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## 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, …, xn−1 is replaced by its ordinal value of xi in the ASCII character set, which is coded into 7 bits; for example

The ASCII character plaintext x0, x1, …, xn−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, …, yn−1) consists of n 7-bit vectors where yi is the XOR of the ith plaintext block xi and the block of key si

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) = cN + cN−1z + ··· + c1zN−1 + c0zN, 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 ...

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