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mpi_exp_mod: improve documentation

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Signed-off-by: Janos Follath <janos.follath@arm.com>
This commit is contained in:
Janos Follath 2022-11-22 21:08:33 +00:00
parent 7d89d351e6
commit 3321b5842c
1 changed files with 8 additions and 2 deletions
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10
library/bignum_core.c
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@ -629,6 +629,10 @@ static void exp_mod_precompute_window( const mbedtls_mpi_uint *A,
* As in other bignum functions, assume that AN_limbs and E_limbs are nonzero.
*
* RR must contain 2^{2*biL} mod N.
*
* The algorithm is a variant of Left-to-right k-ary exponentiation: HAC 14.82
* (The difference is that the body in our loop processes a single bit instead
* of a full window.)
*/
int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
@ -693,7 +697,7 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
/* Square */
mbedtls_mpi_core_montmul( X, X, X, AN_limbs, N, AN_limbs, mm, temp );
/* Insert next exponent bit into window */
/* Move to the next bit of the exponent */
if( E_bit_index == 0 )
{
--E_limb_index;
@ -703,6 +707,7 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
{
--E_bit_index;
}
/* Insert next exponent bit into window */
++window_bits;
window <<= 1;
window |= ( E[E_limb_index] >> E_bit_index ) & 1;
@ -717,7 +722,8 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
mbedtls_mpi_core_ct_uint_table_lookup( Wselect, Wtable,
AN_limbs, welem, window );
/* Multiply X by the selected element. */
mbedtls_mpi_core_montmul( X, X, Wselect, AN_limbs, N, AN_limbs, mm, temp );
mbedtls_mpi_core_montmul( X, X, Wselect, AN_limbs, N, AN_limbs, mm,
temp );
window = 0;
window_bits = 0;
}