society which serves the National Secu---uh, no. Seriously, we're the
good guys, and we've done what we can to ensure the completeness and
accuracy of this document, but in a field of military and commercial
importance like cryptography you have to expect that some people and
organizations consider their interests more important than open
scientific discussion. Trust only what you can verify firsthand.
And don't sue us.

Many people have contributed to this FAQ. In alphabetical order:
Eric Bach, Steve Bellovin, Dan Bernstein, Nelson Bolyard, Carl Ellison,
Jim Gillogly, Mike Gleason, Doug Gwyn, Luke O'Connor, Tony Patti,
William Setzer. We apologize for any omissions.

If you have suggestions, comments, or criticism, please let the current
editors know by sending e-mail to  We don't
assume that this FAQ is at all complete at this point.

Archives: sci.crypt has been archived since October 1991 on, though these archives are available only to U.S. and
Canadian users. Please contact if you know of
other archives.

The sections of this FAQ are available via anonymous FTP to 
as /pub/usenet/news.answers/cryptography-faq/part[xx].  The Cryptography 
FAQ is posted to the newsgroups sci.crypt, sci.answers, and news.answers 
every 21 days.

* What is cryptology? Cryptography? Plaintext? Ciphertext? Encryption? Key?

  The story begins: When Julius Caesar sent messages to his trusted
  acquaintances, he didn't trust the messengers. So he replaced every A
  by a C, every B by a D, and so on through the alphabet. Only someone
  who knew the ``shift by 2'' rule could decipher his messages.

  A cryptosystem or cipher system is a method of disguising messages so
  that only certain people can see through the disguise. Cryptography is
  the art of creating and using cryptosystems. Cryptanalysis is the art
  of breaking cryptosystems---seeing through the disguise even when
  you're not supposed to be able to. Cryptology is the study of both
  cryptography and cryptanalysis.

  The original message is called a plaintext. The disguised message is
  called a ciphertext. Encryption means any procedure to convert
  plaintext into ciphertext. Decryption means any procedure to convert
  ciphertext into plaintext.

  A cryptosystem is usually a whole collection of algorithms. The
  algorithms are labelled; the labels are called keys. For instance,
  Caesar probably used ``shift by n'' encryption for several different
  values of n. It's natural to say that n is the key here.

  The people who are supposed to be able to see through the disguise are
  called recipients. Other people are enemies, opponents, interlopers,
  eavesdroppers, or third parties.

* What is the National Security Agency (NSA)?

  The NSA is the official security body of the U.S. government. It
  was given its charter by President Truman in the late 40's, and
  has continued research in cryptology till the present. The NSA is
  known to be the largest employer of mathematicians in the world,
  and is also the largest purchaser of computer hardware in the
  world. Governments in general have always been prime employers of
  cryptologists. The NSA probably possesses cryptographic expertise many
  years ahead of the public state of the art, and can undoubtedly break
  many of the systems used in practice; but for reasons of national
  security almost all information about the NSA is classified.

  Bamford's book [BAMFD] gives a history of the people and operations of
  the NSA. The following quote from Massey [MAS88] highlights the
  difference between public and private research in cryptography:

  ``... if one regards cryptology as the prerogative of government,
  one accepts that most cryptologic research will be conducted
  behind closed doors. Without doubt, the number of workers engaged
  today in such secret research in cryptology far exceeds that of
  those engaged in open research in cryptology. For only about 10
  years has there in fact been widespread open research in
  cryptology. There have been, and will continue to be, conflicts
  between these two research communities. Open research is common
  quest for knowledge that depends for its vitality on the open
  exchange of ideas via conference presentations and publications in
  scholarly journals. But can a government agency, charged with
  responsibilities of breaking the ciphers of other nations,
  countenance the publication of a cipher that it cannot break? Can
  a researcher in good conscience publish such a cipher that might
  undermine the effectiveness of his own government's code-breakers?
  One might argue that publication of a provably-secure cipher would
  force all governments to behave like Stimson's `gentlemen', but one
  must be aware that open research in cryptography is fraught with
  political and ethical considerations of a severity than in most
  scientific fields. The wonder is not that some conflicts have
  occurred between government agencies and open researchers in
  cryptology, but rather that these conflicts (at least those of which
  we are aware) have been so few and so mild.''

* What are the US export regulations?

  In a nutshell, there are two government agencies which control
  export of encryption software. One is the Bureau of Export
  Administration (BXA) in the Department of Commerce, authorized by
  the Export Administration Regulations (EAR). Another is the Office
  of Defense Trade Controls (DTC) in the State Department, authorized
  by the International Traffic in Arms Regulations (ITAR). As a rule
  of thumb, BXA (which works with COCOM) has less stringent
  requirements, but DTC (which takes orders from NSA) wants to see
  everything first and can refuse to transfer jurisdiction to BXA.

  The newsgroup carries many interesting
  discussions on the laws surrounding cryptographic export, what
  people think about those laws, and many other complex issues which
  go beyond the scope of technical groups like sci.crypt. Make sure to
  consult your lawyer before doing anything which will get you thrown in
  jail; if you are lucky, your lawyer might know a lawyer who has at
  least heard of the ITAR.

* What is TEMPEST?

  TEMPEST is a standard for electromagnetic shielding for computer
  equipment. It was created in response to the discovery that
  information can be read from computer radiation (e.g., from a CRT) at
  quite a distance and with little effort.

  Needless to say, encryption doesn't do much good if the cleartext
  is available this way.