AN3992 Datasheet, PDF (10/22 Pages) STMicroelectronics – Using an STM8L162M8 AES hardware accelerator with a CR95HF to encrypt contactless tag data memory

Firmware description
AN3992
3.3
Key derivation mode
AES is a symmetric cryptography algorithm. The AES hardware accelerator can compute a
derived key from an encryption key that is named “Decryption key”. The decryption key is
pre-computed to speed-up the decryption phase.
In this mode, the AES takes the encryption key as an input and provides the decryption key
as an output. This mode takes 320 clock cycles.
Figure 4. AES hardware accelerator: key derivation mode
Encryption Key=
"ultra-low power.”
AES hardware accelerator:
key derivation mode
Decryption Key=
"91 DF 30 53 7A E9 49 19
92 FF 8D F8 C4 9B B7 4D”
MS19718V1
In the example above, the decryption key, computed from the “Ultralow Power” encryption
key, is “91 DF 30 53 7A E9 49 19 92 FF 8D F8 C4 9B B7 4D" (hex format).
The algorithm given in Figure 5 provides the steps needed to use the AES hardware.
10/22
Doc ID 022369 Rev 3