Block Ciphers: DES and Triple DES

Study Notes

Take fixed-size blocks of plaintext and use a key to generate a block of ciphertext bits, generally of the same size.
Last block needs to be padded with redundant information to make it up to the block size (e.g., 64 bits).

Divides each block into 2 parts: a right and left side.
Substitution: one side (L) is encrypted using an encryption function that uses the right side (R) and a key, while the other side (R) remains unchanged.
Output is then XOR'd with the round key.
Permutation: the two sides are swapped.
Process is repeated for as many rounds as required.

Symmetric-key block cipher.
Uses 16-round Feistel structure.
Block size is 64-bit.
Key length is 64-bit, but has an effective key length of 56 bits (8 bits are not used by the encryption algorithm).
Each round's key is a 48-bit subkey of the 56-bit original key.

Designed to overcome the drawback of DES's small key size.
Users did not want to replace DES due to its widespread adoption and embedded security architectures.
Modified scheme to use DES in a different manner.
Slow compared to other encryption algorithms.

Symmetric key symmetric block cipher.
128-bit data, 128/192/256-bit keys.
Stronger and faster than Triple-DES.
Provides full specification and design details.
Software implementable in C and Java.
Iterative, based on substitution-permutation network.
Computes on bytes rather than bits.
Number of rounds depends on the length of the key (10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys).
Each round uses a different 128-bit round key, calculated from the original AES key.

Take fixed-size blocks of plaintext and use a key to generate a block of ciphertext bits, generally of the same size.
Last block needs to be padded with redundant information to make it up to the block size (e.g., 64 bits).

Divides each block into 2 parts: a right and left side.
Substitution: one side (L) is encrypted using an encryption function that uses the right side (R) and a key, while the other side (R) remains unchanged.
Output is then XOR'd with the round key.
Permutation: the two sides are swapped.
Process is repeated for as many rounds as required.

Symmetric-key block cipher.
Uses 16-round Feistel structure.
Block size is 64-bit.
Key length is 64-bit, but has an effective key length of 56 bits (8 bits are not used by the encryption algorithm).
Each round's key is a 48-bit subkey of the 56-bit original key.

Designed to overcome the drawback of DES's small key size.
Users did not want to replace DES due to its widespread adoption and embedded security architectures.
Modified scheme to use DES in a different manner.
Slow compared to other encryption algorithms.

Symmetric key symmetric block cipher.
128-bit data, 128/192/256-bit keys.
Stronger and faster than Triple-DES.
Provides full specification and design details.
Software implementable in C and Java.
Iterative, based on substitution-permutation network.
Computes on bytes rather than bits.
Number of rounds depends on the length of the key (10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys).
Each round uses a different 128-bit round key, calculated from the original AES key.