Polylphabetic

Use N substitution alphabets.

• Periodical ciphers, with period N.

Example

• Vigenère cipher.

Problems

• Once known the period, is easy to cryptanalyze as N mono-alphabetic ones.

  • The period can be discovered using statistics.

  • Kasiski method.

    • Factoring of distances between equal ciphertext blocks.

  • Coincidence index.

    • Factoring of self-correlation offsets that yield higher coincidences.

Vigenère Cipher

Example of encryption of character M with key S, yielding cryptogram E.

• Decryption is the opposite, E and S yield M.

Cryptanalysis of a Vigenère Cryptogram

Plaintext:

Cipher with the Vigenère square and key "poema"

Kasiski test

• With the text above:

• With the complete poem: