Block Ciphers: DES and Triple DES

Block Ciphers

• 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).

Feistel Block Cipher

• 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.

DES

• 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.

Triple DES

• 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.

AES

• 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.

Block Ciphers

• 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).

Feistel Block Cipher

• 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.

DES

• 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.

Triple DES

• 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.

AES

• 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.