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Separate out low-level mpi_safe_cond_assign

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Separate out a version of mpi_safe_cond_assign that works on
equal-sized limb arrays, without worrying about allocation sizes or
signs.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2020-06-04 19:14:58 +02:00
parent d108d07050
commit 3c44c65fc1
1 changed files with 18 additions and 3 deletions
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21
library/bignum.c
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@ -226,6 +226,22 @@ void mbedtls_mpi_swap( mbedtls_mpi *X, mbedtls_mpi *Y )
memcpy( Y, &T, sizeof( mbedtls_mpi ) );
}
/*
* Conditionally assign dest = src, without leaking information
* about whether the assignment was made or not.
* dest and src must be arrays of limbs of size n.
* assign must be 0 or 1.
*/
static void mpi_safe_cond_assign( size_t n,
mbedtls_mpi_uint *dest,
const mbedtls_mpi_uint *src,
unsigned char assign )
{
size_t i;
for( i = 0; i < n; i++ )
dest[i] = dest[i] * ( 1 - assign ) + src[i] * assign;
}
/*
* Conditionally assign X = Y, without leaking information
* about whether the assignment was made or not.
@ -243,10 +259,9 @@ int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned
X->s = X->s * ( 1 - assign ) + Y->s * assign;
for( i = 0; i < Y->n; i++ )
X->p[i] = X->p[i] * ( 1 - assign ) + Y->p[i] * assign;
mpi_safe_cond_assign( Y->n, X->p, Y->p, assign );
for( ; i < X->n; i++ )
for( i = Y->n; i < X->n; i++ )
X->p[i] *= ( 1 - assign );
cleanup: