Peter A Clarke

The National Institute of Science and Technology has produced the updated Advanced Encryption Standard (AES) on 9 May 2023.

At 46 pages it is the usuallong and detailed publication one expects from NIST.  That said, NIST are excellent publications.

Some relevant matters worth noting:

• A bit is a binary digit —0 or 1.
• a block is a sequence of 128 bits;
• the data input and output for the AES block ciphers are blocks.
• the key is a bit sequence that is typically established beforehand and maintained across many invocations of the block cipher.
• the basic processing unit  is the byte — a sequence of eight bits.
• a byte value is denoted by the concatenation of the eight bits between braces
• when the bits of a byte are denoted by an indexed variable, the convention in this Standard is for the indices to decrease from left to right
• it is also convenient to denote byte values using hexadecimal notation. The 16 hexadecimal characters represent sequences of four bits
• internally, the algorithms for the AES block ciphers are performed on a two-dimensional (fourby-four) array of bytes called the state.
• a word is a sequence of four bytes; a block consists of four words.
• in the algorithms for the AES block ciphers can be expressed in terms of matrix multiplication. In particular, a distinct fxed matrix is specifed for each transformation.
• the core of the algorithms for CIPHER() and INVCIPHER() is a sequence of fxed transformations of the state called a round. Each round requires an additional input called the round key; the round key is a block that is usually represented as a sequence of four words (i.e., 16 bytes)
• an expansion routine, denoted by KEYEXPANSION(), takes the block cipher key as input and generates the round keys as output.
•  the input to KEYEXPANSION() is represented as an array of words, denoted by key, and the output is an expanded array of words, denoted by w, called the key schedule