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[crypto] Use inverse size as effective size for bigint_mod_invert() 

Montgomery reduction requires only the least significant element of an
inverse modulo 2^k, which in turn depends upon only the least
significant element of the invertend.

Use the inverse size (rather than the invertend size) as the effective
size for bigint_mod_invert().  This eliminates around 97% of the loop
iterations for a typical 2048-bit RSA modulus.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
pull/1351/head
Michael Brown 2024-11-27 12:56:22 +00:00
parent 7c2e68cc87
commit 96f385d7a4
2 changed files with 11 additions and 6 deletions
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2
src/include/ipxe/bigint.h
View File

@ -248,7 +248,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
* @v inverse Big integer to hold result
*/
#define bigint_mod_invert( invertend, inverse ) do { \
unsigned int size = bigint_size ( invertend ); \
unsigned int size = bigint_size ( inverse ); \
bigint_mod_invert_raw ( (invertend)->element, \
(inverse)->element, size ); \
} while ( 0 )

15
src/tests/bigint_test.c
View File

@ -596,14 +596,14 @@ void bigint_mod_exp_sample ( const bigint_element_t *base0,
static const uint8_t invertend_raw[] = invertend; \
static const uint8_t expected_raw[] = expected; \
uint8_t inverse_raw[ sizeof ( expected_raw ) ]; \
unsigned int size = \
unsigned int invertend_size = \
bigint_required_size ( sizeof ( invertend_raw ) ); \
bigint_t ( size ) invertend_temp; \
bigint_t ( size ) inverse_temp; \
unsigned int inverse_size = \
bigint_required_size ( sizeof ( inverse_raw ) ); \
bigint_t ( invertend_size ) invertend_temp; \
bigint_t ( inverse_size ) inverse_temp; \
{} /* Fix emacs alignment */ \
\
assert ( bigint_size ( &invertend_temp ) == \
bigint_size ( &inverse_temp ) ); \
bigint_init ( &invertend_temp, invertend_raw, \
sizeof ( invertend_raw ) ); \
DBG ( "Modular invert:\n" ); \
@ -1853,6 +1853,11 @@ static void bigint_test_exec ( void ) {
0xb3, 0xe1, 0x3e, 0xc6, 0x5a, 0x03,
0x51, 0x6f, 0xb7, 0xe3, 0xa5, 0xd6,
0xa1, 0xb9 ) );
bigint_mod_invert_ok ( BIGINT ( 0xfe, 0x43, 0xf6, 0xa0, 0x32, 0x02,
0x47, 0xaa, 0xaa, 0x0e, 0x33, 0x19,
0x2e, 0xe6, 0x22, 0x07 ),
BIGINT ( 0x7b, 0xd1, 0x0f, 0x78, 0x0c, 0x65,
0xab, 0xb7 ) );
bigint_mod_multiply_ok ( BIGINT ( 0x37 ),
BIGINT ( 0x67 ),
BIGINT ( 0x3f ),