cryptanalysis Cryptanalysis (from the Greek ''kryptós'', "hidden", and ''analýein'', "to analyze") refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic sec ...

, contact analysis is the study of the frequency with which certain symbols precede or follow other symbols. The method is used as an aid to breaking

classical cipher In cryptography, a classical cipher is a type of cipher that was used historically but for the most part, has fallen into disuse. In contrast to modern cryptographic algorithms, most classical ciphers can be practically computed and solved by hand. ...

s. Contact analysis is based on the fact that, in any sample of any written language, certain symbols appear adjacent to other symbols with varying frequencies. Moreover, these frequencies are roughly the same for almost all samples of that language, even when the distribution of the symbols themselves differs significantly from normal. This is true regardless of whether the symbols being used are words or letters. In some ciphers, these properties of the natural language plaintext are preserved in the ciphertext, and have the potential to be exploited in a

ciphertext-only attack In cryptography, a ciphertext-only attack (COA) or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. While the attacker has no channel providing access to the ...

. Although in a sense contact analysis can be considered a type of

frequency analysis In cryptanalysis, frequency analysis (also known as counting letters) is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers. Frequency analysis is based on t ...

, most discussions of frequency analysis concern themselves with the simple probabilities of the symbols in the text:

P(X_i=a)

P(X_i=a \cap X_=b)

Contact analysis is based on the

conditional probability In probability theory, conditional probability is a measure of the probability of an event occurring, given that another event (by assumption, presumption, assertion or evidence) has already occurred. This particular method relies on event B occur ...

that certain letters will precede or succeed other letters:

P(X_i=b \mid X_=a)

, or

P(X_i=c \mid X_=a \cap X_=b)

, or even

P(X_i \sub S \mid X_\sub T \cap X_ \sub T)

, where

S

and

T

are subsets of the alphabet being used. Where frequency analysis is based on first-order statistics, contact analysis is based on second or third-order statistics.

References

External links

Statistical Distributions of English Text
{{Cryptography navbox , classical Cryptographic attacks