Computer Security and Cryptography

1.3 CRYPTANALYSIS

Will encipherment provide secrecy? Cryptography is a contest between two adversaries:

• The designer of the system (algorithm, key space, protocol implementation), and
• The opponent, who attempts to circumvent the effect of encipherment.

Can an opponent recover all or part of the plaintext x from the ciphertext y = T_k0(x) and knowledge of the cryptographic system but without the key k₀. Cryptanalysis encompasses all of the techniques to recover the plaintext and/or key from the ciphertext.

The ground rules of this contest were set forth in the nineteenth century by Kerckhoffs¹ in his book “La Cryptographie militare.” Kerckhoffs formulated six attributes that a cryptographic system should enjoy in order for the designer to triumph in the struggle.

K1. The System Should be, if not Theoretically Unbreakable, Unbreakable in Practice.

The term unbreakable is colloquially used to mean that no technique exists to determine the key k or plaintext x from the ciphertext y = T_k(x). It is possible to design an unbreakable system, but it is impractical to use except in situations in which only a modest amount of traffic is exchanged and an alternative secure path for exchanging the key is available.

More relevant is the amount of computational effort – measured by time and memory – needed to produce k and/or x. Claude Shannon's paper [Shannon, 1949] developed a theory of secrecy ...