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New constant-flow function mbedtls_mpi_core_uint_le_mpi

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Compare a single-limb MPI with a multi-limb MPI. This is rather ad hoc, but
will be useful for mbedtls_mpi_core_random.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2022-09-20 18:38:35 +02:00
parent c3902ac661
commit 6f949ea67b
4 changed files with 150 additions and 0 deletions
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21
library/bignum_core.c
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@@ -154,6 +154,27 @@ void mbedtls_mpi_core_bigendian_to_host( mbedtls_mpi_uint *A,
}
}
/* Whether min <= A, in constant time.
* A_limbs must be at least 1. */
unsigned mbedtls_mpi_core_uint_le_mpi( mbedtls_mpi_uint min,
const mbedtls_mpi_uint *A,
size_t A_limbs )
{
/* min <= least significant limb? */
unsigned min_le_lsl = 1 ^ mbedtls_ct_mpi_uint_lt( A[0], min );
/* most significant limbs (excluding 1) are all zero? */
mbedtls_mpi_uint msll_mask = 0;
for( size_t i = 1; i < A_limbs; i++ )
msll_mask |= A[i];
/* The most significant limbs of A are not all zero iff msll_mask != 0. */
unsigned msll_nonzero = mbedtls_ct_mpi_uint_mask( msll_mask ) & 1;
/* min <= A iff the lowest limb of A is >= min or the other limbs
* are not all zero. */
return( min_le_lsl | msll_nonzero );
}
void mbedtls_mpi_core_cond_assign( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
size_t limbs,

16
library/bignum_core.h
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@@ -129,6 +129,22 @@ size_t mbedtls_mpi_core_bitlen( const mbedtls_mpi_uint *A, size_t A_limbs );
void mbedtls_mpi_core_bigendian_to_host( mbedtls_mpi_uint *A,
size_t A_limbs );
/** \brief Compare a machine integer with an MPI.
*
* This function operates in constant time with respect
* to the values of \p min and \p A.
*
* \param min A machine integer.
* \param[in] A An MPI.
* \param A_limbs The number of limbs of \p A.
* This must be at least 1.
*
* \return 1 if \p min is less than or equal to \p A, otherwise 0.
*/
unsigned mbedtls_mpi_core_uint_le_mpi( mbedtls_mpi_uint min,
const mbedtls_mpi_uint *A,
size_t A_limbs );
/**
* \brief Perform a safe conditional copy of an MPI which doesn't reveal
* whether assignment was done or not.