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/*
* Copyright (c) 2013-2020, The PurpleI2P Project
*
* This file is part of Purple i2pd project and licensed under BSD3
*
* See full license text in LICENSE file at top of project tree
*/
#include <openssl/rand.h>
#include "Crypto.h"
#include "Elligator.h"
namespace i2p
{
namespace crypto
{
Elligator2::Elligator2 ()
{
// TODO: share with Ed22519
p = BN_new ();
// 2^255-19
BN_set_bit (p, 255); // 2^255
BN_sub_word (p, 19);
p38 = BN_dup (p); BN_add_word (p38, 3); BN_div_word (p38, 8); // (p+3)/8
p12 = BN_dup (p); BN_sub_word (p12, 1); BN_div_word (p12, 2); // (p-1)/2
p14 = BN_dup (p); BN_sub_word (p14, 1); BN_div_word (p14, 4); // (p-1)/4
A = BN_new (); BN_set_word (A, 486662);
nA = BN_new (); BN_sub (nA, p, A);
BN_CTX * ctx = BN_CTX_new ();
// calculate sqrt(-1)
sqrtn1 = BN_new ();
BN_set_word (sqrtn1, 2);
BN_mod_exp (sqrtn1, sqrtn1, p14, p, ctx); // 2^((p-1)/4
u = BN_new (); BN_set_word (u, 2);
iu = BN_new (); BN_mod_inverse (iu, u, p, ctx);
BN_CTX_free (ctx);
}
Elligator2::~Elligator2 ()
{
BN_free (p); BN_free (p38); BN_free (p12); BN_free (p14);
BN_free (sqrtn1); BN_free (A); BN_free (nA);
BN_free (u); BN_free (iu);
}
bool Elligator2::Encode (const uint8_t * key, uint8_t * encoded, bool highY, bool random) const
{
bool ret = true;
BN_CTX * ctx = BN_CTX_new ();
BN_CTX_start (ctx);
uint8_t key1[32];
for (size_t i = 0; i < 16; i++) // from Little Endian
{
key1[i] = key[31 - i];
key1[31 - i] = key[i];
}
BIGNUM * x = BN_CTX_get (ctx); BN_bin2bn (key1, 32, x);
BIGNUM * xA = BN_CTX_get (ctx); BN_add (xA, x, A); // x + A
BN_sub (xA, p, xA); // p - (x + A)
BIGNUM * uxxA = BN_CTX_get (ctx); // u*x*xA
BN_mod_mul (uxxA, u, x, p, ctx);
BN_mod_mul (uxxA, uxxA, xA, p, ctx);
if (Legendre (uxxA, ctx) != -1)
{
uint8_t randByte = 0; // random highest bits and high y
if (random)
{
RAND_bytes (&randByte, 1);
highY = randByte & 0x01;
}
BIGNUM * r = BN_CTX_get (ctx);
if (highY)
{
BN_mod_inverse (r, x, p, ctx);
BN_mod_mul (r, r, xA, p, ctx);
}
else
{
BN_mod_inverse (r, xA, p, ctx);
BN_mod_mul (r, r, x, p, ctx);
}
BN_mod_mul (r, r, iu, p, ctx);
SquareRoot (r, r, ctx);
bn2buf (r, encoded, 32);
if (random)
encoded[0] |= (randByte & 0xC0); // copy two highest bits from randByte
for (size_t i = 0; i < 16; i++) // To Little Endian
{
uint8_t tmp = encoded[i];
encoded[i] = encoded[31 - i];
encoded[31 - i] = tmp;
}
}
else
ret = false;
BN_CTX_end (ctx);
BN_CTX_free (ctx);
return ret;
}
bool Elligator2::Decode (const uint8_t * encoded, uint8_t * key) const
{
bool ret = true;
BN_CTX * ctx = BN_CTX_new ();
BN_CTX_start (ctx);
uint8_t encoded1[32];
for (size_t i = 0; i < 16; i++) // from Little Endian
{
encoded1[i] = encoded[31 - i];
encoded1[31 - i] = encoded[i];
}
encoded1[0] &= 0x3F; // drop two highest bits
BIGNUM * r = BN_CTX_get (ctx); BN_bin2bn (encoded1, 32, r);
if (BN_cmp (r, p12) <= 0) // r < (p-1)/2
{
// v = -A/(1+u*r^2)
BIGNUM * v = BN_CTX_get (ctx); BN_mod_sqr (v, r, p, ctx);
BN_mod_mul (v, v, u, p, ctx);
BN_add_word (v, 1);
BN_mod_inverse (v, v, p, ctx);
BN_mod_mul (v, v, nA, p, ctx);
BIGNUM * vpA = BN_CTX_get (ctx);
BN_add (vpA, v, A); // v + A
// t = v^3+A*v^2+v = v^2*(v+A)+v
BIGNUM * t = BN_CTX_get (ctx); BN_mod_sqr (t, v, p, ctx);
BN_mod_mul (t, t, vpA, p, ctx);
BN_mod_add (t, t, v, p, ctx);
int legendre = Legendre (t, ctx);
BIGNUM * x = BN_CTX_get (ctx);
if (legendre == 1)
BN_copy (x, v);
else
{
BN_sub (x, p, v);
BN_mod_sub (x, x, A, p, ctx);
}
bn2buf (x, key, 32);
for (size_t i = 0; i < 16; i++) // To Little Endian
{
uint8_t tmp = key[i];
key[i] = key[31 - i];
key[31 - i] = tmp;
}
}
else
ret = false;
BN_CTX_end (ctx);
BN_CTX_free (ctx);
return ret;
}
void Elligator2::SquareRoot (const BIGNUM * x, BIGNUM * r, BN_CTX * ctx) const
{
BIGNUM * t = BN_CTX_get (ctx);
BN_mod_exp (t, x, p14, p, ctx); // t = x^((p-1)/4)
BN_mod_exp (r, x, p38, p, ctx); // r = x^((p+3)/8)
BN_add_word (t, 1);
if (!BN_cmp (t, p))
BN_mod_mul (r, r, sqrtn1, p, ctx);
if (BN_cmp (r, p12) > 0) // r > (p-1)/2
BN_sub (r, p, r);
}
int Elligator2::Legendre (const BIGNUM * a, BN_CTX * ctx) const
{
// assume a < p, so don't check for a % p = 0, but a = 0 only
if (BN_is_zero(a)) return 0;
BIGNUM * r = BN_CTX_get (ctx);
BN_mod_exp (r, a, p12, p, ctx); // r = a^((p-1)/2) mod p
if (BN_is_word(r, 1))
return 1;
else if (BN_is_zero(r))
return 0;
return -1;
}
static std::unique_ptr<Elligator2> g_Elligator;
std::unique_ptr<Elligator2>& GetElligator ()
{
if (!g_Elligator)
{
auto el = new Elligator2();
if (!g_Elligator) // make sure it was not created already
g_Elligator.reset (el);
else
delete el;
}
return g_Elligator;
}
}
}
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