O'Reilly logo

Computer Security and Cryptography by Alan G. Konheim

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

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

image

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

image

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:

image

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

image

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 ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required