51.4.7 Automatic Check

The SHA module features an automatic check of the hash result with the expected hash. A check failure can generate an interrupt if configured in the SHA Interrupt Enable register (SHA_IER).

Automatic check requires the automatic padding feature to be enabled (MSGSIZE and BYTCNT fields must be greater than 0).

There are two methods to configure the expected hash result:

  • if SHA_MR.CHECK = 1, the expected hash result is read from the internal register (IR1). This method cannot be used when HMAC algorithms is selected because this register is already used to store user initial hash values for the second hash processing. IR1 cannot be read by software.
  • If SHA_MR.CHECK = 2, the expected hash result is written in the SHA_IDATARx after the message.

When SHA_MR.CHECK = 2, the method can provide more flexibility of use if a message is stored in system memory together with its expected hash result. A DMA with linked list can be used to ease the transfer of the message and its expected hash result.

Figure 51-3. Message and Expected Hash Result Memory Mapping

The number of 32-bit words of the hash result to check with the expected hash can be selected with SHA_MR.CHKCNT. The status of the check is available in the CHKST field in the SHA Interrupt Status register (SHA_ISR).

An interrupt can be generated (if enabled) when the check is completed. The check occurs several clock cycles after the computation of the requested hash, so the interrupt and the CHECKF bit are set several clock cycles after the DATRDY flag of the SHA_ISR.