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[crypto] Calculate inverse of modulus on demand in bigint_montgomery()

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Reduce the number of parameters passed to bigint_montgomery() by
calculating the inverse of the modulus modulo the element size on
demand.  Cache the result, since Montgomery reduction will be used
repeatedly with the same modulus value.

In all currently supported algorithms, the modulus is a public value
(or a fixed value defined by specification) and so this non-constant
timing does not leak any private information.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2024-12-16 15:09:56 +00:00
parent 24db39fb29
commit 97079553b6
3 changed files with 23 additions and 36 deletions
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11
src/tests/bigint_test.c
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@@ -207,20 +207,17 @@ void bigint_mod_invert_sample ( const bigint_element_t *invertend0,
}
void bigint_montgomery_sample ( const bigint_element_t *modulus0,
const bigint_element_t *modinv0,
bigint_element_t *mont0,
bigint_element_t *result0,
unsigned int size ) {
const bigint_t ( size ) __attribute__ (( may_alias ))
*modulus = ( ( const void * ) modulus0 );
const bigint_t ( 1 ) __attribute__ (( may_alias ))
*modinv = ( ( const void * ) modinv0 );
bigint_t ( 2 * size ) __attribute__ (( may_alias ))
*mont = ( ( void * ) mont0 );
bigint_t ( size ) __attribute__ (( may_alias ))
*result = ( ( void * ) result0 );
bigint_montgomery ( modulus, modinv, mont, result );
bigint_montgomery ( modulus, mont, result );
}
void bigint_mod_exp_sample ( const bigint_element_t *base0,
@@ -631,7 +628,6 @@ void bigint_mod_exp_sample ( const bigint_element_t *base0,
unsigned int size = \
bigint_required_size ( sizeof ( modulus_raw ) ); \
bigint_t ( size ) modulus_temp; \
bigint_t ( 1 ) modinv_temp; \
bigint_t ( 2 * size ) mont_temp; \
bigint_t ( size ) result_temp; \
{} /* Fix emacs alignment */ \
@@ -641,13 +637,10 @@ void bigint_mod_exp_sample ( const bigint_element_t *base0,
bigint_init ( &modulus_temp, modulus_raw, \
sizeof ( modulus_raw ) ); \
bigint_init ( &mont_temp, mont_raw, sizeof ( mont_raw ) ); \
bigint_mod_invert ( &modulus_temp, &modinv_temp ); \
DBG ( "Montgomery:\n" ); \
DBG_HDA ( 0, &modulus_temp, sizeof ( modulus_temp ) ); \
DBG_HDA ( 0, &modinv_temp, sizeof ( modinv_temp ) ); \
DBG_HDA ( 0, &mont_temp, sizeof ( mont_temp ) ); \
bigint_montgomery ( &modulus_temp, &modinv_temp, &mont_temp, \
&result_temp ); \
bigint_montgomery ( &modulus_temp, &mont_temp, &result_temp ); \
DBG_HDA ( 0, &result_temp, sizeof ( result_temp ) ); \
bigint_done ( &result_temp, result_raw, \
sizeof ( result_raw ) ); \