int fe_isnonzero(const fe f)
{
unsigned char s[32];
fe_tobytes(s,f);
return crypto_verify_32(s,zero);
}
void ed25519_key_exchange(unsigned char *shared_secret, const unsigned char *public_key, const unsigned char *private_key) {
unsigned char e[32];
unsigned int i;
fe x1;
fe x2;
fe z2;
fe x3;
fe z3;
fe tmp0;
fe tmp1;
int pos;
unsigned int swap;
unsigned int b;
/* copy the private key and make sure it's valid */
for (i = 0; i < 32; ++i) {
e[i] = private_key[i];
}
e[0] &= 248;
e[31] &= 63;
e[31] |= 64;
/* unpack the public key and convert edwards to montgomery */
/* due to CodesInChaos: montgomeryX = (edwardsY + 1)*inverse(1 - edwardsY) mod p */
fe_frombytes(x1, public_key);
fe_1(tmp1);
fe_add(tmp0, x1, tmp1);
fe_sub(tmp1, tmp1, x1);
fe_invert(tmp1, tmp1);
fe_mul(x1, tmp0, tmp1);
fe_1(x2);
fe_0(z2);
fe_copy(x3, x1);
fe_1(z3);
swap = 0;
for (pos = 254; pos >= 0; --pos) {
b = e[pos / 8] >> (pos & 7);
b &= 1;
swap ^= b;
fe_cswap(x2, x3, swap);
fe_cswap(z2, z3, swap);
swap = b;
/* from montgomery.h */
fe_sub(tmp0, x3, z3);
fe_sub(tmp1, x2, z2);
fe_add(x2, x2, z2);
fe_add(z2, x3, z3);
fe_mul(z3, tmp0, x2);
fe_mul(z2, z2, tmp1);
fe_sq(tmp0, tmp1);
fe_sq(tmp1, x2);
fe_add(x3, z3, z2);
fe_sub(z2, z3, z2);
fe_mul(x2, tmp1, tmp0);
fe_sub(tmp1, tmp1, tmp0);
fe_sq(z2, z2);
fe_mul121666(z3, tmp1);
fe_sq(x3, x3);
fe_add(tmp0, tmp0, z3);
fe_mul(z3, x1, z2);
fe_mul(z2, tmp1, tmp0);
}
fe_cswap(x2, x3, swap);
fe_cswap(z2, z3, swap);
fe_invert(z2, z2);
fe_mul(x2, x2, z2);
fe_tobytes(shared_secret, x2);
}
int curve25519_verify(const unsigned char* signature,
const unsigned char* curve25519_pubkey,
const unsigned char* msg, const unsigned long msg_len)
{
fe mont_x, mont_x_minus_one, mont_x_plus_one, inv_mont_x_plus_one;
fe one;
fe ed_y;
unsigned char ed_pubkey[32];
unsigned long long some_retval;
unsigned char *verifybuf = NULL; /* working buffer */
unsigned char *verifybuf2 = NULL; /* working buffer #2 */
int result;
if ((verifybuf = malloc(msg_len + 64)) == 0) {
result = -1;
goto err;
}
if ((verifybuf2 = malloc(msg_len + 64)) == 0) {
result = -1;
goto err;
}
/* Convert the Curve25519 public key into an Ed25519 public key. In
particular, convert Curve25519's "montgomery" x-coordinate into an
Ed25519 "edwards" y-coordinate:
ed_y = (mont_x - 1) / (mont_x + 1)
NOTE: mont_x=-1 is converted to ed_y=0 since fe_invert is mod-exp
Then move the sign bit into the pubkey from the signature.
*/
fe_frombytes(mont_x, curve25519_pubkey);
fe_1(one);
fe_sub(mont_x_minus_one, mont_x, one);
fe_add(mont_x_plus_one, mont_x, one);
fe_invert(inv_mont_x_plus_one, mont_x_plus_one);
fe_mul(ed_y, mont_x_minus_one, inv_mont_x_plus_one);
fe_tobytes(ed_pubkey, ed_y);
/* Copy the sign bit, and remove it from signature */
ed_pubkey[31] &= 0x7F; /* bit should be zero already, but just in case */
ed_pubkey[31] |= (signature[63] & 0x80);
memmove(verifybuf, signature, 64);
verifybuf[63] &= 0x7F;
memmove(verifybuf+64, msg, msg_len);
/* Then perform a normal Ed25519 verification, return 0 on success */
/* The below call has a strange API: */
/* verifybuf = R || S || message */
/* verifybuf2 = internal to next call gets a copy of verifybuf, S gets
replaced with pubkey for hashing, then the whole thing gets zeroized
(if bad sig), or contains a copy of msg (good sig) */
result = crypto_sign_open(verifybuf2, &some_retval, verifybuf, 64 + msg_len, ed_pubkey);
err:
if (verifybuf != NULL) {
free(verifybuf);
}
if (verifybuf2 != NULL) {
free(verifybuf2);
}
return result;
}