p256.h

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#ifndef SYSTEM_CORE_INCLUDE_MINCRYPT_LITE_P256_H_
#define SYSTEM_CORE_INCLUDE_MINCRYPT_LITE_P256_H_

// Collection of routines manipulating 256 bit unsigned integers.
// Just enough to implement ecdsa-p256 and related algorithms.

#include <stdint.h>

#ifdef __cplusplus
extern "C" {
#endif

#define P256_BITSPERDIGIT 64
#define P256_NDIGITS 4
#define P256_NBYTES 32

// n' such as n * n' = -1 mod (2^64)
#define P256_MONTGOMERY_FACTOR 0xCCD1C8AAEE00BC4F

#define P256_LITERAL(lo,hi) (((uint32_t) (lo)) + (((uint64_t) (hi)) << 32))

typedef int crypton_p256_err;
typedef uint64_t crypton_p256_digit;
typedef int64_t crypton_p256_sdigit;
typedef __uint128_t crypton_p256_ddigit;
typedef __int128_t crypton_p256_sddigit;

// Defining crypton_p256_int as struct to leverage struct assigment.
typedef struct {
  crypton_p256_digit a[P256_NDIGITS];
} crypton_p256_int;

extern const crypton_p256_int crypton_SECP256r1_n;  // Curve order
extern const crypton_p256_int crypton_SECP256r1_p;  // Curve prime
extern const crypton_p256_int crypton_SECP256r1_b;  // Curve param

// Initialize a crypton_p256_int to zero.
void crypton_p256_init(crypton_p256_int* a);

// Clear a crypton_p256_int to zero.
void crypton_p256_clear(crypton_p256_int* a);

// Return bit. Index 0 is least significant.
int crypton_p256_get_bit(const crypton_p256_int* a, int index);

// b := a % MOD
void crypton_p256_mod(
    const crypton_p256_int* MOD,
    const crypton_p256_int* a,
    crypton_p256_int* b);

// c := a * (top_b | b) % MOD
void crypton_p256_modmul(
    const crypton_p256_int* MOD,
    const crypton_p256_int* a,
    const crypton_p256_digit top_b,
    const crypton_p256_int* b,
    crypton_p256_int* c);

// b := 1 / a % MOD
// MOD best be SECP256r1_n
void crypton_p256_modinv(
    const crypton_p256_int* MOD,
    const crypton_p256_int* a,
    crypton_p256_int* b);

// b := 1 / a % MOD
// MOD best be SECP256r1_n
// Faster than crypton_p256_modinv()
void crypton_p256_modinv_vartime(
    const crypton_p256_int* MOD,
    const crypton_p256_int* a,
    crypton_p256_int* b);

// b := a << (n % P256_BITSPERDIGIT)
// Returns the bits shifted out of most significant digit.
crypton_p256_digit crypton_p256_shl(const crypton_p256_int* a, int n, crypton_p256_int* b);

// b := a >> (n % P256_BITSPERDIGIT)
void crypton_p256_shr(const crypton_p256_int* a, int n, crypton_p256_int* b);

int crypton_p256_is_zero(const crypton_p256_int* a);
int crypton_p256_is_odd(const crypton_p256_int* a);
int crypton_p256_is_even(const crypton_p256_int* a);

// Returns -1, 0 or 1.
int crypton_p256_cmp(const crypton_p256_int* a, const crypton_p256_int *b);

// c: = a - b
// Returns -1 on borrow.
int crypton_p256_sub(const crypton_p256_int* a, const crypton_p256_int* b, crypton_p256_int* c);

// c := a + b
// Returns 1 on carry.
int crypton_p256_add(const crypton_p256_int* a, const crypton_p256_int* b, crypton_p256_int* c);

// c := a + (single digit)b
// Returns carry 1 on carry.
int crypton_p256_add_d(const crypton_p256_int* a, crypton_p256_digit b, crypton_p256_int* c);

// ec routines.

// {out_x,out_y} := nG
void crypton_p256_base_point_mul(const crypton_p256_int *n,
                         crypton_p256_int *out_x,
                         crypton_p256_int *out_y);

// {out_x,out_y} := n{in_x,in_y}
void crypton_p256_point_mul(const crypton_p256_int *n,
                    const crypton_p256_int *in_x,
                    const crypton_p256_int *in_y,
                    crypton_p256_int *out_x,
                    crypton_p256_int *out_y);

// {out_x,out_y} := n1G + n2{in_x,in_y}
void crypton_p256_points_mul_vartime(
    const crypton_p256_int *n1, const crypton_p256_int *n2,
    const crypton_p256_int *in_x, const crypton_p256_int *in_y,
    crypton_p256_int *out_x, crypton_p256_int *out_y);

// Return whether point {x,y} is on curve.
int crypton_p256_is_valid_point(const crypton_p256_int* x, const crypton_p256_int* y);

// Outputs big-endian binary form. No leading zero skips.
void crypton_p256_to_bin(const crypton_p256_int* src, uint8_t dst[P256_NBYTES]);

// Reads from big-endian binary form,
// thus pre-pad with leading zeros if short.
void crypton_p256_from_bin(const uint8_t src[P256_NBYTES], crypton_p256_int* dst);

#define P256_DIGITS(x) ((x)->a)
#define P256_DIGIT(x,y) ((x)->a[y])

#define P256_ZERO {{0}}
#define P256_ONE {{1}}

#ifdef __cplusplus
}
#endif

#endif  // SYSTEM_CORE_INCLUDE_MINCRYPT_LITE_P256_H_