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

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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>
This commit is contained in:
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
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@@ -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 )