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[crypto] Expose carry flag from big integer addition and subtraction

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Expose the effective carry (or borrow) out flag from big integer
addition and subtraction, and use this to elide an explicit bit test
when performing x25519 reduction.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2024-11-26 12:53:01 +00:00
parent da6da6eb3b
commit 167a08f089
8 changed files with 140 additions and 85 deletions
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src/crypto/x25519.c
+3 -2
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@@ -564,6 +564,7 @@ void x25519_invert ( const union x25519_oct258 *invertend,
*/
static void x25519_reduce_by ( const x25519_t *subtrahend, x25519_t *value ) {
x25519_t tmp;
int underflow;
/* Conditionally subtract subtrahend
*
@@ -571,8 +572,8 @@ static void x25519_reduce_by ( const x25519_t *subtrahend, x25519_t *value ) {
* time) if the subtraction underflows.
*/
bigint_copy ( value, &tmp );
bigint_subtract ( subtrahend, value );
bigint_swap ( value, &tmp, bigint_msb_is_set ( value ) );
underflow = bigint_subtract ( subtrahend, value );
bigint_swap ( value, &tmp, underflow );
}
/**