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Crypto Glossary

Here's the glossary of crypto terms we passed out in printed form at
the first Cypherpunks meeting in September 1992. Some compromises had
to be made in going from the printed form to the ASCII of this
transmission, so I hope you'll bear with me.

I'm sending it to the entire list because nearly everyone who hears
about it says "Is it online?" and wants a copy. If you don't want it,
discard it.

I'm not going to be maintaining the "Cypherpunks FAQ," so don't send
me corrections or additions.


--Tim May


Compiled by Tim May ([email protected]) and Eric Hughes
([email protected]), circa September 1992.

Major Branches of Cryptology (as we see it)

-	(these sections will introduce the terms in context, 
though complete definitions will not be given)

*** Encryption
-	privacy of messages
-	using ciphers and codes to protect the secrecy of 
-	DES is the most common symmetric cipher (same key for 
encryption and decryption)
-	RSA is the most common asymmetric cipher (different 
keys for encryption and decryption)

*** Signatures and Authentication
-	proving who you are
-	proving you signed a document (and not someone else)

*** Untraceable Mail
-	untraceable sending and receiving of mail and messages
-	focus: defeating eavesdroppers and traffic analysis
-	DC protocol (dining cryptographers)

*** Cryptographic Voting
-	focus: ballot box anonymity
-	credentials for voting
-	issues of double voting, security, robustness, efficiency

*** Digital Cash
-	focus: privacy in transactions, purchases
-	unlinkable credentials
-	blinded notes
-	"digital coins" may not be possible

*** Crypto Anarchy
-	using the above to evade government, to bypass tax collection, 
-	a technological solution to the problem of too much 

***	G L O S S A R Y    ***

***	agoric systems -- open, free market systems in which 
voluntary transactions are central. 

***	Alice and Bob -- cryptographic protocols are often made 
clearer by considering parties A and B, or Alice and Bob, 
performing some protocol. Eve the eavesdropper, Paul the 
prover, and Vic the verifier are other common stand-in names.

***	ANDOS -- all or nothing disclosure of secrets.

***	anonymous credential -- a credential which asserts 
some right or privilege or fact without revealing the identity 
of the holder.  This is unlike CA driver's licenses.

***	asymmetric cipher -- same as public key 

***	authentication -- the process of verifying an identity 
or credential, to ensure you are who you said you were.

***	biometric security -- a type of authentication using 
fingerprints, retinal scans, palm prints, or other 
physical/biological signatures of an individual.

***	bit commitment -- e.g., tossing a coin and then 
committing to the value without being able to change the 
outcome. The blob is a cryptographic primitive for this.

***	blinding, blinded signatures -- A signature that the 
signer does not remember having made.  A blind signature is 
always a cooperative protocol and the receiver of the 
signature provides the signer with the blinding information. 

***	blob -- the crypto equivalent of a locked box. A 
cryptographic primitive for bit commitment, with the 
properties that a blobs can represent a 0 or a 1, that others 
cannot tell be looking whether itUs a 0 or a 1, that the creator 
of the blob can "open" the blob to reveal the contents, and that 
no blob can be both a 1 and a 0. An example of this is a flipped 
coin covered by a hand.

***	channel -- the path over which messages are 
transmitted. Channels may be secure or insecure, and may 
have eavesdroppers (or enemies, or disrupters, etc.) who alter 
messages, insert and delete messages, etc. Cryptography is 
the means by which communications over insecure channels 
are protected.

***	chosen plaintext attack -- an attack where the 
cryptanalyst gets to choose the plaintext to be enciphered, 
e.g., when possession of an enciphering machine or algorithm 
is in the possession of the cryptanalyst.

***	cipher -- a secret form of writing, using substitution or 
transposition of characters or symbols.

***	ciphertext -- the plaintext after it has been encrypted.

***	code -- a restricted cryptosystem where words or 
letters of a message are replaced by other words chosen from 
a codebook. Not part of modern cryptology, but still useful.

***	coin flipping -- an important crypto primitive, or 
protocol, in which the equivalent of flipping a fair coin is 
possible. Implemented with blobs.

