Polylphabetic
Last updated
Applied Cryptography
Last updated
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.
Example of encryption of character M with key S, yielding cryptogram E.
Decryption is the opposite, E and S yield M.
Plaintext:
Cipher with the Vigenère square and key "poema"
Kasiski test
With the text above:
With the complete poem:
