diff --git a/ecdsa.c b/ecdsa.c
index 310dad325..81dc2a82f 100644
--- a/ecdsa.c
+++ b/ecdsa.c
@@ -658,7 +658,7 @@ int ecdh_multiply(const ecdsa_curve *curve, const uint8_t *priv_key, const uint8
 	return 0;
 }
 
-void init_k_rfc6979(const uint8_t *priv_key, const uint8_t *hash, rfc6979_state *state) {
+void init_rfc6979(const uint8_t *priv_key, const uint8_t *hash, rfc6979_state *state) {
 	uint8_t bx[2*32];
 	uint8_t buf[32 + 1 + 2*32];
 
@@ -684,18 +684,27 @@ void init_k_rfc6979(const uint8_t *priv_key, const uint8_t *hash, rfc6979_state
 	MEMSET_BZERO(buf, sizeof(buf));
 }
 
-// generate K in a deterministic way, according to RFC6979
-// http://tools.ietf.org/html/rfc6979
-void generate_k_rfc6979(bignum256 *k, rfc6979_state *state)
+// generate next number from deterministic random number generator
+void generate_rfc6979(uint8_t rand[32], rfc6979_state *state)
 {
 	uint8_t buf[32 + 1];
 
 	hmac_sha256(state->k, sizeof(state->k), state->v, sizeof(state->v), state->v);
-	bn_read_be(state->v, k);
 	memcpy(buf, state->v, sizeof(state->v));
 	buf[sizeof(state->v)] = 0x00;
 	hmac_sha256(state->k, sizeof(state->k), buf, sizeof(state->v) + 1, state->k);
 	hmac_sha256(state->k, sizeof(state->k), state->v, sizeof(state->v), state->v);
+	memcpy(rand, buf, 32);
+	MEMSET_BZERO(buf, sizeof(buf));
+}
+
+// generate K in a deterministic way, according to RFC6979
+// http://tools.ietf.org/html/rfc6979
+void generate_k_rfc6979(bignum256 *k, rfc6979_state *state)
+{
+	uint8_t buf[32];
+	generate_rfc6979(buf, state);
+	bn_read_be(buf, k);
 	MEMSET_BZERO(buf, sizeof(buf));
 }
 
@@ -739,7 +748,7 @@ int ecdsa_sign_digest(const ecdsa_curve *curve, const uint8_t *priv_key, const u
 
 #if USE_RFC6979
 	rfc6979_state rng;
-	init_k_rfc6979(priv_key, digest, &rng);
+	init_rfc6979(priv_key, digest, &rng);
 #endif
 
 	bn_read_be(digest, &z);
diff --git a/ecdsa.h b/ecdsa.h
index 163558a7d..596ec1ec6 100644
--- a/ecdsa.h
+++ b/ecdsa.h
@@ -91,7 +91,8 @@ int ecdsa_verify_digest_recover(const ecdsa_curve *curve, uint8_t *pub_key, cons
 int ecdsa_sig_to_der(const uint8_t *sig, uint8_t *der);
 
 // Private
-void init_k_rfc6979(const uint8_t *priv_key, const uint8_t *hash, rfc6979_state *rng);
+void init_rfc6979(const uint8_t *priv_key, const uint8_t *hash, rfc6979_state *rng);
+void generate_rfc6979(uint8_t rand[32], rfc6979_state *rng);
 void generate_k_rfc6979(bignum256 *k, rfc6979_state *rng);
 void generate_k_random(bignum256 *k, const bignum256 *prime);
 
diff --git a/ed25519-donna/ed25519.c b/ed25519-donna/ed25519.c
index 631d15f9d..9a91896b2 100644
--- a/ed25519-donna/ed25519.c
+++ b/ed25519-donna/ed25519.c
@@ -44,6 +44,75 @@ ed25519_publickey(const ed25519_secret_key sk, ed25519_public_key pk) {
 	ge25519_pack(pk, &A);
 }
 