***	collusion -- wherein several participants cooperate to 
deduce the identity of a sender or receiver, or to break a 
cipher. Most cryptosystems are sensitive to some forms of 
collusion. Much of the work on implementing DC Nets, for 
example, involves ensuring that colluders cannot isolate 
message senders and thereby trace origins and destinations 
of mail.

***	computationally secure -- where a cipher cannot be 
broken with available computer resources, but in theory can 
be broken with enough computer resources. Contrast with 
unconditionally  secure.

***	countermeasure -- something you do to thwart an 

***	credential -- facts or assertions about some entity. For 
example, credit ratings, passports, reputations, tax status, 
insurance records, etc.  Under the current system, these 
credentials are increasingly being cross-linked. Blind 
signatures may be used to create anonymous credentials.

***	credential clearinghouse  -- banks, credit agencies, 
insurance companies, police departments, etc., that correlate 
records and decide the status of records. 

***	cryptanalysis -- methods for attacking and breaking 
ciphers and related cryptographic systems. Ciphers may be 
broken, traffic may be analyzed, and passwords may be 
cracked. Computers are of course essential.

***	crypto anarchy -- the economic and political system 
after the deployment of encryption, untraceable e-mail, 
digital pseudonyms, cryptographic voting, and digital cash. A 
pun on "crypto," meaning "hidden," and as when Gore Vidal 
called William F. Buckley a "crypto fascist."

***	cryptography -- another name for cryptology.

***	cryptology -- the science and study of writing, sending, 
receiving, and deciphering secret messages. Includes 
authentication, digital signatures, the hiding of messages 
(steganography), cryptanalysis, and several other fields.

***	cyberspace  -- the electronic domain, the Nets, and 
computer-generated spaces. Some say it is the "consensual 
reality" described in "Neuromancer." Others say it is the phone 
system. Others have work to do.

***	DC protocol, or DC-Net -- the dining cryptographers 
protocol. DC-Nets use multiple participants communicating 
with the DC protocol.

***	DES -- the Data Encryption Standard, proposed in 
1977 by the National Bureau of Standards (now NIST), with 
assistance from the National Security Agency. Based on the 
"Lucifer" cipher developed by Horst Feistel at IBM, DES is a 
secret key cryptosystem that cycles 64-bit blocks of data 
through multiple permutations with a 56-bit key controlling 
the routing. "Diffusion" and "confusion" are combined to form 
a cipher that has not yet been cryptanalyzed (see "DES, 
Security of"). DES is in use for interbank transfers, as a 
cipher inside of several RSA-based systems, and is available 
for PCs.

***	DES, Security of  -- many have speculated that the NSA 
placed a trapdoor (or back door) in DES to allow it to read 
DES-encrypted messages. This has not been proved. It is 
known that the original Lucifer algorithm used a 128-bit key 
and that this key length was shortened to 64 bits (56 bits 
plus 8 parity bits), thus making exhaustive search much 
easier (so far as is known, brute-force search has not been 
done, though it should be feasible today). Shamir and Bihan 
have used a technique called "differential cryptanalysis" to 
reduce the exhaustive search needed for chosen plaintext 
attacks (but with no import for ordinary DES).

***	differential cryptanalysis -- the Shamir-Biham 
technique for cryptanalyzing DES. With a chosen plaintext 
attack, they've reduced the number of DES keys that must be 
tried from about 2^56 to about 2^47 or less. Note, however, 
that rarely can an attacker mount a chosen plaintext attack 
on DES systems.

***	digital cash, digital money -- Protocols for 
transferring value, monetary or otherwise, electronically.  
Digital cash usually refers to systems that are anonymous. 
Digital money systems can be used to implement any quantity 
that is conserved, such as points, mass, dollars, etc.  There 
are many variations of  digital money systems, ranging from 
VISA numbers to blinded signed digital coins.  A topic too 
large for a single glossary entry.

