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/* SPDX-License-Identifier: GPL-2.0
*
* Copyright (C) 2008 Google Inc. All Rights Reserved.
* Copyright (C) 2015-2018 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
*
* Original author: Adam Langley <agl@imperialviolet.org>
*/
typedef u64 limb;
typedef limb felem[5];
typedef __uint128_t u128;
/* Sum two numbers: output += in */
static __always_inline void fsum(limb *output, const limb *in)
{
output[0] += in[0];
output[1] += in[1];
output[2] += in[2];
output[3] += in[3];
output[4] += in[4];
}
/* Find the difference of two numbers: output = in - output
* (note the order of the arguments!)
*
* Assumes that out[i] < 2**52
* On return, out[i] < 2**55
*/
static __always_inline void fdifference_backwards(felem out, const felem in)
{
/* 152 is 19 << 3 */
static const limb two54m152 = (((limb)1) << 54) - 152;
static const limb two54m8 = (((limb)1) << 54) - 8;
out[0] = in[0] + two54m152 - out[0];
out[1] = in[1] + two54m8 - out[1];
out[2] = in[2] + two54m8 - out[2];
out[3] = in[3] + two54m8 - out[3];
out[4] = in[4] + two54m8 - out[4];
}
/* Multiply a number by a scalar: output = in * scalar */
static __always_inline void fscalar_product(felem output, const felem in, const limb scalar)
{
u128 a;
a = ((u128) in[0]) * scalar;
output[0] = ((limb)a) & 0x7ffffffffffffUL;
a = ((u128) in[1]) * scalar + ((limb) (a >> 51));
output[1] = ((limb)a) & 0x7ffffffffffffUL;
a = ((u128) in[2]) * scalar + ((limb) (a >> 51));
output[2] = ((limb)a) & 0x7ffffffffffffUL;
a = ((u128) in[3]) * scalar + ((limb) (a >> 51));
output[3] = ((limb)a) & 0x7ffffffffffffUL;
a = ((u128) in[4]) * scalar + ((limb) (a >> 51));
output[4] = ((limb)a) & 0x7ffffffffffffUL;
output[0] += (a >> 51) * 19;
}
/* Multiply two numbers: output = in2 * in
*
* output must be distinct to both inputs. The inputs are reduced coefficient
* form, the output is not.
*
* Assumes that in[i] < 2**55 and likewise for in2.
* On return, output[i] < 2**52
*/
static __always_inline void fmul(felem output, const felem in2, const felem in)
{
u128 t[5];
limb r0, r1, r2, r3, r4, s0, s1, s2, s3, s4, c;
r0 = in[0];
r1 = in[1];
r2 = in[2];
r3 = in[3];
r4 = in[4];
s0 = in2[0];
s1 = in2[1];
s2 = in2[2];
s3 = in2[3];
s4 = in2[4];
t[0] = ((u128) r0) * s0;
t[1] = ((u128) r0) * s1 + ((u128) r1) * s0;
t[2] = ((u128) r0) * s2 + ((u128) r2) * s0 + ((u128) r1) * s1;
t[3] = ((u128) r0) * s3 + ((u128) r3) * s0 + ((u128) r1) * s2 + ((u128) r2) * s1;
t[4] = ((u128) r0) * s4 + ((u128) r4) * s0 + ((u128) r3) * s1 + ((u128) r1) * s3 + ((u128) r2) * s2;
r4 *= 19;
r1 *= 19;
r2 *= 19;
r3 *= 19;
t[0] += ((u128) r4) * s1 + ((u128) r1) * s4 + ((u128) r2) * s3 + ((u128) r3) * s2;
t[1] += ((u128) r4) * s2 + ((u128) r2) * s4 + ((u128) r3) * s3;
t[2] += ((u128) r4) * s3 + ((u128) r3) * s4;
t[3] += ((u128) r4) * s4;
r0 = (limb)t[0] & 0x7ffffffffffffUL; c = (limb)(t[0] >> 51);
t[1] += c; r1 = (limb)t[1] & 0x7ffffffffffffUL; c = (limb)(t[1] >> 51);
t[2] += c; r2 = (limb)t[2] & 0x7ffffffffffffUL; c = (limb)(t[2] >> 51);
t[3] += c; r3 = (limb)t[3] & 0x7ffffffffffffUL; c = (limb)(t[3] >> 51);
t[4] += c; r4 = (limb)t[4] & 0x7ffffffffffffUL; c = (limb)(t[4] >> 51);
r0 += c * 19; c = r0 >> 51; r0 = r0 & 0x7ffffffffffffUL;
r1 += c; c = r1 >> 51; r1 = r1 & 0x7ffffffffffffUL;
r2 += c;
output[0] = r0;
output[1] = r1;
output[2] = r2;
output[3] = r3;
output[4] = r4;
}
static __always_inline void fsquare_times(felem output, const felem in, limb count)
{
u128 t[5];
limb r0, r1, r2, r3, r4, c;
limb d0, d1, d2, d4, d419;
r0 = in[0];
r1 = in[1];
r2 = in[2];
r3 = in[3];
r4 = in[4];
do {
d0 = r0 * 2;
d1 = r1 * 2;
d2 = r2 * 2 * 19;
d419 = r4 * 19;
d4 = d419 * 2;
t[0] = ((u128) r0) * r0 + ((u128) d4) * r1 + (((u128) d2) * (r3 ));
t[1] = ((u128) d0) * r1 + ((u128) d4) * r2 + (((u128) r3) * (r3 * 19));
t[2] = ((u128) d0) * r2 + ((u128) r1) * r1 + (((u128) d4) * (r3 ));
t[3] = ((u128) d0) * r3 + ((u128) d1) * r2 + (((u128) r4) * (d419 ));
t[4] = ((u128) d0) * r4 + ((u128) d1) * r3 + (((u128) r2) * (r2 ));
r0 = (limb)t[0] & 0x7ffffffffffffUL; c = (limb)(t[0] >> 51);
t[1] += c; r1 = (limb)t[1] & 0x7ffffffffffffUL; c = (limb)(t[1] >> 51);
t[2] += c; r2 = (limb)t[2] & 0x7ffffffffffffUL; c = (limb)(t[2] >> 51);
t[3] += c; r3 = (limb)t[3] & 0x7ffffffffffffUL; c = (limb)(t[3] >> 51);
t[4] += c; r4 = (limb)t[4] & 0x7ffffffffffffUL; c = (limb)(t[4] >> 51);
r0 += c * 19; c = r0 >> 51; r0 = r0 & 0x7ffffffffffffUL;
r1 += c; c = r1 >> 51; r1 = r1 & 0x7ffffffffffffUL;
r2 += c;
} while (--count);
output[0] = r0;
output[1] = r1;
output[2] = r2;
output[3] = r3;
output[4] = r4;
}
/* Load a little-endian 64-bit number */
static inline limb load_limb(const u8 *in)
{
return le64_to_cpu(*(__le64 *)in);
}
static inline void store_limb(u8 *out, limb in)
{
*(__le64 *)out = cpu_to_le64(in);
}
/* Take a little-endian, 32-byte number and expand it into polynomial form */
static inline void fexpand(limb *output, const u8 *in)
{
output[0] = load_limb(in) & 0x7ffffffffffffUL;
output[1] = (load_limb(in + 6) >> 3) & 0x7ffffffffffffUL;
output[2] = (load_limb(in + 12) >> 6) & 0x7ffffffffffffUL;
output[3] = (load_limb(in + 19) >> 1) & 0x7ffffffffffffUL;
output[4] = (load_limb(in + 24) >> 12) & 0x7ffffffffffffUL;
}
/* Take a fully reduced polynomial form number and contract it into a
* little-endian, 32-byte array
*/
static void fcontract(u8 *output, const felem input)
{
u128 t[5];
t[0] = input[0];
t[1] = input[1];
t[2] = input[2];
t[3] = input[3];
t[4] = input[4];
t[1] += t[0] >> 51; t[0] &= 0x7ffffffffffffUL;
t[2] += t[1] >> 51; t[1] &= 0x7ffffffffffffUL;
t[3] += t[2] >> 51; t[2] &= 0x7ffffffffffffUL;
t[4] += t[3] >> 51; t[3] &= 0x7ffffffffffffUL;
t[0] += 19 * (t[4] >> 51); t[4] &= 0x7ffffffffffffUL;
t[1] += t[0] >> 51; t[0] &= 0x7ffffffffffffUL;
t[2] += t[1] >> 51; t[1] &= 0x7ffffffffffffUL;
t[3] += t[2] >> 51; t[2] &= 0x7ffffffffffffUL;
t[4] += t[3] >> 51; t[3] &= 0x7ffffffffffffUL;
t[0] += 19 * (t[4] >> 51); t[4] &= 0x7ffffffffffffUL;
/* now t is between 0 and 2^255-1, properly carried. */
/* case 1: between 0 and 2^255-20. case 2: between 2^255-19 and 2^255-1. */
t[0] += 19;
t[1] += t[0] >> 51; t[0] &= 0x7ffffffffffffUL;
t[2] += t[1] >> 51; t[1] &= 0x7ffffffffffffUL;
t[3] += t[2] >> 51; t[2] &= 0x7ffffffffffffUL;
t[4] += t[3] >> 51; t[3] &= 0x7ffffffffffffUL;
t[0] += 19 * (t[4] >> 51); t[4] &= 0x7ffffffffffffUL;
/* now between 19 and 2^255-1 in both cases, and offset by 19. */
t[0] += 0x8000000000000UL - 19;
t[1] += 0x8000000000000UL - 1;
t[2] += 0x8000000000000UL - 1;
t[3] += 0x8000000000000UL - 1;
t[4] += 0x8000000000000UL - 1;
/* now between 2^255 and 2^256-20, and offset by 2^255. */
t[1] += t[0] >> 51; t[0] &= 0x7ffffffffffffUL;
t[2] += t[1] >> 51; t[1] &= 0x7ffffffffffffUL;
t[3] += t[2] >> 51; t[2] &= 0x7ffffffffffffUL;
t[4] += t[3] >> 51; t[3] &= 0x7ffffffffffffUL;
t[4] &= 0x7ffffffffffffUL;
store_limb(output, t[0] | (t[1] << 51));
store_limb(output+8, (t[1] >> 13) | (t[2] << 38));
store_limb(output+16, (t[2] >> 26) | (t[3] << 25));
store_limb(output+24, (t[3] >> 39) | (t[4] << 12));
}
/* Input: Q, Q', Q-Q'
* Output: 2Q, Q+Q'
*
* x2 z3: long form
* x3 z3: long form
* x z: short form, destroyed
* xprime zprime: short form, destroyed
* qmqp: short form, preserved
*/
static void fmonty(limb *x2, limb *z2, /* output 2Q */
limb *x3, limb *z3, /* output Q + Q' */
limb *x, limb *z, /* input Q */
limb *xprime, limb *zprime, /* input Q' */
const limb *qmqp /* input Q - Q' */)
{
limb origx[5], origxprime[5], zzz[5], xx[5], zz[5], xxprime[5], zzprime[5], zzzprime[5];
memcpy(origx, x, 5 * sizeof(limb));
fsum(x, z);
fdifference_backwards(z, origx); // does x - z
memcpy(origxprime, xprime, sizeof(limb) * 5);
fsum(xprime, zprime);
fdifference_backwards(zprime, origxprime);
fmul(xxprime, xprime, z);
fmul(zzprime, x, zprime);
memcpy(origxprime, xxprime, sizeof(limb) * 5);
fsum(xxprime, zzprime);
fdifference_backwards(zzprime, origxprime);
fsquare_times(x3, xxprime, 1);
fsquare_times(zzzprime, zzprime, 1);
fmul(z3, zzzprime, qmqp);
fsquare_times(xx, x, 1);
fsquare_times(zz, z, 1);
fmul(x2, xx, zz);
fdifference_backwards(zz, xx); // does zz = xx - zz
fscalar_product(zzz, zz, 121665);
fsum(zzz, xx);
fmul(z2, zz, zzz);
}
/* Maybe swap the contents of two limb arrays (@a and @b), each @len elements
* long. Perform the swap iff @swap is non-zero.
*
* This function performs the swap without leaking any side-channel
* information.
