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/* Arithmetic mod p = 2^255-19
* Daniel Beer <dlbeer@gmail.com>, 8 Jan 2014
*
* This file is in the public domain.
*/
#ifndef F25519_H_
#define F25519_H_
#include <stdint.h>
#include <string.h>
/* Field elements are represented as little-endian byte strings. All
* operations have timings which are independent of input data, so they
* can be safely used for cryptography.
*
* Computation is performed on un-normalized elements. These are byte
* strings which fall into the range 0 <= x < 2p. Use f25519_normalize()
* to convert to a value 0 <= x < p.
*
* Elements received from the outside may greater even than 2p.
* f25519_normalize() will correctly deal with these numbers too.
*/
#define F25519_SIZE 32
/* Identity constants */
extern const uint8_t f25519_one[F25519_SIZE];
/* Load a small constant */
void f25519_load(uint8_t *x, uint32_t c);
/* Copy two points */
static inline void f25519_copy(uint8_t *x, const uint8_t *a)
{
memcpy(x, a, F25519_SIZE);
}
/* Normalize a field point x < 2*p by subtracting p if necessary */
void f25519_normalize(uint8_t *x);
/* Compare two field points in constant time. Return one if equal, zero
* otherwise. This should be performed only on normalized values.
*/
uint8_t f25519_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 f25519_select(uint8_t *dst,
const uint8_t *zero, const uint8_t *one,
uint8_t condition);
/* Add/subtract two field points. The three pointers are not required to
* be distinct.
*/
void f25519_add(uint8_t *r, const uint8_t *a, const uint8_t *b);
void f25519_sub(uint8_t *r, const uint8_t *a, const uint8_t *b);
/* Unary negation */
void f25519_neg(uint8_t *r, const uint8_t *a);
/* Multiply two field points. The __distinct variant is used when r is
* known to be in a different location to a and b.
*/
void f25519_mul__distinct(uint8_t *r, const uint8_t *a, const uint8_t *b);
/* Take the reciprocal of a field point. The __distinct variant is used
* when r is known to be in a different location to x.
*/
void f25519_inv__distinct(uint8_t *r, const uint8_t *x);
/* Compute one of the square roots of the field element, if the element
* is square. The other square is -r.
*
* If the input is not square, the returned value is a valid field
* element, but not the correct answer. If you don't already know that
* your element is square, you should square the return value and test.
*/
void f25519_sqrt(uint8_t *r, const uint8_t *x);
#endif
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