***	digital pseudonym -- basically, a "crypto identity." A 
way for individuals to set up accounts with various 
organizations without revealing more information than they 
wish. Users may have several digital pseudonyms, some used 
only once, some used over the course of many years. Ideally, 
the pseudonyms can be linked only at the will of the holder. In 
the simplest form, a public key can serve as a digital 
pseudonym and need not be linked to a physical identity.

***	digital signature --  Analogous to a written signature 
on a document. A modification to a message that only the 
signer can make but that everyone can recognize.  Can  be used 
legally to contract at a distance.

***	digital timestamping -- one function of a digital 
notary public, in which some message (a song, screenplay, lab 
notebook, contract, etc.) is stamped with a time that cannot 
(easily) be forged. 

***	dining cryptographers protocol (aka DC protocol, 
DC nets) -- the untraceable message sending system 
invented by David Chaum. Named after the "dining 
philosophers" problem in computer science, participants form 
circuits and pass messages in such a way that the origin 
cannot be deduced, barring collusion. At the simplest level, 
two participants share a key between them. One of them 
sends some actual message by bitwise exclusive-ORing the 
message with the key, while the other one just sends the key 
itself. The actual message from this pair of participants is 
obtained by XORing the two outputs. However, since nobody 
but the pair knows the original key, the actual message 
cannot be traced to either one of the participants.

***	discrete logarithm problem -- given integers a, n, 
and x, find some integer m such that a^m mod n = x, if m 
exists. Modular exponentiation, the a^m mod n part, is 
straightforward (and special purpose chips are available), but 
the inverse problem is believed to be very hard, in general.  
Thus it is conjectured that modular exponentiation is a one-
way function.

***	DSS, Digital Signature Standard -- the latest NIST 
(National Institute of Standards and Technology, successor to 
NBS) standard for digital signatures. Based on the El Gamal 
cipher, some consider it weak and poor substitute for RSA-
based signature schemes.

***	eavesdropping, or passive wiretapping -- 
intercepting messages without detection. Radio waves may be 
intercepted, phone lines may be tapped, and computers may 
have RF emissions detected. Even fiber optic lines can be 

***	factoring -- Some large numbers are difficult to factor. 
It is conjectured that there are no feasible--i.e."easy," less 
than exponential in size of number-- factoring methods. It is 
also an open problem whether RSA may be broken more easily 
than by factoring the modulus (e.g., the public key might 
reveal information which simplifies the problem). 
Interestingly, though factoring is believed to be "hard", it is 
not known to be in the class of NP-hard problems. Professor 
Janek invented a factoring device, but he is believed to be 

***	information-theoretic security -- "unbreakable" 
security, in which no amount of cryptanalysis can break a 
cipher or system. One time pads are an example (providing the 
pads are not lost nor stolen nor used more than once, of 
course). Same as unconditionally secure.

***	key -- a piece of information needed to encipher or 
decipher a message. Keys may be stolen, bought, lost, etc., 
just as with physical keys.

***	key exchange, or key distribution -- the process of 
sharing a key with some other party, in the case of symmetric 
ciphers, or of distributing a  public key in an asymmetric 
cipher. A major issue is that the keys be exchanged reliably 
and without compromise. Diffie and Hellman devised one such 
scheme, based on the discrete logarithm problem. 

***	known-plaintext attack -- a cryptanalysis of a cipher 
where plaintext-ciphertext pairs are known. This attack 
searches for an unknown key. Contrast with the chosen 
plaintext attack, where the cryptanalyst can also choose the 
plaintext to be enciphered.

***	mail, untraceable  -- a system for sending and 
receiving mail without traceability or observability. 
Receiving mail anonymously can be done with broadcast of the 
mail in encrypted form.  Only the intended recipient (whose 
identity, or true name, may be unknown to the sender) may 
able to decipher the message. Sending mail anonymously 
apparently requires mixes or use of the dining cryptographers 
(DC) protocol.

