CRYPTO FAQ
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 crypt-comments@math.ncsu.edu. We don't
assume that this FAQ is at all complete at this point.
Archives: sci.crypt has been archived since October 1991 on
cl-next2.cl.msu.edu, though these archives are available only to U.S. and
Canadian users. Please contact crypt-comments@math.ncsu.edu if you know of
other archives.
The sections of this FAQ are available via anonymous FTP to rtfm.mit.edu
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 misc.legal.computing 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.