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Make the main loop's logic clearer

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The loop ends when there are no more bits to process, with one twist: when
that happens, we need to clear the window one last time. Since the window
does not start empty (E_limbs==0 is not supported), the loop always starts
with a non-empty window and some bits to process. So it's correct to move
the window clearing logic to the end of the loop. This lets us exit the loop
when the end of the exponent is reached.

It would be clearer not to do the final window clearing inside the loop, so
we wouldn't need to repeat the loop termination condition (end of exponent
reached) inside the loop. However, this requires duplicating the code to
clear the window. Empirically, this causes a significant code size increase,
even if the window clearing code is placed into a function.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2022-11-16 22:06:18 +01:00 committed by Janos Follath
parent c718a3ce94
commit 3b63d09fea
1 changed files with 15 additions and 22 deletions
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37
library/bignum_core.c
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@ -683,29 +683,8 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
mbedtls_mpi_uint window = 0;
size_t window_bits = 0;
while( 1 )
do
{
size_t window_bits_missing = wsize - window_bits;
const int no_more_bits =
( E_bit_index == 0 ) && ( E_limb_index == 0 );
const int window_full =
( window_bits_missing == 0 );
/* Clear window if it's full or if we don't have further bits. */
if( window_full || no_more_bits )
{
if( window_bits == 0 )
break;
/* Select table entry, square and multiply */
mbedtls_mpi_core_ct_uint_table_lookup( Wselect, Wtable,
AN_limbs, welem, window );
mbedtls_mpi_core_montmul( X, X, Wselect, AN_limbs, N, AN_limbs, mm, temp );
window = 0;
window_bits = 0;
continue;
}
/* Square */
mbedtls_mpi_core_montmul( X, X, X, AN_limbs, N, AN_limbs, mm, temp );
@ -722,7 +701,21 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
++window_bits;
window <<= 1;
window |= ( E[E_limb_index] >> E_bit_index ) & 1;
/* Clear window if it's full. Also clear the window at the end,
* when we've finished processing the exponent. */
if( window_bits == wsize ||
( E_bit_index == 0 && E_limb_index == 0 ) )
{
/* Select table entry, square and multiply */
mbedtls_mpi_core_ct_uint_table_lookup( Wselect, Wtable,
AN_limbs, welem, window );
mbedtls_mpi_core_montmul( X, X, Wselect, AN_limbs, N, AN_limbs, mm, temp );
window = 0;
window_bits = 0;
}
}
while( ! ( E_bit_index == 0 && E_limb_index == 0 ) );
/* Convert X back to normal presentation */
const mbedtls_mpi_uint one = 1;