.2 or earlier.  Since we had no
choice but to use the new data formats, because of the need to switch
to RSAREF, we can't do anything about this problem.

Beginning with version 2.4 (which was ViaCrypt's first version)
through at least 2.6, PGP does not allow you to generate RSA keys
bigger than 1024 bits.  The upper limit was always intended to be
1024 bits -- there had to be some kind of upper limit, for
performance and interoperability reasons.  But because of a bug in
earlier versions of PGP, it was possible to generate keys larger than
1024 bits.  These larger keys caused interoperability problems
between different older versions of PGP that used different
arithmetic algorithms with different native word sizes.  On some
platforms, PGP choked on the larger keys.  In addition to these older
key size problems, the 1024-bit limit is now enforced by RSAREF.  A
1024-bit key is very likely to be well out of reach of attacks by
major governments.  In a future version, PGP will support bigger keys.

In general, there is compatibility from version 2.0 upwards through
2.4.  Because new features are added, older versions may not always be
able to handle some files created with newer versions.  Because of
massive changes to all the algorithms and data structures, PGP version
2.0 (and later) is not even slightly compatible with PGP version 1.0,
which no one uses anymore anyway.

Future versions of PGP may have to change the data formats for
messages, signatures, keys and key rings, in order to provide
important new features.  We will endeavor to make future versions
handle keys, signatures, and messages from this version, but this is
not guaranteed.  Future releases may provide conversion utilities to
convert old keys, but you may have to dispose of old messages created
with the old PGP.  Also, this current version may not be able to read
stuff produced from all future versions.  


No data security system is impenetrable.  PGP can be circumvented in
a variety of ways.  In any data security system, you have to ask
yourself if the information you are trying to protect is more
valuable to your attacker than the cost of the attack.  This should
lead you to protecting yourself from the cheapest attacks, while not
worrying about the more expensive attacks.  

Some of the discussion that follows may seem unduly paranoid, but
such an attitude is appropriate for a reasonable discussion of
vulnerability issues. 

Compromised Pass Phrase and Secret Key

Probably the simplest attack is if you leave your pass phrase for
your secret key written down somewhere.  If someone gets it and also
gets your secret key file, they can read your messages and make
signatures in your name.  

Don't use obvious passwords that can be easily guessed, such as the
names of your kids or spouse.  If you make your pass phrase a single
word, it can be easily guessed by having a computer try all the words
in the dictionary until it finds your password.  That's why a pass
phrase is so much better than a password.  A more sophisticated
attacker may have his computer scan a book of famous quotations to
find your pass phrase.  An easy to remember but hard to guess pass
phrase can be easily constructed by some creatively nonsensical
sayings or very obscure literary quotes.  

For further details, see the section "How to Protect Secret Keys from
Disclosure" in the Essential Topics volume of the PGP User's Guide.

Public Key Tampering

A major vulnerability exists if public keys are tampered with.  This
may be the most crucially important vulnerability of a public key
cryptosystem, in part because most novices don't immediately
recognize it.  The importance of this vulnerability, and appropriate
hygienic countermeasures, are detailed in the section "How to Protect
Public Keys from Tampering" in the Essential Topics volume.    

To summarize:  When you use someone's public key, make certain it has
not been tampered with.  A new public key from someone else should be
trusted only if you got it directly from its owner, or if it has been
signed by someone you trust.  Make sure no one else can tamper with
your own public key ring.  Maintain physical control of both your
public key ring and your secret key ring, preferably on your own
personal computer rather than on a remote timesharing system.  Keep a
backup copy of both key rings.

"Not Quite Deleted" Files

Another potential security problem is caused by how most operating
systems delete files.  When you encrypt a file and then delete the
original plaintext file, the operating system doesn't actually
physically erase the data.  It merely marks those disk blocks as
deleted, allowing the space to be reused later.  It's sort of like
discarding sensitive paper documents in the paper recycling bin
instead of the paper shredder.  The disk blocks still contain the
original sensitive data you wanted to erase, and will probably
eventually be overwritten by new data at some point in the future. 
If an attacker reads these deleted disk blocks soon after they have
been deallocated, he could recover your plaintext. 

In fact this could even happen accidentally, if for some reason
something went wrong with the disk and some files were accidentally
deleted or corrupted.  A disk recovery program may be run to recover
the damaged files, but this often means some previously deleted files
are resurrected along with everything else.  Your confidential files
that you thought were gone forever could then reappear and be
inspected by whomever is attempting to recover your damaged disk.  
Even while you are creating the original message with a word
processor or text editor, the editor may be creating multiple
temporary copies of your text on the disk, just because of its
internal workings.  These temporary copies of your text are deleted
by the word processor when it's done, but these sensitive fragments
are still on your disk somewhere.  

Let me tell you a true horror story.  I had a friend, married with
young children, who once had a brief and not very serious affair. 
She wrote a letter to her lover on her word processor, and deleted
the letter after she sent it.  Later, after the affair was over, the
floppy disk got damaged somehow and she had to recover it because it
contained other important documents.  She asked her husband to
salvage the disk, which seemed perfectly safe because she knew she
had deleted the incriminating letter.  Her husband ran a commercial
disk recovery software package to salvage the files.  It recovered
the files alright, including the deleted letter.  He read it, which 
set off a tragic chain of events.  

The only way to prevent the plaintext from reappearing is to somehow
cause the deleted plaintext files to be overwritten.  Unless you know
for sure that all the deleted disk blocks will soon be reused, you
must take positive steps to overwrite the plaintext file, and also
any fragments of it on the disk left by your word processor.  You can
overwrite the original plaintext file after encryption by using the
PGP -w (wipe) option.  You can take care of any fragments of the
plaintext left on the disk by using any of the disk utilities
available that can overwrite all of the unused blocks on a disk.  For
example, the Norton Utilities for MSDOS can do this.

Even if you overwrite the plaintext data on the disk, it may still be
possible for a resourceful and determined attacker to recover the
data.  Faint magnetic traces of the original data remain on the disk
after it has been overwritten.  Special sophisticated disk recovery
hardware can sometimes be used to recover the data.

Viruses and Trojan Horses

Another attack could involve a specially-tailored hostile computer
virus or worm that might infect PGP or your operating system.  This
hypothetical virus could be designed to capture your pass phrase or
secret key or deciphered messages, and covertly write the captured
information to a file or send it through a network to the virus's
owner.  Or it might alter PGP's behavior so that signatures are not
properly checked.  This attack is cheaper than cryptanalytic attacks.

Defending against this falls under the category of defending against
viral infection generally.  There are some moderately capable
anti-viral products commercially available, and there are hygienic
procedures to follow that can greatly reduce the chances of viral
infection.  A complete treatment of anti-viral and anti-worm
countermeasures is beyond the scope of this document.  PGP has no
defenses against viruses, and assumes your own personal computer is a
trustworthy execution environment.  If such a virus or worm actually
appeared, hopefully word would soon get around warning everyone.  

Another similar attack involves someone creating a clever imitation
of PGP that behaves like PGP in most respects, but doesn't work the
way it's supposed to.  For example, it might be deliberately crippled
to not check signatures properly, allowing bogus key certificates to
be accepted.  This "Trojan horse" version of PGP is not hard for an
attacker to create, because PGP source code is widely available, so
anyone could modify the source code and produce a lobotomized zombie
imitation PGP that looks real but does the bidding of its diabolical
master.  This Trojan horse version of PGP could then be widely
circulated, claiming to be from me.  How insidious.

You should make an effort to get your copy of PGP from a reliable
source, whatever that means.  Or perhaps from more than one
independent source, and compare them with a file comparison utility.

There are other ways to check PGP for tampering, using digital
signatures.  If someone you trust signs the executable version of
PGP, vouching for the fact that it has not been infected or tampered
with, you can be reasonably sure that you have a good copy.  You
could use an earlier trusted version of PGP to check the signature on
a later suspect version of PGP.  But this will not help at all if
your operating system is infected, nor will it detect if your
original copy of PGP.EXE has been maliciously altered in such a way
as to compromise its own ability to check signatures.  This test also
assumes that you have a good trusted copy of the public key that you
use to check the signature on the PGP executable.

I recommend you not trust your copy of PGP unless it was originally
distributed by MIT or ViaCrypt, or unless it comes with a digitally
signed endorsement from me.  Every new version comes with one or more
digital signatures in the distribution package, signed by the
originator of that release package.  This is usually someone
representing MIT or ViaCrypt, or whoever released that version. 
Check the signatures on the version that you get.  I have actually
seen several bogus versions of PGP distribution packages, even from
apparantly reliable freeware distribution channels such as CD-ROM
distributors and Compuserve.  Always check the signature when you get
a new version.

Physical Security Breach

A physical security breach may allow someone to physically acquire
your plaintext files or printed messages.  A determined opponent
might accomplish this through burglary, trash-picking, unreasonable
search and seizure, or bribery, blackmail or infiltration of your
staff.  Some of these attacks may be especially feasible against
grassroots political organizations that depend on a largely volunteer
staff.  It has been widely reported in the press that the FBI's
COINTELPRO program used burglary, infiltration, and illegal bugging
against antiwar and civil rights groups.  And look what happened at
the Watergate Hotel.  

Don't be lulled into a false sense of security just because you have
a cryptographic tool.  Cryptographic techniques protect data only
while it's encrypted-- direct physical security violations can still
compromise plaintext data or written or spoken information.  

This kind of attack is cheaper than cryptanalytic attacks on PGP.

Tempest Attacks

Another kind of attack that has been used by well-equipped opponents
involves the remote detection of the electromagnetic signals from
your computer.  This expensive and somewhat labor-intensive attack is
probably still cheaper than direct cryptanalytic attacks.  An
appropriately instrumented van can park near your office and remotely
pick up all of your keystrokes and messages displayed on your
computer video screen.  This would compromise all of your passwords,
messages, etc.  This attack can be thwarted by properly shielding all
of your computer equipment and network cabling so that it does not
emit these signals.  This shielding technology is known as "Tempest",
and is used by some Government agencies and defense contractors.  
There are hardware vendors who supply Tempest shielding commercially,
although it may be subject to some kind of Government licensing.  Now
why do you suppose the Government would restrict access to Tempest

Exposure on Multi-user Systems

PGP was originally designed for a single-user MSDOS machine under
your direct physical control.  I run PGP at home on my own PC, and
unless someone breaks into my house or monitors my electromagnetic
emissions, they probably can't see my plaintext files or secret keys. 

But now PGP also runs on multi-user systems such as Unix and VAX/VMS.
On multi-user systems, there are much greater risks of your plaintext
or keys or passwords being exposed.  The Unix system administrator or
a clever intruder can read your plaintext files, or perhaps even use
special software to covertly monitor your keystrokes or read what's
on your screen.  On a Unix system, any other user can read your
environment information remotely by simply using the Unix "ps"
command.  Similar problems exist for MSDOS machines connected on a
local area network.  The actual security risk is dependent on your
particular situation.  Some multi-user systems may be safe because
all the users are trusted, or because they have system security
measures that are safe enough to withstand the attacks available to
the intruders, or because there just aren't any sufficiently
interested intruders.  Some Unix systems are safe because they are
only used by one user-- there are even some notebook computers
running Unix.  It would be unreasonable to simply exclude PGP from
running on all Unix systems.

PGP is not designed to protect your data while it is in plaintext
form on a compromised system.  Nor can it prevent an intruder from
using sophisticated measures to read your secret key while it is
being used.  You will just have to recognize these risks on
multi-user systems, and adjust your expectations and behavior
accordingly.  Perhaps your situation is such that you should consider
running PGP only on an isolated single-user system under your direct
physical control.  That's what I do, and that's what I recommend.

Traffic Analysis

Even if the attacker cannot read the contents of your encrypted
messages, he may be able to infer at least some useful information by
observing where the messages come from and where they are going, the
size of the messages, and the time of day the messages are sent. 
This is analogous to the attacker looking at your long distance phone
bill to see who you called and when and for how long, even though the
actual content of your calls is unknown to the attacker.  This is
called traffic analysis.  PGP alone does not protect against traffic
analysis.  Solving this problem would require specialized 
communication protocols designed to reduce exposure to traffic
analysis in your communication environment, possibly with some
cryptographic assistance.

