Introducting DES Stream Cipher Modes

Cryptography Tutorials - Herong's Tutorial Examples

∟Introducting DES Stream Cipher Modes

This section describes what are DES encryption stream cipher modes and how CFB and OFB block operation modes can be modified as stream modes.

As we know from previous tutorials, DES algorithm is a block cipher algorithm. But it can be operated in different ways to become stream ciphers.

(FIPS 81) Federal Information Processing Standards Publication 81 published in 1980 defined the following 4 operation modes:

ECB - Electronic Code Book operation mode.
CBC - Cipher Block Chaining operation mode.
CFB - Cipher Feedback operation mode
OFB - Output Feedback operation mode

See http://www.itl.nist.gov/fipspubs/fip81.htm for details.

In FIPS 81, the last two operation modes, CFB and OFB, are defined in ways so that they can be used as stream ciphers.

In order to describe these operation modes, we need to define the following notations:

P = P[1], P[2], P[3], ..., P[i], ... - Representing the original plaintext message, P, being arranged into multiple 64-bit plaintext blocks. P[i] represents plaintext block number i.

E(P[i]) - Representing the DES encryption algorithm applied on a single 64-bit plaintext block, P[i], with a predefined key, k.

C = C[1], C[2], C[3], ..., C[i], ... - Representing the final ciphertext message, C, being regrouped from multiple 64-bit ciphertext blocks. C[i] represents ciphertext block number i.