Advanced Encryption Standard (AES)

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Advanced Encryption Standard (AES) is a symmetric block cipher that encrypts data in 128-bit blocks.

• aka Rijndael block cipher
• developed as the official replacement to DES
• designated as FIPS 197
• has 3 different ciphers of different key sizes
  • AES 128, AES 192, AES 256
  • each uses a 128-bit block size
• not based on Feistel Function
  • instead uses a substitution-permutation matrix
    • uses a 4 x 4 column matrix of bytes
      • called the state
• an encryption round is a set of operations that are consecutively applied to a block during the encryption process
  • number of encryption rounds is determined by the key length
  • longer key → higher number of rounds and larger keyspace
    • both make cipher more resistant to brute force attacks

AES Versions

Version	Key length (bits)	Encryption rounds	Keyspace (number of possible keys)
AES-128	128	10
AES-192	192	12
AES-256	256	14

How It Works

• general steps:
  1. Key expansion
     • round keys are derived from the cipher key using Rijndael’s key schedule
       • generates a different key in each round, based on the original key
       • uses a set of round constants to generate a unique 128-bit round key block for each round
         • along with one extra round key block
  2. Initial round
     • AddRoundKey
       • each byte of the state is combined with the round key using bitwise XOR
         • i.e., the plaintext is arrayed bit by bit in a matrix that is XOR’d with the key
  3. Rounds
     • SubBytes
       • a nonlinear substitution step where each byte is replaced with another according to a lookup table
       • each bytes in the matrix is then fed into a substitution box
       • also transposes the bits as well as substituting them
         • so it is called a permutation box
     • ShiftRows
       • a transposition step where each row of the state is shifted cyclically a certain number of steps
     • MixColumns
       • a mixing operation that operates on the columns of the state, combining the 4 bytes in each column
     • AddRoundKey
       • a step where the key is XOR’d with the matrix again
  4. Final round (no MixColumns)
     • Repeat
       • SubBytes
       • ShiftRows
       • AddRoundKey
• details
  • SubBytes step
    • each byte in the matrix is substituted for another byte using an 8-bit substitution box
      • called the Rijndael S-box
  • ShiftRows step
    • works by shifting the bytes in each row by a certain amount
    • first row is left unchanged
    • second row is shifted one to the left
    • the third row is shifted by two, and so on
  • MixColumns step
    • columns are mixed, similar to shifting rows
    • but rather than just shifting them, they are mixed together
  • AddRoundKey step
    • subkey is XOR’d with the state
    • for each round key, a subkey is derived from the main key using Rijndael’s key schedule
    • each subkey is the same size as the state