RM0090: Reference manual - STMicroelectronics

RM0090 Cryptographic processor (CRYP)
Additional bits should be saved when GCM/GMAC or CCM/CMAC algorithms are
used:
– bits [17:16] in the CRYP_CR register
– context swap registers:
CRYP_CSGCMCCM0..7 for GCM/GMAC or CCM/CMAC algorithm
CRYP_CSGCM0..7 for GCM/GMAC algorithm.
2. Configure and execute the other processing.
3. Context restoration
a) Configure the processor as in Section 20.3.6: Procedure to perform an encryption
or a decryption on page 568, Initialization with the saved configuration. For the
AES-ECB or AES-CBC decryption, the key must be prepared again.
b) If needed, reconfigure the DMA controller to transfer the rest of the message.
c) Enable the processor by setting the CRYPEN bit and, the DMA requests by setting
the DIEN and DOEN bits.
Case of the TDES
Context swapping can be done in the TDES in the same way as in the AES. But as the input
FIFO can contain up to 4 unprocessed blocks and as the processing duration per block is
higher, it can be faster in certain cases to interrupt the processing without waiting for the IN
FIFO to be empty.
1. Context saving
a) Stop DMA transfers on the IN FIFO by clearing the DIEN bit in the CRYP_DMACR
register.
b) Disable the processor by clearing the CRYPEN bit (the processing will stop at the
end of the current block).
c) Wait until the OUT FIFO is empty (OFNE=0 in the CRYP_SR register) and the
BUSY bit is cleared.
d) Stop DMA transfers on the OUT FIFO by writing the DOEN bit to 0 in the
CRYP_DMACR register.
e) Save the current configuration (bits [9:2] and bits 19 in the CRYP_CR register)
and, if not in ECB mode, the initialization vectors. The key value must already be
available in the memory. When needed, save the DMA status (pointers for IN and
OUT messages, number of remaining bytes, etc.). Read back the data loaded in
the IN FIFO that have not been processed and save them in the memory until the
FIFO is empty.
Note: In GCM/GMAC or CCM/CMAC mode, bits [17:16] of the CRYP_CR register should also be
saved.
2. Configure and execute the other processing.
3. Context restoration
a) Configure the processor as in Section 20.3.6: Procedure to perform an encryption
or a decryption on page 568, Initialization with the saved configuration. For the
AES-ECB or AES-CBC decryption, the key must be prepared again.
b) Write the data that were saved during context saving into the IN FIFO.
c) If needed, reconfigure the DMA controller to transfer the rest of the message.
d) Enable the processor by setting the CRYPEN bit and, the DMA requests by setting
the DIEN and DOEN bits.
Doc ID 018909 Rev 3 570/1416