Protecting Against Bogus Timestamps

A somewhat obscure vulnerability of PGP involves dishonest users
creating bogus timestamps on their own public key certificates and
signatures.  You can skip over this section if you are a casual user
and aren't deeply into obscure public key protocols.

There's nothing to stop a dishonest user from altering the date and
time setting of his own system's clock, and generating his own public
key certificates and signatures that appear to have been created at a
different time.  He can make it appear that he signed something
earlier or later than he actually did, or that his public/secret key
pair was created earlier or later.  This may have some legal or
financial benefit to him, for example by creating some kind of 
loophole that might allow him to repudiate a signature.

I think this problem of falsified timestamps in digital signatures is
no worse than it is already in handwritten signatures.  Anyone may
write a date next to their handwritten signature on a contract with
any date they choose, yet no one seems to be alarmed over this state
of affairs.  In some cases, an "incorrect" date on a handwritten
signature might not be associated with actual fraud.  The timestamp
might be when the signator asserts that he signed a document, or
maybe when he wants the signature to go into effect.

In situations where it is critical that a signature be trusted to
have the actual correct date, people can simply use notaries to
witness and date a handwritten signature.  The analog to this in
digital signatures is to get a trusted third party to sign a
signature certificate, applying a trusted timestamp.  No exotic or
overly formal protocols are needed for this.  Witnessed signatures
have long been recognized as a legitimate way of determining when a
document was signed.

A trustworthy Certifying Authority or notary could create notarized
signatures with a trustworthy timestamp.  This would not necessarily
require a centralized authority.  Perhaps any trusted introducer or
disinterested party could serve this function, the same way real
notary publics do now.  When a notary signs other people's
signatures, it creates a signature certificate of a signature
certificate.  This would serve as a witness to the signature the same
way real notaries now witness handwritten signatures.  The notary
could enter the detached signature certificate (without the actual
whole document that was signed) into a special log controlled by the
notary.  Anyone can read this log.  The notary's signature would have
a trusted timestamp, which might have greater credibility or more
legal significance than the timestamp in the original signature.

There is a good treatment of this topic in Denning's 1983 article in
IEEE Computer (see references).  Future enhancements to PGP might
have features to easily manage notarized signatures of signatures,
with trusted timestamps.


An expensive and formidable cryptanalytic attack could possibly be
mounted by someone with vast supercomputer resources, such as a
Government intelligence agency.  They might crack your RSA key by
using some new secret factoring breakthrough.  Perhaps so, but it is
noteworthy that the US Government trusts the RSA algorithm enough in
some cases to use it to protect its own nuclear weapons, according to
Ron Rivest.  And civilian academia has been intensively attacking it
without success since 1978. 

Perhaps the Government has some classified methods of cracking the
IDEA(tm) conventional encryption algorithm used in PGP.  This is
every cryptographer's worst nightmare.  There can be no absolute
security guarantees in practical cryptographic implementations.  

Still, some optimism seems justified.  The IDEA algorithm's designers
are among the best cryptographers in Europe.  It has had extensive
security analysis and peer review from some of the best cryptanalysts
in the unclassified world.  It appears to have some design advantages
over the DES in withstanding differential and linear cryptanalysis,
which have both been used to crack the DES.  

Besides, even if this algorithm has some subtle unknown weaknesses,
PGP compresses the plaintext before encryption, which should greatly
reduce those weaknesses.  The computational workload to crack it is
likely to be much more expensive than the value of the message.

If your situation justifies worrying about very formidable attacks of
this caliber, then perhaps you should contact a data security
consultant for some customized data security approaches tailored to
your special needs.  Boulder Software Engineering, whose address and
phone are given at the end of this document, can provide such

In summary, without good cryptographic protection of your data
communications, it may have been practically effortless and perhaps
even routine for an opponent to intercept your messages, especially
those sent through a modem or E-mail system.  If you use PGP and
follow reasonable precautions, the attacker will have to expend far
more effort and expense to violate your privacy. 

If you protect yourself against the simplest attacks, and you feel
confident that your privacy is not going to be violated by a
determined and highly resourceful attacker, then you'll probably be
safe using PGP.  PGP gives you Pretty Good Privacy.

Legal Issues

Trademarks, Copyrights, and Warranties

"PGP", "Pretty Good Privacy", "Phil's Pretty Good Software", and the
"Pretty Good" label for computer software and hardware products are
all trademarks of Philip R. Zimmermann.

PGP is (c) Copyright Philip R. Zimmermann, 1990-1994.  All rights
reserved.  The PGP User's Guide is also copyright Philip Zimmermann,
1990-1994.  All rights reserved.  These rights include but are not
limited to any foreign language translations of the manual or the
software, and all derivative works of both.

MIT may have a copyright on the particular software distribution
package that they distribute from the MIT FTP site.  This copyright
on the "compilation" of the distribution package in no way implies
that MIT has a copyright on PGP itself, or its user documentation. 

The author assumes no liability for damages resulting from the use of
this software, even if the damage results from defects in this
software, and makes no representations concerning the merchantability
of this software or its suitability for any specific purpose.  It is
provided "as is" without express or implied warranty of any kind. 
Because certain actions may delete files or render them
unrecoverable, the author assumes no responsibility for the loss or
modification of any data.

Patent Rights on the Algorithms

The RSA public key cryptosystem was developed at MIT, which holds a
patent on it (U.S. patent #4,405,829, issued 20 Sep 1983).  A company
in California called Public Key Partners (PKP) holds the exclusive
commercial license to sell and sub-license the RSA public key
cryptosystem.  MIT distributes a freeware version of PGP under the
terms of the RSAREF license from RSA Data Security, Inc. (RSADSI).

Non-US users of earlier versions of PGP should note that the RSA
patent does not apply outside the US, and at least at the time of
this writing, the author is not aware of any RSA patent in any other
country.  Federal agencies may use the RSA algorithm, because the
Government paid for the development of RSA with grants from the
National Science Foundation and the Navy.  But despite the fact of
Government users having free access to the RSA algorithm, Government
use of PGP has additional restrictions imposed by the agreement I
have with ViaCrypt, as explained later.

