Introduction to Information Security

A Simple SPN Cipher

Round 1
Round 2
Round 3
Plaintext Block
Ciphertext Block
Roundkey 1
Roundkey 2
Roundkey 3
The overall plaintext is broken into blocks and each block is encrypted with the cipher
Typical SPN ciphers will have 10-14 rounds
Alice and Bob only need one key, and the cipher will transform that key into subkeys for each round
To decrypt, Alice just does everything in the reverse order

One Feistel Round

Input Left Half
Input Right Half
Round
Function
Output Left Half
Output Right Half
The input to the round is divided in half
The right half is put into a round function with the roundkey
The output of the round function is XORed with the left half
The two halves switch sides to become the input to the next round
Only the left half of the input has been modified
Roundkey

A Simple Feistel Cipher

Plaintext Block
Ciphertext Block
Round 1
Round 2
Round 3
Feistel ciphers need twice as many rounds as SPN ciphers because only half of the input is being encrypted each round
Works the same as SPN ciphers in terms of transforming one key into subkeys and splitting the plaintext into blocks
To decrypt, the ciphertext is sent through the same cipher and the roundkeys are used in reverse order
Roundkey 1
Roundkey 2
Roundkey 3

Modern Ciphers in Practice

• Follow SPN/Feistel structure in general, but with added twists for security
• There are two important ciphers in the history of modern cryptography
• DES (Data Encryption Standard)
• AES (Advanced Encryption Standard)

DES

• U.S. Government recognized the need to have a standardized cipher for secret documents
• DES was developed by IBM in 1976
• Feistel structure
• Key length of 56 bits, block size of 64 bits
• 16 rounds
• Analysis of DES was the beginning of modern cryptographic research