Crypto U4, Block Cipher, Cipher Feedback Mode (CFB)

Encryption Start with an initialization vector (IV). Pass IV and key to the encryption function. XOR the result of the encryption function with the first plaintext block. The result is the first ciphertext block. Pass the resulting ciphertext block and the key to the encryption function. XOR the result of the encryption function with the next plaintext block. The result is the next ciphertext block. Go back to step 5, and repeat until there are no plaintext blocks remaining.

Decryption Start with the initialization vector (IV). Pass IV and key to the encryption function. XOR the result of the encryption function with the first ciphertext block. The result is the first plaintext block. Take the ciphertext block from the previous operation. Pass the ciphertext block and the key to the encryption function. XOR the result of the encryption function with the next ciphertext block. The result is the next plaintext block. Go back to step 5, and repeat until there are no ciphertext blocks remaining.

Error propagation For one error in a block, the error spans at most two blocks. In the first block, only the XOR process will be impacted, so errors only impact the bit positions of the actual error. In the next succeeding block, the encryption function process will be impacted, and the whole block has the potential to be incorrect.

General Properties message dependence limited error propagation no synchronization is required; as long as we have the ciphertext, we have the order only slightly less efficient than ECB mode only encryption algorithm needed padding is unnecessary. The XOR function works without padding.

Variant of cipher feedback mode: Reduced CFB uses 8 bit blocks good for small transfers (such as character-by-character encryption)

Encryption and Decryption

Properties and Error Propagation