/* SPDX-License-Identifier: MIT
 *
 * Copyright (C) 2015-2019 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
 *
 * Curve25519 ECDH functions, based on TweetNaCl but cleaned up.
 */

#include <stdint.h>
#include <string.h>
#include <assert.h>
#include <CommonCrypto/CommonRandom.h>

#include "x25519.h"

typedef int64_t fe[16];

static inline void carry(fe o)
{
    int i;

    for (i = 0; i < 16; ++i) {
        o[(i + 1) % 16] += (i == 15 ? 38 : 1) * (o[i] >> 16);
        o[i] &= 0xffff;
    }
}

static inline void cswap(fe p, fe q, int b)
{
    int i;
    int64_t t, c = ~(b - 1);

    for (i = 0; i < 16; ++i) {
        t = c & (p[i] ^ q[i]);
        p[i] ^= t;
        q[i] ^= t;
    }
}

static inline void pack(uint8_t *o, const fe n)
{
    int i, j, b;
    fe m, t;

    memcpy(t, n, sizeof(t));
    carry(t);
    carry(t);
    carry(t);
    for (j = 0; j < 2; ++j) {
        m[0] = t[0] - 0xffed;
        for (i = 1; i < 15; ++i) {
            m[i] = t[i] - 0xffff - ((m[i - 1] >> 16) & 1);
            m[i - 1] &= 0xffff;
        }
        m[15] = t[15] - 0x7fff - ((m[14] >> 16) & 1);
        b = (m[15] >> 16) & 1;
        m[14] &= 0xffff;
        cswap(t, m, 1 - b);
    }
    for (i = 0; i < 16; ++i) {
        o[2 * i] = t[i] & 0xff;
        o[2 * i + 1] = t[i] >> 8;
    }
}

static inline void unpack(fe o, const uint8_t *n)
{
    int i;

    for (i = 0; i < 16; ++i)
        o[i] = n[2 * i] + ((int64_t)n[2 * i + 1] << 8);
    o[15] &= 0x7fff;
}

static inline void add(fe o, const fe a, const fe b)
{
    int i;

    for (i = 0; i < 16; ++i)
        o[i] = a[i] + b[i];
}

static inline void subtract(fe o, const fe a, const fe b)
{
    int i;

    for (i = 0; i < 16; ++i)
        o[i] = a[i] - b[i];
}

static inline void multmod(fe o, const fe a, const fe b)
{
    int i, j;
    int64_t t[31] = { 0 };

    for (i = 0; i < 16; ++i) {
        for (j = 0; j < 16; ++j)
            t[i + j] += a[i] * b[j];
    }
    for (i = 0; i < 15; ++i)
        t[i] += 38 * t[i + 16];
    memcpy(o, t, sizeof(fe));
    carry(o);
    carry(o);
}

static inline void invert(fe o, const fe i)
{
    fe c;
    int a;

    memcpy(c, i, sizeof(c));
    for (a = 253; a >= 0; --a) {
        multmod(c, c, c);
        if (a != 2 && a != 4)
            multmod(c, c, i);
    }
    memcpy(o, c, sizeof(fe));
}

static void curve25519_shared_secret(uint8_t shared_secret[32], const uint8_t private_key[32], const uint8_t public_key[32])
{
    static const fe a24 = { 0xdb41, 1 };
    uint8_t z[32];
    int64_t r;
    int i;
    fe a = { 1 }, b, c = { 0 }, d = { 1 }, e, f, x;

    memcpy(z, private_key, sizeof(z));

    z[31] = (z[31] & 127) | 64;
    z[0] &= 248;

    unpack(x, public_key);
    memcpy(b, x, sizeof(b));

    for (i = 254; i >= 0; --i) {
        r = (z[i >> 3] >> (i & 7)) & 1;
        cswap(a, b, (int)r);
        cswap(c, d, (int)r);
        add(e, a, c);
        subtract(a, a, c);
        add(c, b, d);
        subtract(b, b, d);
        multmod(d, e, e);
        multmod(f, a, a);
        multmod(a, c, a);
        multmod(c, b, e);
        add(e, a, c);
        subtract(a, a, c);
        multmod(b, a, a);
        subtract(c, d, f);
        multmod(a, c, a24);
        add(a, a, d);
        multmod(c, c, a);
        multmod(a, d, f);
        multmod(d, b, x);
        multmod(b, e, e);
        cswap(a, b, (int)r);
        cswap(c, d, (int)r);
    }
    invert(c, c);
    multmod(a, a, c);
    pack(shared_secret, a);
}

void curve25519_derive_public_key(uint8_t public_key[32], const uint8_t private_key[32])
{
    static const uint8_t basepoint[32] = { 9 };

    curve25519_shared_secret(public_key, private_key, basepoint);
}

void curve25519_generate_private_key(uint8_t private_key[32])
{
    assert(CCRandomGenerateBytes(private_key, 32) == kCCSuccess);
    private_key[31] = (private_key[31] & 127) | 64;
    private_key[0] &= 248;
}