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/* SPDX-License-Identifier: GPL-2.0
*
* Copyright (C) 2015-2018 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
*
* Curve25519 ECDH functions, based on TweetNaCl but cleaned up.
*/
#include <linux/kernel.h>
#include <linux/string.h>
enum { CURVE25519_POINT_SIZE = 32 };
static __always_inline void normalize_secret(u8 secret[CURVE25519_POINT_SIZE])
{
secret[0] &= 248;
secret[31] &= 127;
secret[31] |= 64;
}
typedef s64 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;
s64 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(u8 *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 u8 *n)
{
int i;
for (i = 0; i < 16; ++i)
o[i] = n[2 * i] + ((s64)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;
s64 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));
}
bool curve25519_tweetnacl(u8 shared_secret[CURVE25519_POINT_SIZE], const u8 private_key[CURVE25519_POINT_SIZE], const u8 public_key[CURVE25519_POINT_SIZE])
{
static const fe a24 = { 0xdb41, 1 };
u8 z[32];
s64 r;
int i;
fe a = { 1 }, b, c = { 0 }, d = { 1 }, e, f, x;
memcpy(z, private_key, sizeof(z));
normalize_secret(z);
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, r);
cswap(c, d, 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, r);
cswap(c, d, r);
}
invert(c, c);
multmod(a, a, c);
pack(shared_secret, a);
return true;
}
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