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Cryptanalysis

Cryptanalysis. codes & ciphers. Introduction. Cryptanalysis is the process of analyzing information to discover a hidden meaning. Code – one word is substituted for another Example: “The eagle is in the nest” could mean “My mom is home.”

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Cryptanalysis

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  1. Cryptanalysis codes & ciphers

  2. Introduction • Cryptanalysis is the process of analyzing information to discover a hidden meaning. • Code – one word is substituted for another Example: “The eagle is in the nest” could mean “My mom is home.” • Cipher – a message that has been encrypted, one letter at a time Example: “I am in trouble” might translate “09-0113-0914-20181521021205.”

  3. Clues for Cryptanalysis • Context: • Who? • When? • Where? • Before & after • Anything unusual • Cribs • Frequency analysis • Look for common letters or words. • Look for anagrams. • When all else fails, use brute force!

  4. Codes • A code is like another language that only you and your buddies can understand. • Book code • words of a message are replaced by page number references • sender and receiver must have the same book, and the exact same edition • a dictionary works well • Word transposition • Whole words can be transposed, in alphabetical order of a keyword. This method is not very secure. Code words should be used in place of sensitive information.

  5. Types of Ciphers • Transposition • The plaintext is re-arranged to create the ciphertext.

  6. Types of Ciphers • Simple substitution • One character of ciphertext is substituted for one character of the plaintext. • Polygraphic substitution • The message is enciphered in blocks of letters. • One of the most common types of polygraphic substitution is digraphic substitution, which deals with letters in pairs. • A technique called fractionation can be used to split each plaintext letter into two or more parts (usually numbers). Alternately, some systems condense two or more letters into a single ciphertext character. As a result, polygraphic substitution may produce a ciphertext of different length than the original plaintext.

  7. Types of Ciphers • Monoalphabetic • Uses one alphabet to create the ciphertext • May include homophones and null characters • Polyalphabetic • Uses multiple alphabet tables • The tables to be used are determined by a keyword.

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