Glossary - Cryptology Unlocked - page 490

Congruence In number theory, a subsection of mathematics, the term denotes equations
with remainders when divided by integers: a b

mod n '), which means that the integers, a and b , leave the same remainder when
divided by the integer module, n (see Section 4.5.3).

≡ b (mod n) (also written as ' a =

Cryptanalysis The art of decrypting an encrypted message without knowing the secret
key, or, more generally, bypassing the secret-key defense without knowing this
key (e.g., by forging a digitally signed document). This is the counterpart of
cryptography.

Cryptographic protocol Cryptography implemented in practice, where several parties
walk through a well-defined sequence of actions. Cryptographic protocols can
be broken, just like cryptographic algorithms; their formalization is difficult (see
Chapter 6).

Cryptography The art of designing encryption algorithms; the counterpart of crypt-
analysis. Cryptography without knowing cryptanalysis is not meaningful (but that
doesn't hold for the opposite!).

Cryptology Term used for cryptanalysis and cryptography together.

Daemon A program in UNIX that runs continually in the background, even when no
user is working at the computer. For example, a daemon listens in on a network
and establishes a connection when it hears a request. Other daemons adjust the
system clock, or monitor the mouse port, or handle print jobs, or restrict user
access to certain software, and some intercept passwords!

Denial-of-service attack A special type of attack against digital communication aimed
at disturbing the traffic of messages in an untraceable way, or using cryptological
means to prevent communication (e.g., by maliciously swapping a key to pre-
vent further communication, but not by physically cutting a cable between two
computers). Section 6.5.1 uses an example.

DES (Digital Encryption Standard) Still one of the most widely used encryption meth-
ods (Sections 4.3 and 4.4). DES is a product algorithm using 56-bit keys and
64-bit blocks.

Dictionary attack An attack against a reduced key space, where the structure of this
key space can be described by a known basic set of keys (a dictionary ), and by
possibly modifying these keys. The most insightful example is the Crack program
(Section 3.3).

Diffie-Hellman key exchange A cryptographic protocol for session-key distribution
based on discrete logarithms (see Section 6.1.1).