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Change mbedtls_mpi_cmp_mpi_ct to check less than

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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.
This commit is contained in:
Janos Follath
2019-10-11 14:21:53 +01:00
parent 1fc97594da
commit 0e5532d6cf
5 changed files with 71 additions and 70 deletions
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6
library/ecp.c
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@ -2803,7 +2803,7 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
{
/* SEC1 3.2.1: Generate d such that 1 <= n < N */
int count = 0;
int cmp = 0;
unsigned cmp = 0;
/*
* Match the procedure given in RFC 6979 (deterministic ECDSA):
@ -2829,13 +2829,13 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
if( ++count > 30 )
return( MBEDTLS_ERR_ECP_RANDOM_FAILED );
ret = mbedtls_mpi_cmp_mpi_ct( d, &grp->N, &cmp );
ret = mbedtls_mpi_lt_mpi_ct( d, &grp->N, &cmp );
if( ret != 0 )
{
goto cleanup;
}
}
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 || cmp >= 0 );
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 || cmp != 1 );
}
#endif /* ECP_SHORTWEIERSTRASS */