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Tools for breaking the substitution cipher,
Sašo Kodrič

**Abstract of the diploma thesis:**
Digital communication is becoming an essential part of modern way of
life. In some cases it is desired, that message stays secret, while
in some other cases it is required to be revealed. Simplified ,
c ryptography is a theory that studies techniques to enable secret
communication with the use of encryption and the use of decryption to
obta in b ack the original message. This thesis deals with cryptoanalys
is, in particular, it explains mathematical background for breaking the
substitution cipher based on statistics of pairs of letters. It presents
our implementation of algorithms, which encrypt a clear text with the use
of random substitution cipher , calculate the frequencies of pairs of
letters in corpus and encrypted text and stores them in two matrices .
It permutes the substitution cipher in matrix of the encrypted text
and calculates the distan ce with corresponding matrix of corpus. The
algorithm is running, until the distance is decreasing. As result we
get substitution cipher, which was used to encrypt the clear text.

**Keywords:**
cryptography, cryptoanalysis, substitution cipher