RM0082
AS_Cryptographic co-processor (C3)
Doc ID 018672 Rev 1
389/844
21.9.4
AES START instruction
The AES START instruction can be applied with 3 different modes of operation:
●
ECB
●
CBC
●
CTR
21.9.5 ECB
The AES START ECB instruction is 2 words long. This instruction is used to set the key for
the following operations. The length of the key is encoded in the first instruction word, while
the second word represents the Source Address for the key.
Bit ‘a’ in the above table is used to set the operation to perform:
Bits 15 to 0 in the first instruction word (cccc in
) represent the length in Bytes of
the key.
21.9.6 CBC
The AES START CBC instruction is 3 words long. This instruction is used to set the key and
the initialization vector for the following operations. The length of the key is encoded in the
first instruction word, the second word represents the Source Address for the key and the
third word represents the Source Address for the Initialization Vector.
Bit ‘a’ in the above table is used to set the operation to perform and has the same encoding
as in the ECB instruction (
). Bits 15 to 0 in the first instruction word (cccc in
) represent the length in Bytes of the key.
Table 324.
AES ECB START instruction bit encoding
W#
Bit Encoding
1
xxxx 01xa 000x xxxx cccc cccc cccc cccc
2
32 bit Source Address for the key
Table 325.
AES ECB Bit ‘a’ encoding
Bit #24 - a
Operation
0
DES
1
3DES
Table 326.
AES CBC START instruction bit encoding
W#
Bit Encoding
1
xxxx 10xa 001x xxxx cccc cccc cccc cccc
2
32 bit Source Address for the key
3
32 bit Source Address for the IV