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

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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