RM0090: Reference manual - STMicroelectronics

RM0090 Hash processor (HASH)
Once this is done, writing into HASH_STR with bit DCAL = 1 starts the processing of the last
entered block of message by the hash processor. This processing consists in:
● Automatically performing the message padding operation: the purpose of this operation
is to make the total length of a padded message a multiple of 512. The HASH
sequentially processes blocks of 512 bits when computing the message digest
● Computing the final message digest
When the DMA is enabled, it provides the information to the hash processor when it is
transferring the last data word. Then the padding and digest computation are performed
automatically as if DCAL had been written to 1.
22.3.4 Message padding
Message padding consists in appending a “1” followed by m “0”s followed by a 64-bit integer
to the end of the original message to produce a padded message block of length 512. The
“1” is added to the last word written into the HASH_DIN register at the bit position defined by
the NBLW bitfield, and the remaining upper bits are cleared (“0”s).
Example: let us assume that the original message is the ASCII binary-coded form of “abc”,
of length L = 24:
byte 0 byte 1 byte 2 byte 3
01100001 01100010 01100011 UUUUUUUU
NBLW has to be loaded with the value 24: a “1” is appended at bit location 24 in the bit string
(starting counting from left to right in the above bit string), which corresponds to bit 31 in the
HASH_DIN register (little-endian convention):
01100001 01100010 01100011 1UUUUUUU
Since L = 24, the number of bits in the above bit string is 25, and 423 “0”s are appended,
making now 448. This gives (in hexadecimal, big-endian format):
61626380 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000
The L value, in two-word representation (that is 00000000 00000018) is appended. Hence
the final padded message in hexadecimal:
61626380 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000000
00000000 00000000 00000000 00000028
If the HASH is programmed to use the little-endian byte input format, the above message
has to be entered by doing the following steps:
1. 0xUU636261 is written into the HASH_DIN register (where ‘U’ means don’t care)
2. 0x18 is written into the HASH_STR register (the number of valid bits in the last word
written into the HASH_DIN register is 24, as the original message length is 24 bits)
3. 0x10 is written into the HASH_STR register to start the message padding and digest
computation. When NBLW ≠ 0x00, the message padding puts a “1” into the HASH_DIN
register at the bit position defined by the NBLW value, and inserts “0”s at bit locations
[31:(NBLW+1)]. When NBLW == 0x00, the message padding inserts one new word
Doc ID 018909 Rev 3 600/1416