I wrote my PGP software from scratch, with my own independently
developed implementation of the RSA algorithm.  Before publishing PGP
in 1991, I got a formal written legal opinion from a patent attorney
with extensive experience in software patents.  I'm convinced that
publishing PGP the way I did does not violate patent law.

Not only did PKP acquire the exclusive patent rights for the RSA
cryptosystem, but they also acquired the exclusive rights to three
other patents covering other public key schemes invented by others at
Stanford University, also developed with federal funding.  This
one company claims to have a legal lock in the USA on nearly all
practical public key cryptosystems.  They even appear to be claiming
patent rights on the very concept of public key cryptography,
regardless of what clever new original algorithms are independently
invented by others.  I find such a comprehensive monopoly troubling,
because I think public key cryptography is destined to become a
crucial technology in the protection of our civil liberties and
privacy in our increasingly connected society.  At the very least,
it places these vital tools at risk by affording to the Government
a single pressure point of influence.

Beginning with PGP version 2.5 (distributed by MIT, the holders of
the original RSA patent), the freeware version of PGP uses the RSAREF
subroutine library to perform its RSA calculations, under the RSAREF
license, which allows noncommercial use in the USA.  RSAREF is a
subroutine package from RSA Data Security Inc, that implements the
RSA algorithm.  The RSAREF subroutines are used instead of PGP's
original subroutines to implement the RSA functions in PGP.  See the
RSAREF license for terms and conditions of use of RSAREF

PGP 2.5 was released by MIT for a brief test period in May, 1994
before releasing 2.6.  PGP 2.5 was released under the 16 March, 1994
RSAREF license, which is a perpetual license, so it may legally be
used forever in the US.  But it would be better for PGP's legal and
political future for users in the United States to upgrade to version
2.6 or later to facilitate the demise of PGP 2.3a and earlier
versions.  Also, PGP 2.5 has bugs that are corrected in 2.6, and 2.5
will not read the new data format after September 1, 1994.  (See the
section on Compatibility with Previous and Future Versions of PGP.)

The PGP 2.0 release was a joint effort of an international team of
software engineers, implementing enhancements to the original PGP
with design guidance from me.  It was released by Branko Lankester in
The Netherlands and Peter Gutmann in New Zealand, out of reach of US
patent law.  Although released only in Europe and New Zealand, it
spontaneously spread to the USA without help from me or the PGP
development team.

The IDEA(tm) conventional block cipher used by PGP is covered by a
patent in Europe, held by ETH and a Swiss company called Ascom-Tech
AG.  The US Patent number is 5,214,703, and the European patent
number is EP 0 482 154 B1.  IDEA(tm) is a trademark of Ascom-Tech AG.
There is no license fee required for noncommercial use of IDEA.
Commercial users of IDEA may obtain licensing details from Dieter
Profos, Ascom Tech AG, Teleservices Section, Postfach 151, 4502
Solothurn, Switzerland, Tel +41 65 242885, Fax +41 65 235761.   

Ascom-Tech AG has granted permission for the freeware version PGP to
use the IDEA cipher in non-commercial uses, everywhere.  In the US
and Canada, all commercial or Government users must obtain a licensed
version from ViaCrypt, who has a license from Ascom-Tech for the IDEA

Ascom-Tech has recently been changing its policies regarding the use
of IDEA in PGP for commercial use outside the US, and that policy
still seems to be in flux.  They tell me that their current thinking
is as follows:  They will allow commercial users of PGP outside the
US or Canada to use IDEA in PGP without paying royalties to
Ascom-Tech, because it is not currently possible for commercial users
to buy a licensed version of PGP outside the US or Canada.  If the
legal situation in the USA changes in the future, so that users
outside the US or Canada can buy a licensed version of PGP (either
from ViaCrypt, or from me, or from a foreign enterprise licensed by
me), then Ascom-Tech will begin enforcing its patent licensing
policies on commercial users who are in a position to buy a licensed
version of PGP.  To get a more up-to-date report on this, contact
Ascom-Tech AG.

The ZIP compression routines in PGP come from freeware source code,
with the author's permission.  I'm not aware of any patents on the
compression algorithms used in the ZIP routines.

Freeware Status and Restrictions

PGP is not shareware, it's freeware.  Published as a community
service.  Giving PGP away for free will encourage far more people to
use it, which will have a greater social impact.  Feel free to
disseminate the complete unmodified PGP release package as widely as
possible, but be careful not to violate U.S. export controls if you
live in the USA.  Give it to all your friends.  If you have access to
any electronic Bulletin Board Systems, please upload the complete PGP
executable object release package to as many BBS's as possible.

You may also disseminate the source code release package.  PGP's
source code is published to assist public scrutiny of PGP to show that
it has no hidden weaknesses or back doors, and to help people to find
bugs and report them.  Recompile it and port it to new target
machines.  Experiment with the code and learn from it.

I place no restraints on your modifying the source code for your own
use.  However, do not distribute a modified version of PGP under the
name "PGP" without first getting permission from me.  Please respect
this restriction.  PGP's reputation for cryptographic integrity
depends on maintaining strict quality control on PGP's cryptographic
algorithms and protocols.  Beyond that, ad hoc "improvements" to PGP
can affect interoperability, which creates user confusion and
compatability problems that could damage PGP's (and my own)
reputation and undermine the good will earned by the PGP trademark.

This has already started to happen, which is why I'm making a point
of it here.  This creates technical support headaches, and I get
phone calls from confused users who run into problems either because
they have a mutant strain of PGP, or are trying to process a key,
signature, or message that came from an incompatible mutant strain of
PGP.  The source code to PGP was not published to help spawn these
mutant strains.

If you want to distribute a modified version of PGP, or use a modified
version to send messages to other people, you should name the program
in such a way that no one could mistake it for PGP.  The messages,
signatures, and keys it produces must also be labeled in such a way
that no one could mistake them for material produced by PGP.  If you
feel you must modify your copy of PGP, and there is any chance that
the modified version could escape into the environment, please contact
me first to discuss some easy methods for how to prevent people from
confusing your version with the standard PGP.  Perhaps we'll even
decide that your changes are appropriate for incorporating into the
standard PGP release.