+int
+ed25519_cosi_combine_publickeys(ed25519_public_key res, const ed25519_public_key *pks, size_t n) {
+	size_t i = 0;
+	ge25519 P;
+	ge25519_pniels sump;
+	ge25519_p1p1 sump1;
+
+	if (n == 1) {
+		memcpy(res, pks, sizeof(ed25519_public_key));
+		return 0;
+	}
+	if (!ge25519_unpack_negative_vartime(&P, pks[i++]))
+		return -1;
+	ge25519_full_to_pniels(&sump, &P);
+	while (i < n-1) {
+		if (!ge25519_unpack_negative_vartime(&P, pks[i++]))
+			return -1;
+		ge25519_pnielsadd(&sump, &P, &sump);
+	}
+	if (!ge25519_unpack_negative_vartime(&P, pks[i++]))
+		return -1;
+	ge25519_pnielsadd_p1p1(&sump1, &P, &sump, 0);
+	ge25519_p1p1_to_partial(&P, &sump1);
+	curve25519_neg(P.x, P.x);
+	ge25519_pack(res, &P);
+	return 0;
+}
+
+void
+ed25519_cosi_combine_signatures(ed25519_signature res, const ed25519_public_key R, const ed25519_cosi_signature *sigs, size_t n) {
+	bignum256modm s, t;
+	size_t i;
+
+	i = 0;
+	expand256_modm(s, sigs[i++], 32);
+	while (i < n) {
+		expand256_modm(t, sigs[i++], 32);
+		add256_modm(s, s, t);
+	}
+	memcpy(res, R, 32);
+	contract256_modm(res + 32, s);
+}
+
+void
+ed25519_cosi_sign(const unsigned char *m, size_t mlen, const ed25519_secret_key sk, const ed25519_secret_key nonce, const ed25519_public_key R, const ed25519_public_key pk, ed25519_cosi_signature sig) {
+	bignum256modm r, S, a;
+	hash_512bits extsk, extnonce, hram;
+
+	ed25519_extsk(extsk, sk);
+	ed25519_extsk(extnonce, nonce);
+
+	/* r = nonce */
+	expand256_modm(r, extnonce, 32);
+
+	/* S = H(R,A,m).. */
+	ed25519_hram(hram, R, 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(sig, S);
+}
+
 void
 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;
@@ -75,7 +144,7 @@ ed25519_sign(const unsigned char *m, size_t mlen, const ed25519_secret_key sk, c
 	/* S = (r + H(R,A,m)a) */
 	add256_modm(S, S, r);
 
-	/* S = (r + H(R,A,m)a) mod L */	
+	/* S = (r + H(R,A,m)a) mod L */
 	contract256_modm(RS + 32, S);
 }
 
diff --git a/ed25519-donna/ed25519.h b/ed25519-donna/ed25519.h
index 96a2727b4..2fa64e402 100644
--- a/ed25519-donna/ed25519.h
+++ b/ed25519-donna/ed25519.h
@@ -5,6 +5,7 @@
 extern "C" {
 #endif
 
+typedef unsigned char ed25519_cosi_signature[32];
 typedef unsigned char ed25519_signature[64];
 typedef unsigned char ed25519_public_key[32];
 typedef unsigned char ed25519_secret_key[32];
@@ -12,6 +13,10 @@ typedef unsigned char ed25519_secret_key[32];
 typedef unsigned char curve25519_key[32];
 
 void ed25519_publickey(const ed25519_secret_key sk, ed25519_public_key pk);
+
+int ed25519_cosi_combine_publickeys(ed25519_public_key res, const ed25519_public_key *pks, size_t n);
+void ed25519_cosi_combine_signatures(ed25519_signature res, const ed25519_public_key R, const ed25519_cosi_signature *sigs, size_t n);
+void ed25519_cosi_sign(const unsigned char *m, size_t mlen, const ed25519_secret_key key, const ed25519_secret_key nonce, const ed25519_public_key R, const ed25519_public_key pk, ed25519_cosi_signature sig);
 int ed25519_sign_open(const unsigned char *m, size_t mlen, const ed25519_public_key pk, const ed25519_signature RS);
 void ed25519_sign(const unsigned char *m, size_t mlen, const ed25519_secret_key sk, const ed25519_public_key pk, ed25519_signature RS);
 
diff --git a/tests.c b/tests.c
index 97e0ec7ed..a9c37ab01 100644
--- a/tests.c
+++ b/tests.c
@@ -1287,7 +1287,7 @@ END_TEST
 
