AES hardware accelerator operation modes

AN3270

12/23

Doc ID 17919 Rev 2

EK is a 16-byte buffer containing the Encryption Key.

PT is a 16-byte containing the Plain Text.

CT is 16-byte containing the Cipher Text.

Note:

Refer to the AES_EncryptDecrypt example available in the STM8L15x_StdPeriph_Lib

firmware package for an example of implementation in C language.

1.5

Decryption mode vs. key derivation and decryption mode

The AES hardware accelerator support two methods for decrypting a cipher text:

●

Key derivation and decryption mode which requires the encryption key.

●

Decryption mode which requires the decryption key that must previously be computed

using the key derivation mode.

The former takes 1228 clock cycles for one 128-bit data block but the latter takes only 892

clock cycles for the same data block length.

Key derivation and decryption modes are useful when the data length is greater than 128-

bit. In the first step the decryption key is computed using key derivation mode then data are

decrypted using decryption mode.

Example: The length of the data to be encrypted is 160 bytes (128-bit * 10). The formulas

below can be used to determine the processing time for both methods:

●

Key derivation and decryption mode method:

processing time = number of 128-bit blocks * processing time of single block = 10 *

1228 = 12280 clock cycles

●

Decryption mode + Key derivation mode method:

processing time = key derivation processing time + number of 128-bit blocks *

processing time of single block = 320 + 10 *892 = 9240 clock cycles

This shows that it is better to use the combined key derivation and decryption mode

when processing multiple blocks.