tree 9c788bb073f82b7288ff5181bdce444eeefbb534
parent 8461c0e2a88120a53e97f8d544755855b2feac6a
author Janos Follath <janos.follath@arm.com> 1570800113 +0100
committer Janos Follath <janos.follath@arm.com> 1573475256 +0000

Change mbedtls_mpi_cmp_mpi_ct to check less than

The signature of mbedtls_mpi_cmp_mpi_ct() meant to support using it in
place of mbedtls_mpi_cmp_mpi(). This meant full comparison functionality
and a signed result.

To make the function more universal and friendly to constant time
coding, we change the result type to unsigned. Theoretically, we could
encode the comparison result in an unsigned value, but it would be less
intuitive.

Therefore we won't be able to represent the result as unsigned anymore
and the functionality will be constrained to checking if the first
operand is less than the second. This is sufficient to support the
current use case and to check any relationship between MPIs.

The only drawback is that we need to call the function twice when
checking for equality, but this can be optimised later if an when it is
needed.