***	minimum disclosure proofs  -- another name for zero 
knowledge proofs, favored by Chaum.

***	mixes -- David Chaum's term for a box which performs 
the function of mixing, or decorrelating, incoming and 
outgoing electronic mail messages. The box also strips off 
the outer envelope (i.e., decrypts with its private key) and 
remails the message to the address on the inner envelope. 
Tamper-resistant modules may be used to prevent cheating 
and forced disclosure of the mapping between incoming and 
outgoing mail. A sequence of many remailings effectively 
makes tracing sending and receiving impossible. Contrast this 
with the software version, the DC protocol.

***	modular exponentiation  -- raising an integer to the 
power of another integer, modulo some integer. For integers 
a, n, and m, a^m mod n. For example, 5^3 mod 100 = 25. Modular 
exponentiation can be done fairly quickly with a sequence of 
bit shifts and adds, and special purpose chips have been 
designed. See also discrete logarithm.

***	National Security Agency (NSA)  -- the largest 
intelligence agency, responsible for making and breaking 
ciphers, for intercepting communications, and for ensuring 
the security of U.S. computers. Headquartered in Fort Meade, 
Maryland, with many listening posts around the world.  The 
NSA funds cryptographic research and advises other agencies 
about cryptographic matters. The NSA once obviously had the 
world's leading cryptologists, but this may no longer be the 

***	negative credential -- a credential that you possess 
that you don't want any one else to know, for example, a 
bankruptcy filing.  A formal version of a negative reputation.

***	NP-complete -- a large class of difficult problems.  
"NP" stands for nondeterministic polynomial time, a class of 
problems thought in general not to have feasible algorithms 
for their solution.  A problem is "complete"  if  any other NP 
problem may be reduced to that problem.   Many important 
combinatorial and algebraic problems are NP-complete: the 
traveling salesman problem, the Hamiltonian cycle problem, 
the word problem, and on and on.

***	oblivious transfer -- a cryptographic primitive that 
involves the probabilistic transmission of bits. The sender 
does not know if the bits were received.

***	one-time pad -- a string of randomly-selected bits or 
symbols which is combined with a plaintext message to 
produce the ciphertext. This combination may be shifting 
letters some amount, bitwise exclusive-ORed, etc.). The 
recipient, who also has a copy of the one time pad, can easily 
recover the plaintext. Provided the pad is only used once and 
then destroyed, and is not available to an eavesdropper, the 
system is perfectly secure, i.e., it is information-
theoretically secure. Key distribution (the pad)  is obviously a 
practical concern, but consider CD-ROM's.

***	one-way function -- a function which is easy to 
compute in one direction but hard to find any inverse for, e.g. 
modular exponentiation, where the inverse problem is known 
as the discrete logarithm problem. Compare the special case 
of trap door one-way functions.  An example of  a one-way 
operation is multiplication: it is  easy to multiply two 
prime numbers of 100 digits to produce a 200-digit number, 
but  hard to factor that 200-digit number. 

***	P ?=? NP  -- Certainly the most  important unsolved 
problem in complexity theory. If P = NP, then cryptography as 
we know it today does not exist.  If P = NP,  all NP problems 
are "easy." 

***	padding -- sending extra messages to confuse 
eavesdroppers and to defeat traffic analysis.   Also adding 
random bits to a message to be enciphered.

***	plaintext -- also called cleartext, the text that is to be 

***	Pretty Good Privacy (PGP)  -- Phillip ZimmermanUs 
implementation of RSA, recently upgraded to version 2.0, 
with more robust components and several new features. RSA 
Data Security has threatened PZ so he no longer works on it.  
Version 2.0 was written by a consortium of non-U.S. hackers.

***	prime numbers -- integers with no factors other than 
themselves and 1. The number of primes is unbounded.  About 
1% of the 100 decimal digit numbers are prime.  Since there 
are about 10^70 particles in the universe, there are about 
10^23  100 digit primes for each and every particle in the 

***	probabilistic encryption  -- a scheme by Goldwasser, 
Micali, and Blum that allows multiple ciphertexts for the 
same plaintext, i.e., any given plaintext may have many 
ciphertexts if the ciphering is repeated. This protects against 
certain types of known ciphertext attacks on RSA.

