Computer Security and Cryptography

8.9 CRIBBING OF STREAM ENCIPHERED ASCII PLAINTEXT

The stream encipherment of ASCII character plaintext is performed in three steps:

Step 1: Each letter of the character plaintext x₀, x₁, …, x_n−1 is replaced by its ordinal value of x_i in the ASCII character set, which is coded into 7 bits; for example

The ASCII character plaintext x₀, x₁, …, x_n−1 of n characters is transformed into a sequence of n 7-bit vectors, the (0, 1)-plaintext

Step 2: The LFSR with initial state s(0) = (s₀(0), s₀(1), …, s₀(N − 1)) generates the key stream, a sequence of 7n-bits (s₀(0), s₀(l), …, s₀(7n − 1)), which are grouped into n 7-bit blocks:

Step 3: The ciphertext y = (y₀, y₁, …, y_n−1) consists of n 7-bit vectors where y_i is the XOR of the ith plaintext block x_i and the block of key s_i

The key of an LFSR encipherment system has three components:

1. The number of stages N of the LFSR,
2. The characteristic polynomial p(z) = c_N + c_N−1z + ··· + c₁z^N−1 + c₀z^N, and
3. The initial state s(0) = (s₀(0), s₀(1), …, s₀(N−1)).

We formulate the cribbing of ASCII character plaintext as:

Given:

A plaintext crib ...