Advanced Encryption Standard (AES) is a popular and widely adopted symmetric key encryption algorithm. AES uses repeat cycles or "rounds". There are 10, 12, or 14 rounds for keys of 128, 192, and 256 bits, respectively. Each round of Algorithm consists of four steps:
- subBytes: for each byte in the array, use its value as an index into a fixed 256-element lookup table, and replace its value in the state by the byte value stored at that location in the table. You can find the table and the inverse table on the web.
- shiftRows: Let Ri denote the ith row in state. Shift R0 in the state left 0 bytes (i.e., no change); shift R1 left 1 byte; shift R2 left 2 bytes; shift R3 left 3 bytes. These are circular shifts. They do not affect the individual byte values themselves. Shift left for decryption.
- mixColumns: for each column of the state, replace the column by its value multiplied by a fixed 4 x 4 matrix of integers (in a particular Galois Field). This is a relatively complex step, but if you utilize the BitVector library demonstrated in the sessional class it will be simple matrix multiplication. Note that the inverse operation multiplies by a different matrix.
- addRoundkey: XOR the state with a 128-bit round key derived from the original key K by a recursive process. The final round is slightly different from the others. Implementation details can be found in the presentation slide shared in the repo.
Decrytion is almost opposite algo of Encryption.