***	proofs of identity -- proving who you are, either your 
true name, or your digital identity. Generally, possession of 
the right key is sufficient proof (guard your key!). Some work 
has been done on "is-a-person" credentialling agencies, using 
the so-called Fiat-Shamir protocol...think of this as a way to 
issue unforgeable digital passports. Physical proof of identity 
may be done with biometric security methods. Zero knowledge 
proofs of identity reveal nothing beyond the fact that the 
identity is as claimed. This has obvious uses for computer 
access, passwords, etc.

***	protocol -- a formal procedure for solving some 
problem. Modern cryptology is mostly about the study of 
protocols for many problems, such as coin-flipping, bit 
commitment (blobs), zero knowledge proofs, dining 
cryptographers, and so on.

***	public key -- the key distributed publicly to potential 
message-senders. It may be published in a phonebook-like 
directory or otherwise sent. A major concern is the validity 
of this public key to guard against spoofing or impersonation.

***	public key cryptosystem -- the modern breakthrough 
in cryptology, designed by Diffie and Hellman, with 
contributions from several others. Uses trap door one-way 
functions so that encryption may be done by anyone with 
access to the "public key" but decryption may be done only by 
the holder of the "private key." Encompasses public key 
encryption, digital signatures, digital cash, and many other 
protocols and applications.

***	public key encryption -- the use of modern 
cryptologic methods to provided message security and 
authentication. The RSA algorithm is the most widely used 
form of public key encryption, although other systems exist. 
A public key may be freely published, e.g., in phonebook-like 
directories, while the corresponding private key is closely 

***	public key patents  -- M.I.T. and Stanford, due to the 
work of Rivest, Shamir, Adleman, Diffie, Hellman, and Merkle, 
formed Public Key Partners to license the various public key, 
digital signature, and RSA patents. These patents, granted in 
the early 1980s, expire in the between 1998 and 2002. PKP 
has licensed RSA Data Security Inc., of Redwood City, CA, 
which handles the sales, etc.

***	quantum cryptography -- a system based on quantum-
mechanical principles. Eavesdroppers alter the quantum state 
of the system and so are detected. Developed by Brassard and 
Bennett, only small laboratory demonstrations have been 

***	reputations -- the trail of positive and negative 
associations and judgments that some entity accrues. Credit 
ratings, academic credentials, and trustworthiness are all 
examples. A digital pseudonym will accrue these reputation 
credentials based on actions, opinions of others, etc. In 
crypto anarchy, reputations and agoric systems will be of 
paramount importance. There are many fascinating issues of 
how reputation-based systems work, how credentials can be 
bought and sold, and so forth.

***	RSA -- the main public key encryption algorithm, 
developed by Ron Rivest, Adi Shamir, and Kenneth Adleman. It 
exploits the difficulty of factoring large numbers to create a 
private key and public key. First invented in 1978, it remains 
the core of modern public key systems. It is usually much 
slower than DES, but special-purpose modular exponentiation 
chips will likely speed it up. A popular scheme for speed is to 
use RSA to transmit session keys and then a high-speed 
cipher like DES for the actual message text.
***	Description -- Let p and q be large primes, typically with more than 
100 digits. Let n = pq and find some e such that e is relatively prime to (p 
- 1)(q - 1). The set of numbers p, q, and e is the private key for RSA. The 
set of numbers n and e forms the public key (recall that knowing n is not 
sufficient to easily find p and q...the factoring problem).  A message M is 
encrypted by computing M^e mod n. The owner of the private key can 
decrypt the encrypted message by exploiting number theory results, as 
follows. An integer d is computed such that ed =1 (mod (p - 1)(q - 1)). 
Euler proved a theorem that M^(ed) = M mod n and so M^(ed) mod n = M. 
This means that in some sense the integers e and d are "inverses" of each 
other. [If this is unclear, please see one of the many texts and articles on 
public key encryption.]

***	secret key cryptosystem -- A system which uses the 
same key to encrypt and decrypt traffic at each end of a 
communication link.  Also called a symmetric or one-key 
system.  Contrast with public key cryptosystem.

