Question

Simplified AES (S-AES) is based on GF(2⁴). What is the block length and the structure of the state array? Express them in bits.

Answer 1

At the encryption site, S-AES takes a 16-bit plaintext and creates a 16-bit ciphertext; at the decryption site, S-AES takes a 16-bit ciphertext and creates a 16-bit plaintext. The same 16-bit cipher key is used for both encryption and decryption.

Nibble: A nibble is a group of 4 bits that can be treated as a single entity, a row matrix of 4 bits, or a column matrix of 4 bits. When treated as a row matrix, the bits are inserted into the matrix from left to right; when treated as a column matrix, the bits are inserted into the matrix from top to bottom. We use a lowercase bold letter n to refer to a nibble. Note that a nibble is actually a single hexadecimal digit.

In S-AES, a data block is also referred to as a state. We use an uppercase bold letter S to refer to a state. States, like blocks, are made of 16 bits, but normally they are treated as matrices of 4 nibbles. In this case, each element of a state is referred to as sr,c, where r (0 to 1) defines the row and the c (0 to 1) defines the column. At the beginning of the cipher, nibbles in a data block are inserted into a state column by column, and in each column, from top to bottom. At the end of the cipher, nibbles in the state are extracted in the same way.