Quadratic Ciphers - Introduction to Cryptography with Java Applets - page 199

The plaintext is now large enough so that when squared modulo n , we obtain a residue

2 . An attacker is now forced to compute the modular square root.

different from P

Java Algorithm I have written methods to encrypt and decrypt using Rabin. To ensure
only the correct message is returned out of the four possible roots obtained in the decryp-
tion phase, I add 4 bytes of redundancy to the beginning of each block; that is, before a
block is encrypted, the first 4 bytes of the blocks are repeated at the front. (See Figure 10.2.)
Then, to protect against attacks commonly used on Rabin ciphers, I add 4 bytes of salt
to the head of each block before encrypting it. This means a different ciphertext will be pro-
duced each time the message is encrypted. (See Figure 10.3.)
Of course, after decrypting, the receiver knows these bytes are simply random data, and
throws them out.
I also use PKCS#5 padding, and so that the addition of redundancy and salt does not
further restrict the block size, I do not include it in the padding. Thus, the maximum plain-
text block size (including salt and redundancy) is 255 + 8 = 263 bytes. (See Figure 10.4.)

FIGURE 10.2

Repeat first 4 bytes

HI !

............

HI ! HOW

FIGURE 10.3

Random Data

$% @ &

............

HI ! HI !

FIGURE 10.4

Message + Padding
255

Salt
4

Redundancy
4

263 bytes