SAMA5D41_14 Datasheet, PDF (1598/1776 Pages) ATMEL Corporation – Atmel | SMART ARM-based Embedded MPU

Figure 50-7. Single GHASHH Block Diagram (AADLEN ≤ 0x10 and CLEN = 0)
GHASH
IDATA
GF128Mult(H)
GHASH
It is possible to process a message with only AAD setting the CLEN field to ‘0’ in the AES_CLENR, this can be
used for J₀ generation when len(IV) ≠ 96 for instance.
Example: Processing J₀ when len(IV) ≠ 96
To process J₀ = GHASHH(IV || 0^s+64 || [len(IV)]64) perform the following steps:
1. In AES_MR set OPMOD to GCM and GTAGEN to ‘0’ (configuration as usual for the rest).
2. Set KEYW in AES_KEYWRx and wait until DATRDY bit of AES_ISR is set (GCM hash subkey generation
complete); use interrupt if needed. After the GCM hash subkey generation is complete the GCM hash
subkey can be read or overwritten with specific value in the AES_GCMHRx (see Section 50.4.5.2 “Key
Writing and Automatic Hash Subkey Calculation” for details).
3. Set AADLEN field with ‘len(IV || 0^s+64 || [len(IV)]64)’ in AES_AADLENR and CLEN field to ‘0’ in AES_CLENR.
This will allow running a GHASHH only.
4. Fill the IDATA field of AES_IDATARx with the message to process (IV || 0^s+64 || [len(IV)]64) according to the
SMOD configuration used. If Manual Mode or Auto Mode is used, the DATRDY bit indicates when a
GHASHH step is over (use interrupt if needed).
5. Read the GHASH field of AES_GHASHRx to obtain the J₀ value.
Note: The GHASH value can be overwritten at any time by writing the GHASH field value of AES_GHASHRx, used to
perform a GHASHH with an initial value for GHASH (write GHASH field between step 3 and step 4 in this case).
Processing a Single GF128 Multiplication
The AES can also be used to process a single multiplication in the Galois field on 128 bits (GF128) using a single
GHASHH with custom H value (see Figure 50-7).
To run a GF128 multiplication (A x B) perform the following steps:
1. In AES_MR set OPMOD to GCM and GTAGEN to ‘0’ (configuration as usual for the rest).
2. Set AADLEN field with 0x10 (16 bytes) in AES_AADLENR and CLEN field to ‘0’ in AES_CLENR. This will
allow running a single GHASHH.
3. Fill the H field of the AES_GCMHRx with B value.
4. Fill the IDATA field of AES_IDATARx with the A value according to the SMOD configuration used. If Manual
Mode or Auto Mode is used, the DATRDY bit indicates when a GHASHH computation is over (use interrupt if
needed).
5. Read the GHASH field of AES_GHASHRx to obtain the result.
Note: The GHASH field of AES_GHASHRx can be initialized with a value C between step 3 and step 4 to run a ((A XOR C) x
B) GF128 multiplication.
1598
SAMA5D4 Series [DATASHEET]
Atmel-11238A-ATARM-SAMA5D4-Datasheet_30-Sep-14