ed25519.c

/*
	Public domain by Andrew M. <liquidsun@gmail.com>

	Ed25519 reference implementation using Ed25519-donna
*/


/* define ED25519_SUFFIX to have it appended to the end of each public function */
#if !defined(ED25519_SUFFIX)
#define ED25519_SUFFIX 
#endif

#define ED25519_FN3(fn,suffix) fn##suffix
#define ED25519_FN2(fn,suffix) ED25519_FN3(fn,suffix)
#define ED25519_FN(fn)         ED25519_FN2(fn,ED25519_SUFFIX)

#include "ed25519-donna.h"
#include "ed25519.h"
#include "ed25519-randombytes.h"
#include "ed25519-hash.h"

/*
	Generates a (extsk[0..31]) and aExt (extsk[32..63])
*/

DONNA_INLINE static void
ed25519_extsk(hash_512bits extsk, const ed25519_secret_key sk) {
	ed25519_hash(extsk, sk, 32);
	extsk[0] &= 248;
	extsk[31] &= 127;
	extsk[31] |= 64;
}

static void
ed25519_hram(hash_512bits hram, const ed25519_signature RS, const ed25519_public_key pk, const unsigned char *m, size_t mlen) {
	ed25519_hash_context ctx;
	ed25519_hash_init(&ctx);
	ed25519_hash_update(&ctx, RS, 32);
	ed25519_hash_update(&ctx, pk, 32);
	ed25519_hash_update(&ctx, m, mlen);
	ed25519_hash_final(&ctx, hram);
}

void
ED25519_FN(ed25519_publickey) (const ed25519_secret_key sk, ed25519_public_key pk) {
	bignum256modm a;
	ge25519 ALIGN(16) A;
	hash_512bits extsk;

	/* A = aB */
	ed25519_extsk(extsk, sk);
	expand256_modm(a, extsk, 32);
	ge25519_scalarmult_base_niels(&A, ge25519_niels_base_multiples, a);
	ge25519_pack(pk, &A);
}


void
ED25519_FN(ed25519_sign) (const unsigned char *m, size_t mlen, const ed25519_secret_key sk, const ed25519_public_key pk, ed25519_signature RS) {
	ed25519_hash_context ctx;
	bignum256modm r, S, a;
	ge25519 ALIGN(16) R;
	hash_512bits extsk, hashr, hram;

	ed25519_extsk(extsk, sk);

	/* r = H(aExt[32..64], m) */
	ed25519_hash_init(&ctx);
	ed25519_hash_update(&ctx, extsk + 32, 32);
	ed25519_hash_update(&ctx, m, mlen);
	ed25519_hash_final(&ctx, hashr);
	expand256_modm(r, hashr, 64);

	/* R = rB */
	ge25519_scalarmult_base_niels(&R, ge25519_niels_base_multiples, r);
	ge25519_pack(RS, &R);

	/* S = H(R,A,m).. */
	ed25519_hram(hram, RS, pk, m, mlen);
	expand256_modm(S, hram, 64);

	/* S = H(R,A,m)a */
	expand256_modm(a, extsk, 32);
	mul256_modm(S, S, a);

	/* S = (r + H(R,A,m)a) */
	add256_modm(S, S, r);

	/* S = (r + H(R,A,m)a) mod L */	
	contract256_modm(RS + 32, S);
}

int
ED25519_FN(ed25519_sign_open) (const unsigned char *m, size_t mlen, const ed25519_public_key pk, const ed25519_signature RS) {
	ge25519 ALIGN(16) R, A;
	hash_512bits hash;
	bignum256modm hram, S;
	unsigned char checkR[32];

	if ((RS[63] & 224) || !ge25519_unpack_negative_vartime(&A, pk))
		return -1;

	/* hram = H(R,A,m) */
	ed25519_hram(hash, RS, pk, m, mlen);
	expand256_modm(hram, hash, 64);

	/* S */
	expand256_modm(S, RS + 32, 32);

	/* SB - H(R,A,m)A */
	ge25519_double_scalarmult_vartime(&R, &A, hram, S);
	ge25519_pack(checkR, &R);

	/* check that R = SB - H(R,A,m)A */
	return ed25519_verify(RS, checkR, 32) ? 0 : -1;
}

#include "ed25519-donna-batchverify.h"

/*
	Fast Curve25519 basepoint scalar multiplication
*/

void
ED25519_FN(curved25519_scalarmult_basepoint) (curved25519_key pk, const curved25519_key e) {
	curved25519_key ec;
	bignum256modm s;
	bignum25519 ALIGN(16) yplusz, zminusy;
	ge25519 ALIGN(16) p;
	size_t i;

	/* clamp */
	for (i = 0; i < 32; i++) ec[i] = e[i];
	ec[0] &= 248;
	ec[31] &= 127;
	ec[31] |= 64;

	expand_raw256_modm(s, ec);

	/* scalar * basepoint */
	ge25519_scalarmult_base_niels(&p, ge25519_niels_base_multiples, s);

	/* u = (y + z) / (z - y) */
	curve25519_add(yplusz, p.y, p.z);
	curve25519_sub(zminusy, p.z, p.y);
	curve25519_recip(zminusy, zminusy);
	curve25519_mul(yplusz, yplusz, zminusy);
	curve25519_contract(pk, yplusz);
}