RSA Security 5.2.2 manual Stream Ciphers, 6Output Feedback Mode OFB

Cryptography Overview

Initialization Vector (IV)

Block Cipher

Key (K)

1st message

block

1st cipher

XORblock

2nd message

block

Block Cipher

Key (K)

XOR	2nd cipher
	block

Figure 3-6Output Feedback Mode (OFB)

Stream Ciphers

A stream cipher processes the input data one unit at a time. A unit of data is generally a byte, or sometimes even a bit. In this way, encryption or decryption can execute on a variable length of input. The algorithm does not have to wait for a specified amount of data to be input before processing, nor does it have to append and encrypt extra bytes.

RC4

The RC4 cipher is a symmetric stream-encryption algorithm developed by Ronald Rivest and proprietary to RSA Security. It is actually a keyed pseudo-random sequence. It uses the provided key to produce a pseudo-random number sequence which is then XORed with the input data. This means that the encryption and decryption operations are identical.

The number of key bits is variable and ranges from eight to 2048 bits. Using the RC4 cipher with a key size of less than 40 bits is not recommended.

Because RC4 encryption is an XOR between the message bytes and the pseudo- random byte stream generated from the key, the same key should not be used more than once. Otherwise, if some of the bytes of one input message are known (or easy to

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