9.6 CHAINING

The DES only specifies the encipherment a block of 64 bits. DES can be extended to encipher plaintext of arbitrary length in two ways.

The Standard Extension of DES divides the plaintext image into 8-byte blocks

image

and enciphers each block separately

image

TABLE 9.19 Trace of DES

image

There remains the question of how to handle the encipherment of plaintext whose length is not a multiple of 8n bytes. More importantly, there are instances in which the encipherment as defined above reveals structure in the plaintext. For example, when we encipher a file containing a picture, the outline of the picture might be detectable in the ciphertext. Also, stereotyped preambles of plaintext messages, like Dear Mr./Ms. or To : may be visible in the ciphertext. In order to hide the repetitive nature of plaintext and stereotyped preambles, chaining was introduced.

The record chained encipherment of plaintext x = (x(0), x(1), …, x(n−1), x(n)) of length 8n + k bytes with 0 ≤ k < 8 by DES is defined as follows:

  1. A nonsecret and randomly chosen 8-byte y(−1) initial chaining value (ICV) prefixes the ciphertext. ...

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