*/
static void swap_conditional(limb a[5], limb b[5], limb iswap)
{
unsigned int i;
const limb swap = -iswap;
for (i = 0; i < 5; ++i) {
const limb x = swap & (a[i] ^ b[i]);
a[i] ^= x;
b[i] ^= x;
}
}
/* Calculates nQ where Q is the x-coordinate of a point on the curve
*
* resultx/resultz: the x coordinate of the resulting curve point (short form)
* n: a little endian, 32-byte number
* q: a point of the curve (short form)
*/
static void cmult(limb *resultx, limb *resultz, const u8 *n, const limb *q)
{
limb a[5] = {0}, b[5] = {1}, c[5] = {1}, d[5] = {0};
limb *nqpqx = a, *nqpqz = b, *nqx = c, *nqz = d, *t;
limb e[5] = {0}, f[5] = {1}, g[5] = {0}, h[5] = {1};
limb *nqpqx2 = e, *nqpqz2 = f, *nqx2 = g, *nqz2 = h;
unsigned int i, j;
memcpy(nqpqx, q, sizeof(limb) * 5);
for (i = 0; i < 32; ++i) {
u8 byte = n[31 - i];
for (j = 0; j < 8; ++j) {
const limb bit = byte >> 7;
swap_conditional(nqx, nqpqx, bit);
swap_conditional(nqz, nqpqz, bit);
fmonty(nqx2, nqz2,
nqpqx2, nqpqz2,
nqx, nqz,
nqpqx, nqpqz,
q);
swap_conditional(nqx2, nqpqx2, bit);
swap_conditional(nqz2, nqpqz2, bit);
t = nqx;
nqx = nqx2;
nqx2 = t;
t = nqz;
nqz = nqz2;
nqz2 = t;
t = nqpqx;
nqpqx = nqpqx2;
nqpqx2 = t;
t = nqpqz;
nqpqz = nqpqz2;
nqpqz2 = t;
byte <<= 1;
}
}
memcpy(resultx, nqx, sizeof(limb) * 5);
memcpy(resultz, nqz, sizeof(limb) * 5);
}
static void crecip(felem out, const felem z)
{
felem a, t0, b, c;
/* 2 */ fsquare_times(a, z, 1); // a = 2
/* 8 */ fsquare_times(t0, a, 2);
/* 9 */ fmul(b, t0, z); // b = 9
/* 11 */ fmul(a, b, a); // a = 11
/* 22 */ fsquare_times(t0, a, 1);
/* 2^5 - 2^0 = 31 */ fmul(b, t0, b);
/* 2^10 - 2^5 */ fsquare_times(t0, b, 5);
/* 2^10 - 2^0 */ fmul(b, t0, b);
/* 2^20 - 2^10 */ fsquare_times(t0, b, 10);
/* 2^20 - 2^0 */ fmul(c, t0, b);
/* 2^40 - 2^20 */ fsquare_times(t0, c, 20);
/* 2^40 - 2^0 */ fmul(t0, t0, c);
/* 2^50 - 2^10 */ fsquare_times(t0, t0, 10);
/* 2^50 - 2^0 */ fmul(b, t0, b);
/* 2^100 - 2^50 */ fsquare_times(t0, b, 50);
/* 2^100 - 2^0 */ fmul(c, t0, b);
/* 2^200 - 2^100 */ fsquare_times(t0, c, 100);
/* 2^200 - 2^0 */ fmul(t0, t0, c);
/* 2^250 - 2^50 */ fsquare_times(t0, t0, 50);
/* 2^250 - 2^0 */ fmul(t0, t0, b);
/* 2^255 - 2^5 */ fsquare_times(t0, t0, 5);
/* 2^255 - 21 */ fmul(out, t0, a);
}
static bool curve25519_donna(u8 mypublic[CURVE25519_POINT_SIZE], const u8 secret[CURVE25519_POINT_SIZE], const u8 basepoint[CURVE25519_POINT_SIZE])
{
limb bp[5], x[5], z[5], zmone[5];
u8 e[32];
memcpy(e, secret, 32);
normalize_secret(e);
fexpand(bp, basepoint);
cmult(x, z, e, bp);
crecip(zmone, z);
fmul(z, x, zmone);
fcontract(mypublic, z);
memzero_explicit(e, sizeof(e));
memzero_explicit(bp, sizeof(bp));
memzero_explicit(x, sizeof(x));
memzero_explicit(z, sizeof(z));
memzero_explicit(zmone, sizeof(zmone));
return true;
}
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