AES
The AES parameters, the 4 × 4 state, the four operations of a round, the last round without MixColumns, and what sets AES apart from DES.
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AES: what is it?
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A symmetric block cipher standardised in 2001 to succeed DES, derived from the Rijndael algorithm.
It is the current standard of symmetric encryption. The block is always 128 bits; the key is 128, 192 or 256 bits, for 10, 12 or 14 rounds.
Row index 2 of the state is
00 11 22 33. What does it become after ShiftRows?22 33 00 1111 22 33 0033 00 11 2200 11 22 33
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22 33 00 11Row index 2 is shifted 2 cells to the left.
11 22 33 00would be row index 1,33 00 11 22row index 3, and00 11 22 33row index 0, which does not move.The AES block is … bits, held in a state of … bytes.
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The AES block is 128 bits, held in a state of 4 × 4 bytes.
16 bytes of 8 bits make 128 bits. The state is filled column by column with the bytes of the block.
Substitution-permutation network: what is it?
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A cipher structure that repeats rounds alternating substitutions (confusion), permutations (diffusion) and key addition, over the whole block.
Unlike a Feistel network, the whole state is transformed at each round, so each round has to be invertible to decrypt.
An application encrypts its backups with AES-256, but its key is made of the first 32 characters of the password typed by the user, padded with zeros. What is the flaw?
- The key has very little entropy: the password can be guessed without attacking AES
- AES-256 is broken as soon as the key is made of printable characters, because the bytes are too regular
- A 32-byte key needs 14 rounds of encryption, whereas the library only does 10
- AES in practice refuses keys that contain null bytes, so the backup stays readable
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The key has very little entropy: the password can be guessed without attacking AES
AES is no stronger than its key: a short password is found by guessing. The key should be derived by a slow, salted function (PBKDF2, scrypt, Argon2), not copied.