***	smart cards -- a computer chip embedded in credit 
card.  They can hold cash, credentials, cryptographic keys, 
etc. Usually these are built with some degree of tamper-
resistance. Smart cards may perform part of a crypto 
transaction, or all of it. Performing part of it may mean 
checking the computations of a more powerful computer, e.g., 
one in an ATM.

***	spoofing, or masquerading -- posing as another user. 
Used for stealing passwords, modifying files, and  stealing 
cash. Digital signatures and other authentication methods are 
useful to prevent this. Public keys must be validated and 
protected to ensure that others don't substitute their own 
public keys which users may then unwittingly use.

***	steganography -- a part of cryptology dealing with 
hiding messages and obscuring who is sending and receiving 
messages. Message traffic is often padded to reduce the 
signals that would otherwise come from a sudden beginning 
of messages.

***	symmetric cipher -- same as private key 
***	tamper-responding modules, tamper-resistant 
modules (TRMs) -- sealed boxes or modules which are hard 
to open, requiring extensive probing and usually leaving ample 
evidence that the tampering has occurred. Various protective 
techniques are used, such as special metal or oxide layers on 
chips, armored coatings, embedded optical fibers, and other 
measures to thwart analysis. Popularly called "tamper-proof 
boxes." Uses include: smart cards, nuclear weapon initiators, 
cryptographic key holders, ATMs, etc.

***	tampering, or active wiretapping -- interfering with 
messages and possibly modifying them. This may compromise 
data security, help to break ciphers, etc.  See also spoofing.

***	token -- some representation, such as ID cards, subway 
tokens, money, etc., that indicates possession of some 
property or value. 

***	traffic analysis -- determining who is sending or 
receiving messages by analyzing packets, frequency of 
packets, etc. A part of steganography. Usually handled with 
traffic padding.

***	transmission rules -- the protocols for determining 
who can send messages in a DC protocol, and when. These 
rules are needed to prevent collision and deliberate jamming 
of the channels.

***	trap messages -- dummy messages in DC Nets which 
are used to catch jammers and disrupters. The messages 
contain no private information and are published in a blob 
beforehand so that the trap message can later be opened to 
reveal the disrupter. (There are many strategies to explore 

***	trap-door -- In cryptography, a piece of secret 
information that allows the holder of a private key to invert a 
normally hard to invert function.

***	trap-door one way functions -- functions which are 
easy to compute in both the forward and reverse direction but 
for which the disclosure of an algorithm to compute the 
function in the forward direction does not provide 
information on how to compute the function in the reverse 
direction. More simply put, trap-door one way functions are 
one way for all but the holder of the secret information. The 
RSA algorithm is the best-known example of such a function.

***	unconditional security -- same as information-
theoretic security, that is, unbreakable except by loss or 
theft of the key.

***	unconditionally  secure -- where no amount of 
intercepted ciphertext is enough to allow the cipher to be 
broken, as with the use of a one-time pad cipher. Contrast 
with computationally secure.

***	voting, cryptographic -- Various schemes have been 
devised for anonymous, untraceable voting. Voting schemes 
should have several properties: privacy of the vote, security 
of the vote (no multiple votes), robustness against disruption 
by jammers or disrupters, verifiability (voter has confidence 
in the results), and efficiency.

***	zero knowledge proofs -- proofs in which no 
knowledge of the actual proof is conveyed. Peggy the Prover 
demonstrates to Sid the Skeptic that she is indeed in 
possession of some piece of knowledge without actually 
revealing any of that knowledge. This is useful for access to 
computers, because eavesdroppers or dishonest sysops cannot 
steal the knowledge given. Also called minimum disclosure 
proofs. Useful for proving possession of some property, or 
credential, such as age or voting status, without revealing 
personal information.

Timothy C. May         | Crypto Anarchy: encryption, digital money,  
[email protected]       | anonymous networks, digital pseudonyms, zero
408-688-5409           | knowledge, reputations, information markets, 
W.A.S.T.E.: Aptos, CA  | black markets, collapse of governments.
Higher Power: 2^756839 | PGP Public Key: by arrangement.