fprime.h

   1 /* Arithmetic in prime fields
   2  * Daniel Beer <dlbeer@gmail.com>, 10 Jan 2014
   3  *
   4  * This file is in the public domain.
   5  */
   6
   7 #ifndef FPRIME_H_
   8 #define FPRIME_H_
   9
  10 #include <stdint.h>
  11 #include <string.h>
  12
  13 /* Maximum size of a field element (or a prime). Field elements are
  14  * always manipulated and stored in normalized form, with 0 <= x < p.
  15  * You can use normalize() to convert a denormalized bitstring to normal
  16  * form.
  17  *
  18  * Operations are constant with respect to the value of field elements,
  19  * but not with respect to the modulus.
  20  *
  21  * The modulus is a number p, such that 2p-1 fits in FPRIME_SIZE bytes.
  22  */
  23 #define FPRIME_SIZE             32
  24
  25 /* Load a large constant */
  26 void fprime_from_bytes(uint8_t *x,
  27                        const uint8_t *in, size_t len,
  28                        const uint8_t *modulus);
  29
  30 /* Copy an element */
  31 static inline void fprime_copy(uint8_t *x, const uint8_t *a)
  32 {
  33         memcpy(x, a, FPRIME_SIZE);
  34 }
  35
  36 /* Compare two field points in constant time. Return one if equal, zero
  37  * otherwise. This should be performed only on normalized values.
  38  */
  39 uint8_t fprime_eq(const uint8_t *x, const uint8_t *y);
  40
  41 /* Conditional copy. If condition == 0, then zero is copied to dst. If
  42  * condition == 1, then one is copied to dst. Any other value results in
  43  * undefined behaviour.
  44  */
  45 void fprime_select(uint8_t *dst,
  46                    const uint8_t *zero, const uint8_t *one,
  47                    uint8_t condition);
  48
  49 /* Add one value to another. The two pointers must be distinct. */
  50 void fprime_add(uint8_t *r, const uint8_t *a, const uint8_t *modulus);
  51
  52 /* Multiply two values to get a third. r must be distinct from a and b */
  53 void fprime_mul(uint8_t *r, const uint8_t *a, const uint8_t *b,
  54                 const uint8_t *modulus);
  55
  56 #endif