Cryptanalysis seeks to decrypt a message when the key is not known by using different types of analysis to determine the key. Analytical techniques used in cryptanalysis include: frequency analysis of letters, index of coincidence and analysis of two- and three-letter combinations.

Letter Frequencies/Letter Combinations: Letters in the English language occur in certain frequencies and by comparing the distributions of letter frequencies of cipher text with that of plain text, one can determine the type of cipher used.

Index of Coincidence: The index of coincidence is calculated by comparing the encrypted message with that of a shifted encrypted message to determine the percentage of letters that match up. If the message is in plaintext or encrypted by certain methods the index is normally around 6.6%, while for messages encrypted with other ciphers it is around 3.8%.