Also, you should note that official executable versions of PGP are
always released signed by the PGP developers, so you can verify their
authenticity.  If you find a corrupted copy of PGP, or notice one
being distributed, please contact the people doing the distribution
and suggest that they replace this with an authentic version.

Some older versions of PGP were published under the terms of the
General Public License (GPL), a license designed by the Free Software
Foundation to protect the status of free software.  Newer freeware
versions of PGP are no longer published under the GPL.  The RSAREF
licensing terms are more stringent than those of the GPL.  But even
if a version of PGP is published without RSAREF, in a situation or
place where the RSA patent does not apply, I still do not want the
GPL to apply to PGP, for a variety of reasons, not the least of which
is because the GPL is not optimal for protecting PGP from being
republished with ad-hoc "improvements".

Outside the United States, the RSA patent is not in force, so PGP
users there are free to use implementations of PGP that do not rely
on RSAREF and its restrictions.  Canadians may use PGP without using
RSAREF, and there are legal ways to export PGP to Canada.  In Canada,
where RSAREF is not needed, it is easy to modify and recompile the
current PGP source code to perform the RSA calculations without using
the RSAREF library, just as it was done in PGP 2.3a.  In such a case,
this modified PGP may be re-released under the identical licensing
terms as the current official freeware PGP release, but without the
RSAREF-specific restrictions.  It may not be re-released under the
GPL, as certain older versions were.  And this manual must accompany
it.  That modified version of PGP may not be used in environments
where RSAREF would be needed.

Restrictions on Commercial Use of PGP

The freeware version of PGP is for personal, non-commercial use.  For
commercial use in the USA or Canada, contact ViaCrypt in Phoenix,
Arizona (phone 602 944-0773, or email

I made an agreement with ViaCrypt in the summer of 1993 to license the
exclusive commercial rights to PGP, so that there would be a way for
corporations to use PGP without risk of a patent infringement lawsuit
from PKP.  For PGP to succeed in the long term as a viable industry
standard, the legal stigma associated with the RSA patent rights had
to be resolved.  ViaCrypt had already obtained a patent license from
PKP to make, use, and sell products that practice the RSA patents.
ViaCrypt offered a way out of the patent quagmire for PGP to penetrate
the corporate environment.  They could sell a fully-licensed version
of PGP, but only if I licensed it to them under these terms.  So we
entered into an agreement to do that, opening the door for PGP's
future in the commercial sector, which was necessary for PGP's
long-term political future.

Therefore, regardless of the complexities and partially overlapping
restrictions from all the other terms and conditions imposed by the
various patent and copyright licenses (RSA, RSAREF, and IDEA) from
various third parties, an additional overriding restriction on PGP
usage is imposed by my own agreement with ViaCrypt: The freeware
version of PGP is only for personal, non-commercial use -- all other
users in the USA and Canada must obtain a fully licensed version of
PGP from ViaCrypt.  The restrictions imposed by my agreement with
ViaCrypt do not apply outside the USA or Canada.

Finally, if you want to turn PGP into a commercial product and make
money selling it, then we must agree on a way for me to also make
money on it.  If you use PGP in such a manner that you must pay
patent royalties or any other software licensing fees to the patent
holders for any cryptographic algorithms used by PGP, then we must
agree on a way for me to also be paid in some manner.  Buying PGP
from ViaCrypt is one way to meet this requirement.

Other Licensing Restrictions

Under no circumstances may PGP be distributed without the PGP
documentation, including this PGP User's Guide.  And, assuming this is
an RSAREF version of PGP, the RSAREF license agreement must be kept
with it.  You must also keep the copyright, patent, and trademark
notices on PGP and its documentation.

The standard freeware PGP release is primarily distributed in
electronic form, as a single compressed archive file, containing a
collection of files in a "shrink-wrapped" package.  This package
should not be broken up and the components separately distributed --
in the interests of quality control, we want to make it difficult for
users to obtain PGP without getting the full release package.


In the USA, PGP is available for free from the Massachusetts Institute
of Technology, under the restrictions described above.

The primary release site for PGP is the Massachusetts Institute of
Technology, at their FTP site "", in the /pub/PGP
directory.  You may obtain free copies or updates to PGP from this
site, or any other Internet FTP site or BBS that PGP has spread to.
Don't ask me for a copy directly from me, especially if you live
outside the US or Canada.  I recommend that you not use any modified
version of PGP that comes from any other source, other than MIT,
ViaCrypt, or me, unless it is accompanied by a signed endorsement
from me personally.  You can get the official release software from
many other distribution sites "downstream" from MIT.  Hopefully, all
these other sites are adhering to US export controls.

The PGP version 2.6.1 executable object release package for MSDOS
contains the PGP executable software, documentation, RSAREF license,
sample key rings including my own public key, and signatures for the
software and this manual, all in one PKZIP compressed file called  The PGP source release package for MSDOS contains all
the C source files in one PKZIP compressed file called 
The filename for the release package is derived from the version
number of the release.

Export Controls

The U.S. Government has made it illegal in most cases to export good
cryptographic technology, and that may include PGP.  They regard this
kind of software just like they regard munitions.  This is determined
not by legislation, but by administrative policies of the State
Department, Defense Department and Commerce Department.

The U.S. Government is using export restrictions as a means of
suppressing both domestic and foreign availability of cryptographic
technology.  In particular, it is trying to suppress the emergence of
an international standard for cryptographic protocols, until it can
establish the Escrowed Encryption Standard (the Clipper chip) as the
dominant standard.

Any export restrictions on PGP are imposed by the US Government. 
This does not imply that I or MIT agree with these restrictions.  We
just comply with them.  We do not impose additional licensing
restrictions of our own on the use of PGP outside of the US, other
than those restrictions that already apply inside the US.  PGP may be
subject to export controls.  Anyone wishing to export it should first
consult the State Department's Office of Defense Trade Controls.

I will not export this software out of the US or Canada in cases when
it is illegal to do so under US controls, and I urge other people not
to export it on their own.  If you live outside the US or Canada, I
urge you not to violate US export laws by getting any version of PGP
in a way that violates those laws.  Since thousands of domestic users
got the first version after its initial publication, it somehow
leaked out of the US and spread itself widely abroad, like dandelion
seeds blowing in the wind.

Starting with PGP version 2.0 through version 2.3a, the release point
of the software has been outside the US, on publicly-accessible
computers in Europe.  Each release was electronically sent back into
the US and posted on publicly-accessible computers in the US by PGP
privacy activists in foreign countries.  There are some restrictions
in the US regarding the import of munitions, but I'm not aware of any
cases where this was ever enforced for importing cryptographic
software into the US.  I imagine that a legal action of that type
would be quite a spectacle of controversy.

ViaCrypt PGP is sold in the United States and Canada and is not for
export.  The following language was supplied by the US Government to
ViaCrypt for inclusion in the ViaCrypt PGP documentation:  "PGP is
export restricted by the Office of Export Administration, United
States Department of Commerce and the Offices of Defense Trade
Controls and Munitions Control, United States Department of State. 
PGP cannot be exported or reexported, directly or indirectly, (a)
without all export or reexport licenses and governmental approvals
required by any applicable laws, or (b) in violation of any
prohibition against the export or reexport of any part of PGP."  The
Government may take the position that the freeware PGP versions are
also subject to those controls.

The freeware PGP versions 2.5 and 2.6 were released through a posting
on a controlled FTP site maintained by MIT.  This site has
restrictions and limitations which have been used on other FTP sites
to comply with export control requirements with respect to other
encryption software such as Kerberos and software from RSA Data
Security, Inc.  I urge you not to do anything which would weaken
those controls or facilitate any improper export of PGP.

Although PGP has become a worldwide de facto standard for E-mail
encryption, and is widely available overseas, I still get calls from
people outside the US who ask me if it is legal to use it in their
own country, for versions that are already available there.  Please
don't contact me to ask me if it is legal to use PGP in your country
if you live outside the US.  That question is not up to me.  I've got
enough legal problems of my own with export control issues, without
getting involved in giving you legal advice over my phone.  It might
even put me at some legal risk to simply answer a question like that
for a foreigner.  If this question concerns you, ask someone else,
like a lawyer.

You may have a need to use PGP in a commercial application outside
the US or Canada.  Unfortunately, at the time of this writing, there
is no current commercial source for PGP outside the US or Canada.  I
am trying to find a US-legal way to make a commercially licensed
version available abroad, but right now the US export restrictions
make that difficult without putting me at legal risk.  This situation
may change.

Some foreign governments impose serious penalties on anyone inside
their country for merely using encrypted communications.  In some
countries they might even shoot you for that.  But if you live in
that kind of country, perhaps you need PGP even more.

Philip Zimmermann's Legal Situation

At the time of this writing, I am the target of a US Customs criminal
investigation in the Northern District of California.  A criminal
investigation is not a civil lawsuit.  Civil lawsuits do not involve
prison terms.  My defense attorney has been told by the Assistant US
Attorney that the area of law of interest to the investigation has to
do with the export controls on encryption software.  The federal
mandatory sentencing guidelines for this offense are 41 to 51 months
in a federal prison.  US Customs appears to be taking the position
that electronic domestic publication of encryption software is the
same as exporting it.  The prosecutor has issued a number of federal
grand jury subpoenas.  It may be months before a decision is reached
on whether to seek indictment.  This situation may change at any
time, so this description may be out of date by the time you read
it.  Watch the news for further developments.  If I am indicted and
this goes to trial, it will be a major test case.

I have a legal defense fund set up for this case.  So far, no other
organization is doing the fundraising for me, so I am depending on
people like you to contribute directly to this cause.  If you care
about the future of your civil liberties in the information age, then
perhaps you will care about this case.  The legal fees are expensive,
the meter is running, and I need your help.  The fund is run by my
lead defense attorney, Phil Dubois, here in Boulder.  Please send
your contributions to:

   Philip L. Dubois, Lawyer
   2305 Broadway
   Boulder, Colorado 80304 USA
   Phone (303) 444-3885

You can also phone in your donation and put it on Mastercard or Visa.
If you want to be really cool, you can use Internet E-mail to send in
your contribution, encrypting your message with PGP so that no one
can intercept your credit card number.  Include in your E-mail
message your Mastercard or Visa number, expiration date, name on the
card, and amount of donation.  Then sign it with your own key and
encrypt it with Phil Dubois's public key (his key is included in the
standard PGP distribution package, in the "keys.asc" file).  Put a
note on the subject line that this is a donation to my legal defense
fund, so that Mr. Dubois will decrypt it promptly.  Please don't send
a lot of casual encrypted E-mail to him -- I'd rather he use his
valuable time to work on my case.

If you want to read some press stories to find out why this is an
important case, see the following references:

  1)  William Bulkeley, "Cipher Probe", Wall Street Journal, Thursday
      28 April 1994, front page.
  2)  John Cary, "Spy vs. Computer Nerd:  The Fight Over Data
      Security", Business Week, 4 Oct 1993, page 43.
  3)  Jon Erickson, "Cryptography Fires Up the Feds", Dr. Dobb's
      Journal, December 1993, page 6.
  4)  John Markoff, "Federal Inquiry on Software Examines Privacy
      Programs", New York Times, Tuesday 21 Sep 1993, page C1.
  5)  Kurt Kleiner, "Punks and Privacy", Mother Jones Magazine, 
      Jan/Feb 1994, page 17.
  6)  Steven Levy, "Battle of the Clipper Chip", New York Times
      Magazine, Sunday 12 Jun 1994, page 44.
  7)  Steven Levy, "Crypto Rebels", WIRED, May/Jun 1993, page 54.
  8)  John Markoff, "Cyberspace Under Lock and Key", New York Times,
      Sunday 13 Feb 1994.
  9)  Philip Elmer-DeWitt, "Who Should Keep the Keys", Time, 14 Mar
      1994, page 90.

There are a great many other articles on PGP from around the world. 
I'm keeping a scrapbook.

Other Sources of Information on PGP

Where to Get a Commercial Version of PGP

To get a fully licensed version of PGP for use in the USA or Canada,

   9033 North 24th Avenue, Suite 7
   Phoenix, Arizona 85021  USA
   Phone: (602) 944-0773, or (800) 536-2664 
   Fax: (602) 943-2601

ViaCrypt has a version of PGP for MSDOS, and a number of Unix
platforms.  They also have a Windows shell version, and other 
versions are under development, including Macintosh.  If you have a
need to use PGP in a commercial or Government setting, and ViaCrypt
has a version of PGP for your hardware platform, you should get
ViaCrypt PGP.

ViaCrypt has obtained all the necessary licenses from PKP, Ascom-Tech
AG, and Philip Zimmermann to sell PGP for use in commercial or
government environments.  ViaCrypt PGP is every bit as secure as the
freeware PGP, and is entirely compatible in both directions with the
freeware version of PGP.  ViaCrypt PGP is the perfect way to get a
fully licensed version of PGP into your corporate environment.

If you work in a large company and you are a fan of PGP, I urge you
to try to persuade your company to buy lots of copies of PGP from
ViaCrypt.  Not just because that will earn royalties for me.  If
ViaCrypt can make PGP a commercial success, it will go a long way
toward cementing PGP's political future as an unstoppable standard
for E-mail encryption in the corporate world.  The corporate world is
where the money is, and that affects public policy like nothing
else.  And that includes Government policy to suppress strong

Reporting PGP Bugs

Bugs in PGP should be reported via E-mail to MIT, the official
distribution site of PGP.  The E-mail address for bug reports is  MIT will forward a copy of your bug report to me. 
When you report bugs, be sure to specify what machine and operating
system you are using and what version of PGP you have, and provide
enough detail to reproduce the problem.  It would also be a good idea
to find out if you have the latest version of PGP, in case the bug
has already been fixed.  Also, it's a good idea to make sure it
really is a bug before you report it.  RTFM.

Fan Mail, Updates, and News

After all this work I have to admit I wouldn't mind getting some fan
mail for PGP, to gauge its popularity.  Let me know what you think
about it and how many of your friends use it.  Bug reports and
suggestions for enhancing PGP are welcome, too.  Perhaps a future PGP
release will reflect your suggestions.  

This project has not been funded and the project has nearly eaten me
alive.  This means you usually won't get a reply to your mail, unless
you only need a short written reply and you include a stamped
self-addressed envelope.  But I often do reply to E-mail.  Please
keep it in English, as my foreign language skills are weak.  If you
call and I'm not in, it's best to just try again later.  I usually
don't return long distance phone calls, unless you leave a message
that I can call you collect, and even then I might not return your
call.  If you need any significant amount of my time, I am available
on a paid consulting basis, and I always return those calls.

The most inconvenient mail I get is for some well-intentioned person
to send me a few dollars asking me for a copy of PGP.  I don't send 
it to them because I'd rather avoid any legal problems with PKP.  Or
worse, sometimes these requests are from foreign countries, and I
would be risking a violation of US cryptographic export control
laws.  Even if there were no legal hassles involved in sending PGP to
them, they usually don't send enough money to make it worth my time.
I'm just not set up as a low cost low volume mail order business.  I
can't just ignore the request and keep the money, because they
probably regard the money as a fee for me to fulfill their request.
If I return the money, I might have to get in my car and drive down
to the post office and buy some postage stamps, because these
requests rarely include a stamped self-addressed envelope.  And I
have to take the time to write a polite reply that I can't do it.  If
I postpone the reply and set the letter down on my desk, it might be
buried within minutes and won't see the light of day again for
months.  Multiply these minor inconveniences by the number of
requests I get, and you can see the problem.  Isn't it enough that
the software is free?  It would be nicer if people could try to get
PGP from any of the myriad other sources.  If you don't have a modem,
ask a friend to get it for you.  If you can't find it yourself, I
don't mind answering a quick phone call.

If anyone wants to volunteer to improve PGP, please let me know.  It
could certainly use some more work.  Some features were deferred to
get it out the door.  A number of PGP users have since donated their
time to port PGP to Unix on Sun SPARCstations, to Ultrix, to VAX/VMS,
to OS/2, to the Amiga, and to the Atari ST.  Perhaps you can help
port it to some new environments.  But please let me know if you plan
to port or add enhancements to PGP, to avoid duplication of effort,
and to avoid starting with an obsolete version of the source code.  

Because so many foreign language translations of PGP have been
produced, most of them are not distributed with the regular PGP
release package because it would require too much disk space. 
Separate language translation "kits" are available from a number of
independent sources, and are sometimes available separately from the
same distribution centers that carry the regular PGP release
software.  These kits include translated versions of the file 
LANGUAGE.TXT, PGP.HLP, and the PGP User's Guide.  If you want to
produce a translation for your own native language, contact me first
to get the latest information and standard guidelines, and to find
out if it's been translated to your language already.  To find out
where to get a foreign language kit for your language, you might
check on the Internet newsgroups, or get it from Mike Johnson

If you have access to the Internet, watch for announcements of new
releases of PGP on the Internet newsgroups "sci.crypt" and PGP's own
newsgroup, "".  If you want to know where to get PGP,
MIT is the primary FTP distribution site (  Or ask
Mike Johnson ( for a list of Internet FTP sites and BBS
phone numbers.

Computer-Related Political Groups

PGP is a very political piece of software.  It seems appropriate to
mention here some computer-related activist groups.  Full details on
these groups, and how to join them, is provided in a separate
document file in the PGP release package.

The Electronic Privacy Information Center (EPIC) is a public interest
research center in Washington, DC.  It was established in 1994 to
focus public attention on emerging privacy issues relating to the
National Information Infrastructure, such as the Clipper Chip, the
Digital Telephony proposal, medical record privacy, and the sale of
consumer data.  EPIC is sponsored by the Fund for Constitutional
Government and Computer Professionals for Social Responsibility. 
EPIC publishes the EPIC Alert and EPIC Reports, pursues Freedom of
Information Act litigation, and conducts policy research on emerging
privacy issues.  For more information email, or write
EPIC, 666 Pennsylvania Ave., SE, Suite 301, Washington, DC 20003.
+1 202 544 9240 (tel), +1 202 547 5482 (fax).

