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Simplify exponent bit selection

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Use indices instead of mutating data to extract the bits of the exponent.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2022-11-16 20:42:09 +01:00 committed by Janos Follath
parent d83b5cb504
commit c718a3ce94
1 changed files with 17 additions and 20 deletions
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37
library/bignum_core.c
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@ -675,21 +675,20 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
/* X = 1 (in Montgomery presentation) initially */
memcpy( X, Wtable, AN_limbs * ciL );
/* Start from the end of exponent buffer */
E += E_limbs;
size_t limb_bits_remaining = 0;
/* We'll process the bits of E from most significant
* (limb_index=E_limbs-1, E_bit_index=biL-1) to least significant
* (limb_index=0, E_bit_index=0). */
size_t E_limb_index = E_limbs;
size_t E_bit_index = 0;
mbedtls_mpi_uint window = 0;
size_t window_bits = 0;
/* Will be initialized properly in the first loop iteration */
mbedtls_mpi_uint cur_limb = 0;
while( 1 )
{
size_t window_bits_missing = wsize - window_bits;
const int no_more_bits =
( limb_bits_remaining == 0 ) && ( E_limbs == 0 );
( E_bit_index == 0 ) && ( E_limb_index == 0 );
const int window_full =
( window_bits_missing == 0 );
@ -707,24 +706,22 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
continue;
}
/* Load next exponent limb if necessary */
if( limb_bits_remaining == 0 )
{
--E;
cur_limb = *E;
--E_limbs;
limb_bits_remaining = biL;
}
/* Square */
mbedtls_mpi_core_montmul( X, X, X, AN_limbs, N, AN_limbs, mm, temp );
/* Insert next exponent bit into window */
window <<= 1;
window |= ( cur_limb >> ( biL - 1 ) );
cur_limb <<= 1;
if( E_bit_index == 0 )
{
--E_limb_index;
E_bit_index = biL - 1;
}
else
{
--E_bit_index;
}
++window_bits;
--limb_bits_remaining;
window <<= 1;
window |= ( E[E_limb_index] >> E_bit_index ) & 1;
}
/* Convert X back to normal presentation */