 #define test_deterministic(KEY, MSG, K) do {	  \
 	sha256_Raw((uint8_t *)MSG, strlen(MSG), buf); \
-	init_k_rfc6979(fromhex(KEY), buf, &rng); \
+	init_rfc6979(fromhex(KEY), buf, &rng); \
 	generate_k_rfc6979(&k, &rng); \
 	bn_write_be(&k, buf); \
 	ck_assert_mem_eq(buf, fromhex(K), 32); \
@@ -2650,6 +2650,56 @@ START_TEST(test_ed25519) {
 }
 END_TEST
 
+START_TEST(test_ed25519_cosi) {
+	int MAXN=10;
+	ed25519_secret_key keys[MAXN];
+	ed25519_public_key pubkeys[MAXN];
+	ed25519_secret_key nonces[MAXN];
+	ed25519_public_key Rs[MAXN];
+	ed25519_cosi_signature sigs[MAXN];
+	unsigned char msg[32];
+	rfc6979_state rng;
+	int res;
+
+	init_rfc6979(fromhex("26c76712d89d906e6672dafa614c42e5cb1caac8c6568e4d2493087db51f0d36"),
+				 fromhex("26659c1cf7321c178c07437150639ff0c5b7679c7ea195253ed9abda2e081a37"), &rng);
+
+	for (int N = 1; N < 11; N++) {
+		ed25519_public_key pk;
+		ed25519_public_key R;
+		ed25519_signature sig;
+		/* phase 0: create priv/pubkeys and combine pubkeys */
+		for (int j = 0; j < N; j++) {
+			generate_rfc6979(keys[j], &rng);
+			ed25519_publickey(keys[j], pubkeys[j]);
+		}
+		res = ed25519_cosi_combine_publickeys(pk, pubkeys, N);
+		ck_assert_int_eq(res, 0);
+
+		generate_rfc6979(msg, &rng);
+
+		/* phase 1: create nonces, commitments (R values) and combine commitments */
+		for (int j = 0; j < N; j++) {
+			generate_rfc6979(nonces[j], &rng);
+			ed25519_publickey(nonces[j], Rs[j]);
+		}
+		res = ed25519_cosi_combine_publickeys(R, Rs, N);
+		ck_assert_int_eq(res, 0);
+
+		/* phase 2: sign and combine signatures */
+		for (int j = 0; j < N; j++) {
+			ed25519_cosi_sign(msg, sizeof(msg), keys[j], nonces[j], R, pk, sigs[j]);
+		}
+
+		ed25519_cosi_combine_signatures(sig, R, sigs, N);
+
+		/* check signature */
+		res = ed25519_sign_open(msg, sizeof(msg), pk, sig);
+		ck_assert_int_eq(res, 0);
+	}
+}
+END_TEST
+
 static void test_bip32_ecdh_init_node(HDNode *node, const char *seed_str, const char *curve_name) {
 	hdnode_from_seed((const uint8_t *)seed_str, strlen(seed_str), curve_name, node);
 	hdnode_fill_public_key(node);
@@ -3040,6 +3090,10 @@ Suite *test_suite(void)
 	tcase_add_test(tc, test_ed25519);
 	suite_add_tcase(s, tc);
 
+	tc = tcase_create("ed25519_cosi");
+	tcase_add_test(tc, test_ed25519_cosi);
+	suite_add_tcase(s, tc);
+
 	tc = tcase_create("script");
 	tcase_add_test(tc, test_output_script);
 	suite_add_tcase(s, tc);