The Electronic Frontier Foundation (EFF) was founded in 1990 to
assure freedom of expression in digital media, with a particular
emphasis on applying the principles embodied in the US Constitution
and the Bill of Rights to computer-based communication.  They can be
reached in Washington DC, at (202) 347-5400.  Internet E-mail address:

Computer Professionals For Social Responsibility (CPSR) empowers
computer professionals and computer users to advocate for the
responsible use of information technology and empowers all who use
computer technology to participate in public policy debates on the
impacts of computers on society.  They can be reached at:
(415) 322-3778 in Palo Alto, E-mail address

The League for Programming Freedom (LPF) is a grass-roots
organization of professors, students, businessmen, programmers and
users dedicated to bringing back the freedom to write programs.  They
regard patents on computer algorithms as harmful to the US software
industry (and so do I!).  They can be reached at (617) 433-7071. 
E-mail address:

For more details on these groups, see the accompanying document in
the PGP release package.

Recommended Readings

Introductory Readings

1)  Bruce Schneier, "Applied Cryptography: Protocols, Algorithms, and
    Source Code in C", John Wiley & Sons, 1993
    (This book is a watershed work on the subject.)
2)  Dorothy Denning, "Cryptography and Data Security", Addison-Wesley,
    Reading, MA 1982
3)  Dorothy Denning, "Protecting Public Keys and Signature Keys",
    IEEE Computer, Feb 1983
4)  Martin E. Hellman, "The Mathematics of Public-Key Cryptography," 
    Scientific American, Aug 1979
5)  Steven Levy, "Crypto Rebels", WIRED, May/Jun 1993, page 54.
    (A "must-read" article on PGP and other related topics.)
6)  Steven Levy, "Battle of the Clipper Chip", New York Times
    Magazine, Sunday 12 Jun 1994, page 44. (Great article, great
7)  William Bulkeley, "Cipher Probe", Wall Street Journal, 28 April
    1994, front page.  (An article on PGP and Zimmermann.)

Other Readings

8)  Ronald Rivest, "The MD5 Message Digest Algorithm", MIT Laboratory
    for Computer Science, 1991
9)  Xuejia Lai, "On the Design and Security of Block Ciphers", 
    ETH Series on Information Processing (Ed. J. L. Massey),
    Vol. 1, Hartung-Gorre Verlag, Konstanz, Switzerland, 1992
10) Philip Zimmermann, "A Proposed Standard Format for RSA 
    Cryptosystems", Advances in Computer Security, Vol III, edited by
    Rein Turn, Artech House, 1988
11) Paul Wallich, "Electronic Envelopes", Scientific American, Feb
    1993, page 30.  (An article on PGP)
12) William Stallings, "Pretty Good Privacy", BYTE, July 1994, page
13) Philip Zimmermann, "The Official PGP User's Guide", MIT Press,
    1994 (in press)
14) Philip Zimmermann, "PGP Source Code and Internals", MIT Press,
    1994 (in press)

To Contact the Author

Philip Zimmermann may be reached at:

Boulder Software Engineering
3021 Eleventh Street
Boulder, Colorado 80304  USA
Phone (303) 541-0140 (voice)  (10:00am - 7:00pm Mountain Time)
Fax available, if you arrange it via voice line.

Appendix A:  Where to Get PGP

The following describes how to get the freeware public key
cryptographic software PGP (Pretty Good Privacy) from an anonymous
FTP site on Internet, or from other sources.  

PGP has become a worldwide de facto standard for E-mail encryption.
PGP has sophisticated key management, an RSA/conventional hybrid 
encryption scheme, message digests for digital signatures, data
compression before encryption, and good ergonomic design.  PGP is
well featured and fast, and has excellent user documentation.  Source
code is free.

The Massachusetts Institute of Technology is the distributor of PGP
version 2.6, for distribution in the USA only.  It is available from
"," a controlled FTP site that has restrictions and
limitations, similar to those used by RSA Data Security, Inc., to comply
with export control requirements.  The software resides in the directory

A reminder:  Set mode to binary or image when doing an FTP transfer.
And when doing a kermit download to your PC, specify 8-bit binary
mode at both ends.

There are two compressed archive files in the standard release, with
the file name derived from the release version number.  For PGP
version 2.6.1, you must get which contains the MSDOS
binary executable and the PGP User's Guide, and you can optionally
get which contains all the source code.  These files can
be decompressed with the MSDOS shareware archive decompression
utility PKUNZIP.EXE, version 1.10 or later.  For Unix users who lack
an implementation of UNZIP, the source code can also be found in the
compressed tar file pgp261s.tar.Z.

If you don't have any local BBS phone numbers handy, here is a BBS
you might try.  The Catacombs BBS, operated by Mike Johnson in
Longmont, Colorado, has PGP available for download by people in the US
or Canada only.  The BBS phone number is 303-772-1062.  Mike
Johnson's voice phone number is 303 772-1773, and his E-mail address
is  Mike also has PGP available on an Internet FTP site
for users in the US or Canada only; the site name is, in
directory /mpj/, and you must read the README.MPJ file to get it.

To get a fully licensed version of PGP for use in the USA or Canada,
contact ViaCrypt in Phoenix, Arizona.  Their phone number is
602-944-0773.  ViaCrypt has obtained all the necessary licenses from
PKP, Ascom-Tech AG, and Philip Zimmermann to sell PGP for use in
commercial or Government environments.  ViaCrypt PGP is every bit as
secure as the freeware PGP, and is entirely compatible in both
directions with the freeware version of PGP.  ViaCrypt PGP is the
perfect way to get a fully licensed version of PGP into your
corporate or Government environment.

Here are a few people and their E-mail addresses or phone numbers you
can contact in some countries to get information on local PGP 
availability for versions earlier than 2.5:

Peter Gutmann                 Hugh Kennedy
New Zealand                   Germany

Branko Lankester              Miguel Angel Gallardo   
+31 2159 42242                (341) 474 38 09
The Netherlands               Spain

Hugh Miller                   Colin Plumb
(312) 508-2727                Toronto, Ontario, Canada

Jean-loup Gailly