Crypto U4, Block Cipher, Cipher Block Chaining Mode (CBC)

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Encryption

Start with an initialization

vector (IV).

XOR the IV with the first block of

plaintext.

Pass the result and the key to the

encryption function.

The result of the operation is the

first ciphertext block.

XOR the resulting ciphertext block

with the next plaintext block.

Pass the result and the key to the

encryption function.

The result of the operation is the

next ciphertext block.

Go back to step 5 and repeat until

there are no more plaintext blocks remaining.

Decryption

Take the first ciphertext block.

Pass the ciphertext block and the

key to the decryption function.

XOR the result of the operation

with the initialization vector (IV).

The result is the first plaintext

block.

Pass the next ciphertext block and

the key to the decryption function.

XOR the result of the operation

with the ciphertext block from the previous decryption step.

The result is the next plaintext

block.

Go back to step 5 and repeat until

there are no more ciphertext blocks remaining.

Error Propagation

When there is a 1 bit error, only

two blocks are impacted; one right after the other.

Block with the error is badly

impacted because the faulty ciphertext is processed with decryption

function.

Succeeding block has error in the

position where the ciphertext had the original error because it is

only impacted by the XOR process.

Properties

message dependence

limited error propagation

no synchronization necessary

can be used to provide data origin

identification

requires encryption AND decryption

requires padding

Encryption and Decryption

Error Propagation