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/* Arithmetic in prime fields
* Daniel Beer <dlbeer@gmail.com>, 10 Jan 2014
*
* This file is in the public domain.
*/
#ifndef FPRIME_H_
#define FPRIME_H_
#include <stdint.h>
#include <string.h>
/* Maximum size of a field element (or a prime). Field elements are
* always manipulated and stored in normalized form, with 0 <= x < p.
* You can use normalize() to convert a denormalized bitstring to normal
* form.
*
* Operations are constant with respect to the value of field elements,
* but not with respect to the modulus.
*
* The modulus is a number p, such that 2p-1 fits in FPRIME_SIZE bytes.
*/
#define FPRIME_SIZE 32
/* Load a large constant */
void fprime_from_bytes(uint8_t *x,
const uint8_t *in, size_t len,
const uint8_t *modulus);
/* Copy an element */
static inline void fprime_copy(uint8_t *x, const uint8_t *a)
{
memcpy(x, a, FPRIME_SIZE);
}
/* Compare two field points in constant time. Return one if equal, zero
* otherwise. This should be performed only on normalized values.
*/
uint8_t fprime_eq(const uint8_t *x, const uint8_t *y);
/* Conditional copy. If condition == 0, then zero is copied to dst. If
* condition == 1, then one is copied to dst. Any other value results in
* undefined behaviour.
*/
void fprime_select(uint8_t *dst,
const uint8_t *zero, const uint8_t *one,
uint8_t condition);
/* Add one value to another. The two pointers must be distinct. */
void fprime_add(uint8_t *r, const uint8_t *a, const uint8_t *modulus);
/* Multiply two values to get a third. r must be distinct from a and b */
void fprime_mul(uint8_t *r, const uint8_t *a, const uint8_t *b,
const uint8_t *modulus);
#endif
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