Secret Key Cryptography

6 Countermeasures

This chapter covers

• Double encipherment
• Null characters and null bits
• Homophones
• Hiding messages within images or computer files

To recap section 5.9, polyalphabetic ciphers can be solved by a two-step process. First, the period or the key length is determined using the Kasiski Method or the Index of Coincidence. This separates the ciphertext into several smaller texts, each enciphered by just one letter of the key. Second, these individual texts are deciphered using the standard methods for simple substitution ciphers, frequency and contacts.

Let’s turn it around. What can the cryptologist do to prevent a polyalphabetic cipher from being cracked by this two-step process? We will look at a few countermeasures.

6.1 Double encipherment ...