Constant-time crypto state machine tag comparison

[jovanbulck]

Documenting here a known issue (I spotted some time ago already): the hardware-level crypto state machine does not properly verify tags in constant time.

Depending on the application and usage, this could be dangerously exploited in a timing side channel to brute-force expected tags (eg used by sancus_verify) in linear time byte-per-byte.

Note this is not a problem when calling sancus_tag and then doing the provided tag comparison in constant-time software (eg as done in VulCAN).

Solution. Crypto state machine should always behave constant-time and only check the tag_ok signal after mem_done has finished:

https://github.com/sancus-tee/sancus-core/blob/master/core/rtl/verilog/crypto/crypto_control.v#L157

        VERIFY_TAG:        next_state = tag_ok      ? VERIFY_TAG_WAIT   : FAIL;
        VERIFY_TAG_WAIT:   next_state = mem_done    ? SUCCESS           :
                                        wrap_busy   ? VERIFY_TAG_WAIT   : VERIFY_TAG;