4 files changed, 47 insertions, 6 deletions
diff --git a/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.c b/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.c
index 17f8268..97cdb3d 100644
--- a/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.c
+++ b/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.c
@@ -71,6 +71,9 @@ static fastd_protocol_config_t * protocol_init(void) {
 	ecc_25519_scalarmult_base(&work, &protocol_config->key.secret);
 	ecc_25519_store_packed(&protocol_config->key.public.int256, &work);
 
+	if (!divide_key(&protocol_config->key.secret))
+		exit_error("invalid secret key");
+
 	return protocol_config;
 }
 
diff --git a/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.h b/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.h
index 8dd8456..e2034bd 100644
--- a/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.h
+++ b/src/protocols/ec25519_fhmqvc/ec25519_fhmqvc.h
@@ -138,3 +138,26 @@ static inline void hexdump(char out[65], const unsigned char d[32]) {
 static inline bool is_session_valid(const protocol_session_t *session) {
 	return (session->method && session->method->provider->session_is_valid(session->method_state));
 }
+
+
+/** Devides a secret key by 8 (for some optimizations) */
+static inline bool divide_key(ecc_int256_t *key) {
+	uint8_t c = 0, c2;
+	ssize_t i;
+
+	for (i = 31; i >= 0; i--) {
+		c2 = key->p[i] << 5;
+		key->p[i] = (key->p[i] >> 3) | c;
+		c = c2;
+	}
+
+	return (c == 0);
+}
+
+/** Multiplies a point by 8 */
+static inline void octuple_point(ecc_25519_work_t *p) {
+	ecc_25519_work_t work;
+	ecc_25519_double(&work, p);
+	ecc_25519_double(&work, &work);
+	ecc_25519_double(p, &work);
+}
diff --git a/src/protocols/ec25519_fhmqvc/handshake.c b/src/protocols/ec25519_fhmqvc/handshake.c
index ee93e7a..b1c6242 100644
--- a/src/protocols/ec25519_fhmqvc/handshake.c
+++ b/src/protocols/ec25519_fhmqvc/handshake.c
@@ -192,7 +192,7 @@ static bool make_shared_handshake_key(bool initiator, const keypair_t *handshake
 	if (!ecc_25519_load_packed(&workXY, &peer_handshake_key->int256))
 		return false;
 
-	if (!fastd_protocol_ec25519_fhmqvc_check_key(&workXY))
+	if (ecc_25519_is_identity(&workXY))
 		return false;
 
 	if (initiator) {
@@ -235,6 +235,18 @@ static bool make_shared_handshake_key(bool initiator, const keypair_t *handshake
 	}
 
 	ecc_25519_add(&work, &workXY, &work);
+
+	/*
+	  Both our secret keys have been divided by 8 before, so we multiply
+	  the point with 8 here to compensate.
+
+	  By multiplying with 8, we prevent small-subgroup attacks (8 is the order
+	  of the curves twist, see djb's Curve25519 paper). While the factor 8 should
+	  be in the private keys anyways, the reduction modulo the subgroup order (in ecc_25519_gf_*)
+	  will only preserve it if the point actually lies on our subgroup.
+	*/
+	octuple_point(&work);
+
 	ecc_25519_scalarmult(&work, &s, &work);
 
 	if (ecc_25519_is_identity(&work))
diff --git a/src/protocols/ec25519_fhmqvc/state.c b/src/protocols/ec25519_fhmqvc/state.c
index 79bb6e2..cfb9028 100644
--- a/src/protocols/ec25519_fhmqvc/state.c
+++ b/src/protocols/ec25519_fhmqvc/state.c
@@ -46,12 +46,15 @@ static void init_protocol_state(void) {
 
 /** Generates a new ephemeral keypair */
 static void new_handshake_key(keypair_t *key) {
-		fastd_random_bytes(key->secret.p, SECRETKEYBYTES, false);
-		ecc_25519_gf_sanitize_secret(&key->secret, &key->secret);
+	fastd_random_bytes(key->secret.p, SECRETKEYBYTES, false);
+	ecc_25519_gf_sanitize_secret(&key->secret, &key->secret);
 
-		ecc_25519_work_t work;
-		ecc_25519_scalarmult_base(&work, &key->secret);
-		ecc_25519_store_packed(&key->public.int256, &work);
+	ecc_25519_work_t work;
+	ecc_25519_scalarmult_base(&work, &key->secret);
+	ecc_25519_store_packed(&key->public.int256, &work);
+
+	if (!divide_key(&key->secret))
+		exit_bug("generated invalid ephemeral key");
 }
 
 /**