71 lines
2.1 KiB
C
71 lines
2.1 KiB
C
/* Arithmetic in prime fields
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* Daniel Beer <dlbeer@gmail.com>, 10 Jan 2014
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*
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* This file is in the public domain.
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*/
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#ifndef FPRIME_H_
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#define FPRIME_H_
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#include <stdint.h>
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#include <string.h>
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/* Maximum size of a field element (or a prime). Field elements are
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* always manipulated and stored in normalized form, with 0 <= x < p.
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* You can use normalize() to convert a denormalized bitstring to normal
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* form.
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*
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* Operations are constant with respect to the value of field elements,
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* but not with respect to the modulus.
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*
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* The modulus is a number p, such that 2p-1 fits in FPRIME_SIZE bytes.
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*/
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#define FPRIME_SIZE 32
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/* Useful constants */
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extern const uint8_t fprime_zero[FPRIME_SIZE];
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extern const uint8_t fprime_one[FPRIME_SIZE];
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/* Load a small constant */
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void fprime_load(uint8_t *x, uint32_t c);
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/* Load a large constant */
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void fprime_from_bytes(uint8_t *x,
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const uint8_t *in, size_t len,
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const uint8_t *modulus);
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/* Copy an element */
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static inline void fprime_copy(uint8_t *x, const uint8_t *a)
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{
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memcpy(x, a, FPRIME_SIZE);
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}
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/* Normalize a field element */
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void fprime_normalize(uint8_t *x, const uint8_t *modulus);
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/* Compare two field points in constant time. Return one if equal, zero
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* otherwise. This should be performed only on normalized values.
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*/
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uint8_t fprime_eq(const uint8_t *x, const uint8_t *y);
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/* Conditional copy. If condition == 0, then zero is copied to dst. If
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* condition == 1, then one is copied to dst. Any other value results in
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* undefined behaviour.
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*/
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void fprime_select(uint8_t *dst,
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const uint8_t *zero, const uint8_t *one,
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uint8_t condition);
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/* Add one value to another. The two pointers must be distinct. */
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void fprime_add(uint8_t *r, const uint8_t *a, const uint8_t *modulus);
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void fprime_sub(uint8_t *r, const uint8_t *a, const uint8_t *modulus);
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/* Multiply two values to get a third. r must be distinct from a and b */
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void fprime_mul(uint8_t *r, const uint8_t *a, const uint8_t *b,
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const uint8_t *modulus);
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/* Compute multiplicative inverse. r must be distinct from a */
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void fprime_inv(uint8_t *r, const uint8_t *a, const uint8_t *modulus);
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#endif
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