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Move mbedtls_cf_mpi_uint_lt function to the constant-time module

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Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>
This commit is contained in:
gabor-mezei-arm
2021-09-27 12:55:33 +02:00
parent 84dc02c8f5
commit 3f90fd540a
3 changed files with 55 additions and 35 deletions
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35
library/bignum.c
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@ -1247,41 +1247,6 @@ int mbedtls_mpi_cmp_mpi( const mbedtls_mpi *X, const mbedtls_mpi *Y )
return( 0 );
}
/** Decide if an integer is less than the other, without branches.
*
* \param x First integer.
* \param y Second integer.
*
* \return 1 if \p x is less than \p y, 0 otherwise
*/
static unsigned mbedtls_cf_mpi_uint_lt( const mbedtls_mpi_uint x,
const mbedtls_mpi_uint y )
{
mbedtls_mpi_uint ret;
mbedtls_mpi_uint cond;
/*
* Check if the most significant bits (MSB) of the operands are different.
*/
cond = ( x ^ y );
/*
* If the MSB are the same then the difference x-y will be negative (and
* have its MSB set to 1 during conversion to unsigned) if and only if x<y.
*/
ret = ( x - y ) & ~cond;
/*
* If the MSB are different, then the operand with the MSB of 1 is the
* bigger. (That is if y has MSB of 1, then x<y is true and it is false if
* the MSB of y is 0.)
*/
ret |= y & cond;
ret = ret >> ( sizeof( mbedtls_mpi_uint ) * 8 - 1 );
return (unsigned) ret;
}
/*
* Compare signed values in constant time
*/