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[crypto] Extract bigint_reduce_supremum() from bigint_mod_exp()

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Calculating the Montgomery constant (R^2 mod N) is done in our
implementation by zeroing the double-width representation of N,
subtracting N once to give (R^2 - N) in order to obtain a positive
value, then reducing this value modulo N.

Extract this logic from bigint_mod_exp() to a separate function
bigint_reduce_supremum(), to allow for reuse by other code.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-01-10 13:44:13 +00:00
parent 83ba34076a
commit d88eb0a193
2 changed files with 44 additions and 7 deletions
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21
src/include/ipxe/bigint.h
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@@ -236,9 +236,21 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
* @v value Big integer to be reduced
*/
#define bigint_reduce( modulus, value ) do { \
unsigned int size = bigint_size (modulus); \
bigint_reduce_raw ( (modulus)->element, \
(value)->element, size ); \
unsigned int size = bigint_size (modulus); \
bigint_reduce_raw ( (modulus)->element, (value)->element, \
size ); \
} while ( 0 )
/**
* Reduce supremum of big integer representation
*
* @v modulus0 Big integer modulus
* @v result0 Big integer to hold result
*/
#define bigint_reduce_supremum( modulus, result ) do { \
unsigned int size = bigint_size (modulus); \
bigint_reduce_supremum_raw ( (modulus)->element, \
(result)->element, size ); \
} while ( 0 )
/**
@@ -385,6 +397,9 @@ void bigint_multiply_raw ( const bigint_element_t *multiplicand0,
bigint_element_t *result0 );
void bigint_reduce_raw ( bigint_element_t *modulus0, bigint_element_t *value0,
unsigned int size );
void bigint_reduce_supremum_raw ( bigint_element_t *modulus0,
bigint_element_t *value0,
unsigned int size );
void bigint_mod_invert_raw ( const bigint_element_t *invertend0,
bigint_element_t *inverse0, unsigned int size );
int bigint_montgomery_relaxed_raw ( const bigint_element_t *modulus0,