field.h

/*
 * Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
 * Copyright 2014 Cryptography Research, Inc.
 *
 * Licensed under the OpenSSL license (the "License").  You may not use
 * this file except in compliance with the License.  You can obtain a copy
 * in the file LICENSE in the source distribution or at
 * https://www.openssl.org/source/license.html
 *
 * Originally written by Mike Hamburg
 */

#ifndef HEADER_FIELD_H
# define HEADER_FIELD_H

# include "internal/constant_time_locl.h"
# include <string.h>
# include <assert.h>
# include "word.h"

# define NLIMBS (64/sizeof(word_t))
# define X_SER_BYTES 56
# define SER_BYTES 56

# if defined(__GNUC__) || defined(__clang__)
#  define INLINE_UNUSED __inline__ __attribute__((__unused__,__always_inline__))
#  define RESTRICT __restrict__
#  define ALIGNED __attribute__((aligned(32)))
# else
#  define INLINE_UNUSED ossl_inline
#  define RESTRICT
#  define ALIGNED
# endif

typedef struct gf_s {
    word_t limb[NLIMBS];
} ALIGNED gf_s, gf[1];

/* RFC 7748 support */
# define X_PUBLIC_BYTES  X_SER_BYTES
# define X_PRIVATE_BYTES X_PUBLIC_BYTES
# define X_PRIVATE_BITS  448

static INLINE_UNUSED void gf_copy(gf out, const gf a)
{
    *out = *a;
}

static INLINE_UNUSED void gf_add_RAW(gf out, const gf a, const gf b);
static INLINE_UNUSED void gf_sub_RAW(gf out, const gf a, const gf b);
static INLINE_UNUSED void gf_bias(gf inout, int amount);
static INLINE_UNUSED void gf_weak_reduce(gf inout);

void gf_strong_reduce(gf inout);
void gf_add(gf out, const gf a, const gf b);
void gf_sub(gf out, const gf a, const gf b);
void gf_mul(gf_s * RESTRICT out, const gf a, const gf b);
void gf_mulw_unsigned(gf_s * RESTRICT out, const gf a, uint32_t b);
void gf_sqr(gf_s * RESTRICT out, const gf a);
mask_t gf_isr(gf a, const gf x); /** a^2 x = 1, QNR, or 0 if x=0.  Return true if successful */
mask_t gf_eq(const gf x, const gf y);
mask_t gf_lobit(const gf x);
mask_t gf_hibit(const gf x);

void gf_serialize(uint8_t *serial, const gf x, int with_highbit);
mask_t gf_deserialize(gf x, const uint8_t serial[SER_BYTES], int with_hibit,
                      uint8_t hi_nmask);

# include "f_impl.h"            /* Bring in the inline implementations */

# ifndef LIMBPERM
#  define LIMBPERM(i) (i)
# endif
# define LIMB_MASK(i) (((1)<<LIMB_PLACE_VALUE(i))-1)

static const gf ZERO = {{{0}}}, ONE = {{{1}}};

/* Square x, n times. */
static ossl_inline void gf_sqrn(gf_s * RESTRICT y, const gf x, int n)
{
    gf tmp;
    assert(n > 0);
    if (n & 1) {
        gf_sqr(y, x);
        n--;
    } else {
        gf_sqr(tmp, x);
        gf_sqr(y, tmp);
        n -= 2;
    }
    for (; n; n -= 2) {
        gf_sqr(tmp, y);
        gf_sqr(y, tmp);
    }
}

# define gf_add_nr gf_add_RAW

/* Subtract mod p.  Bias by 2 and don't reduce  */
static ossl_inline void gf_sub_nr(gf c, const gf a, const gf b)
{
    gf_sub_RAW(c, a, b);
    gf_bias(c, 2);
    if (GF_HEADROOM < 3)
        gf_weak_reduce(c);
}

/* Subtract mod p. Bias by amt but don't reduce.  */
static ossl_inline void gf_subx_nr(gf c, const gf a, const gf b, int amt)
{
    gf_sub_RAW(c, a, b);
    gf_bias(c, amt);
    if (GF_HEADROOM < amt + 1)
        gf_weak_reduce(c);
}

/* Mul by signed int.  Not constant-time WRT the sign of that int. */
static ossl_inline void gf_mulw(gf c, const gf a, int32_t w)
{
    if (w > 0) {
        gf_mulw_unsigned(c, a, w);
    } else {
        gf_mulw_unsigned(c, a, -w);
        gf_sub(c, ZERO, c);
    }
}

/* Constant time, x = is_z ? z : y */
static ossl_inline void gf_cond_sel(gf x, const gf y, const gf z, mask_t is_z)
{
    size_t i;

    for (i = 0; i < NLIMBS; i++) {
#if ARCH_WORD_BITS == 32
        x[0].limb[i] = constant_time_select_32((uint32_t)is_z,
                                               (uint32_t)(z[0].limb[i]),
                                               (uint32_t)(y[0].limb[i]));
#else
        /* Must be 64 bit */
        x[0].limb[i] = constant_time_select_64((uint64_t)is_z,
                                               (uint64_t)(z[0].limb[i]),
                                               (uint64_t)(y[0].limb[i]));
#endif
    }
}

/* Constant time, if (neg) x=-x; */
static ossl_inline void gf_cond_neg(gf x, mask_t neg)
{
    gf y;
    gf_sub(y, ZERO, x);
    gf_cond_sel(x, x, y, neg);
}

/* Constant time, if (swap) (x,y) = (y,x); */
static ossl_inline void gf_cond_swap(gf x, gf_s * RESTRICT y, mask_t swap)
{
    size_t i;

    for (i = 0; i < NLIMBS; i++) {
#if ARCH_WORD_BITS == 32
        constant_time_cond_swap_32((uint32_t)swap, (uint32_t *)&(x[0].limb[i]),
                                   (uint32_t *)&(y->limb[i]));
#else
        /* Must be 64 bit */
        constant_time_cond_swap_64((uint64_t)swap, (uint64_t *)&(x[0].limb[i]),
                                   (uint64_t *)&(y->limb[i]));
#endif
    }
}

#endif                          /* HEADER_FIELD_H */