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Advanced Encryption Standard (AES)#

The Advanced Encryption Standard (AES), (original name Rijndael) is a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology (NIST) in 2001.

AES operates on block sizes of \(32 \times 32\) bits.

Key-length Number of Rounds
128 bit 10
192 bit 12
256 bit 14

Algorithm#

AES 128 High Level Overview

  1. Key Expansion: Create individual 128bit round keys from the cipher key

  2. AddRoundKey: each byte of the block is combined with a block of the round key using bitwise xor.

  3. Rounds:

    1. SubBytes: substitutes every byte of the state with an S-box entry
    2. ShiftRows: cyclically left shift every row
    3. MixColumns: takes the four bytes of each column as input and outputs four new bytes, where each input byte affects all four output bytes. Each column is transformed by multiplication with a fixed \(4 \times 4\) Matrix.
    4. AddRoundKey: XOR of the 128bit round key and the 128bit block.

  4. Final Round: (no MixColumns)

    1. SubBytes
    2. ShiftRows
    3. AddRoundKey

Modes of Operation#

ECB#

Electronic Codebook (ECB)

The message is divided into blocks, and each block is encrypted separately.

Electronic Codebook (ECB) Encryption Electronic Codebook (ECB) Decryption

CBC#

Cipher Block Chaining (CBC)

Each block of plaintext is XORed with the previous ciphertext block before being encrypted.

CFB#

Cipher Feedback (CFB)

A close relative of CBC. Transforms a block cipher into a self-synchronizing stream cipher.

OFB#

Output Feedback (OFB)

Transforms a block cipher into a synchronous stream cipher. It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext.

CTR#

Counter (CTR)

Counter mode turns a block cipher into a stream cipher. It generates the next keystream block by encrypting successive values of a "counter".

Test Vectors#

mode = ECB-AES128.Encrypt
in   = 6bc1bee22e409f96e93d7e117393172a
key  = 2b7e151628aed2a6abf7158809cf4f3c
out  = 3ad77bb40d7a3660a89ecaf32466ef97

mode = CBC-AES128.Encrypt
in   = 6bc1bee22e409f96e93d7e117393172a
key  = 2b7e151628aed2a6abf7158809cf4f3c
iv   = 000102030405060708090a0b0c0d0e0f
out  = 7649abac8119b246cee98e9b12e9197d

References#