RSA Security 5.2.2 manual 5Cipher Feedback CFB Mode

Cryptography Overview

Initialization	Block Cipher B
Vector (IV)
	1
	Key (K)

Block Cipher B2

Key (K)

1st message block P1

1st cipher

XOR

block C1

2nd message block P2

XOR	2nd cipher
	block C2

Figure 3-5Cipher Feedback (CFB) Mode

To encrypt plaintext using CFB mode:

1.Generate your key and your IV.

2.Encrypt the IV with the key to get a block of output, B1.

3.XOR B1 with the first block of your plaintext, P1, to get the first block of ciphertext, C1.

4.Encrypt C1 with the key to get the second block of output, B2.

5.XOR B2 with the second block of your plaintext message, P2, to get the second block of ciphertext, C2.

6.Repeat Steps 4 and 5 until the entire text is encrypted.

To decrypt the ciphertext, the decryptor uses the same (secret) key and initialization vector and follows the same sequence of steps.

CFB mode does not require padding. If your data length is not a multiple of the block size, simply truncate the final block of output to be the same size as the final segment of the data, and then XOR it. You can use CFB mode to encrypt a stream of data.

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