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[crypto] Add bigint_swap() to conditionally swap big integers

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Add a helper function bigint_swap() that can be used to conditionally
swap a pair of big integers in constant time.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2024-01-19 12:34:02 +00:00
parent 13e390d54e
commit bac13ba1f6
3 changed files with 94 additions and 0 deletions
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25
src/crypto/bigint.c
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@@ -50,6 +50,31 @@ static struct profiler bigint_mod_multiply_rescale_profiler __profiler =
static struct profiler bigint_mod_multiply_subtract_profiler __profiler =
{ .name = "bigint_mod_multiply.subtract" };
/**
* Conditionally swap big integers (in constant time)
*
* @v first0 Element 0 of big integer to be conditionally swapped
* @v second0 Element 0 of big integer to be conditionally swapped
* @v size Number of elements in big integers
* @v swap Swap first and second big integers
*/
void bigint_swap_raw ( bigint_element_t *first0, bigint_element_t *second0,
unsigned int size, int swap ) {
bigint_element_t mask;
bigint_element_t xor;
unsigned int i;
/* Construct mask */
mask = ( ( bigint_element_t ) ( ! swap ) - 1 );
/* Conditionally swap elements */
for ( i = 0 ; i < size ; i++ ) {
xor = ( mask & ( first0[i] ^ second0[i] ) );
first0[i] ^= xor;
second0[i] ^= xor;
}
}
/**
* Perform modular multiplication of big integers
*