tls-server: Consider supported signature algorithms when selecting key/certificate

This won't work if the client doesn't send a `signature_algorithms`
extension.  But since the default is SHA1/RSA, most will send it to at
least announce stronger hash algorithms if not ECDSA.

src/libtls/tls_crypto.c

@@ -23,6 +23,7 @@
 
 #include <utils/debug.h>
 #include <plugins/plugin_feature.h>
+#include <collections/hashtable.h>
 
 ENUM_BEGIN(tls_cipher_suite_names, TLS_NULL_WITH_NULL_NULL,
 								   TLS_DH_anon_WITH_3DES_EDE_CBC_SHA,
@@ -2423,3 +2424,87 @@ tls_named_group_t tls_ec_group_to_curve(diffie_hellman_group_t group)
 	}
 	return 0;
 }
+
+/**
+ * See header.
+ */
+key_type_t tls_signature_scheme_to_key_type(tls_signature_scheme_t sig)
+{
+	int i;
+
+	for (i = 0; i < countof(schemes); i++)
+	{
+		if (schemes[i].sig == sig)
+		{
+			return key_type_from_signature_scheme(schemes[i].params.scheme);
+		}
+	}
+	return 0;
+}
+
+/**
+ * Hashtable hash function
+ */
+static u_int hash_key_type(key_type_t *type)
+{
+	return chunk_hash(chunk_from_thing(*type));
+}
+
+/**
+ * Hashtable equals function
+ */
+static bool equals_key_type(key_type_t *key1, key_type_t *key2)
+{
+	return *key1 == *key2;
+}
+
+CALLBACK(filter_key_types, bool,
+	void *data, enumerator_t *orig, va_list args)
+{
+	key_type_t *key_type, *out;
+
+	VA_ARGS_VGET(args, out);
+
+	if (orig->enumerate(orig, NULL, &key_type))
+	{
+		*out = *key_type;
+		return TRUE;
+	}
+	return FALSE;
+}
+
+CALLBACK(destroy_key_types, void,
+	hashtable_t *ht)
+{
+	ht->destroy_function(ht, (void*)free);
+}
+
+/*
+ * See header.
+ */
+enumerator_t *tls_get_supported_key_types(tls_version_t min_version,
+										  tls_version_t max_version)
+{
+	hashtable_t *ht;
+	key_type_t *type, lookup;
+	int i;
+
+	ht = hashtable_create((hashtable_hash_t)hash_key_type,
+						  (hashtable_equals_t)equals_key_type, 4);
+	for (i = 0; i < countof(schemes); i++)
+	{
+		if (schemes[i].min_version <= max_version &&
+			schemes[i].max_version >= min_version)
+		{
+			lookup = key_type_from_signature_scheme(schemes[i].params.scheme);
+			if (!ht->get(ht, &lookup))
+			{
+				type = malloc_thing(key_type_t);
+				*type = lookup;
+				ht->put(ht, type, type);
+			}
+		}
+	}
+	return enumerator_create_filter(ht->create_enumerator(ht),
+									filter_key_types, ht, destroy_key_types);
+}

src/libtls/tls_crypto.h

@@ -691,4 +691,24 @@ int tls_crypto_get_supported_groups(diffie_hellman_group_t **groups);
  */
 tls_named_group_t tls_ec_group_to_curve(diffie_hellman_group_t group);
 
+/**
+ * Get the key type from a TLS signature scheme
+ *
+ * @param sig			TLS signature algorithm scheme
+ * @return				type of a key
+ */
+key_type_t tls_signature_scheme_to_key_type(tls_signature_scheme_t sig);
+
+/**
+ * Create an enumerator over supported key types within a specific TLS version range
+ *
+ * Enumerates over key_type_t
+ *
+ * @param min_version	minimum negotiated TLS version
+ * @param max_version	maximum negotiated TLS version
+ * @return				hashtable of key types
+ */
+enumerator_t *tls_get_supported_key_types(tls_version_t min_version,
+										  tls_version_t max_version);
+
 #endif /** TLS_CRYPTO_H_ @}*/

src/libtls/tls_server.c

@@ -169,71 +169,109 @@ struct private_tls_server_t {
 	bool curves_received;
 };
 
+/**
+ * Create an array of an intersection of server and peer supported key types
+ */
+static array_t *create_common_key_types(chunk_t hashsig,
+										tls_version_t version_min,
+										tls_version_t version_max)
+{
+	array_t *key_types;
+	enumerator_t *enumerator;
+	key_type_t v, lookup;
+	uint16_t sig_scheme;
+
+	key_types = array_create(sizeof(key_type_t), 8);
+	enumerator = tls_get_supported_key_types(version_min, version_max);
+	while (enumerator->enumerate(enumerator, &v))
+	{
+		bio_reader_t *reader;
+
+		reader = bio_reader_create(hashsig);
+		while (reader->remaining(reader) &&
+			   reader->read_uint16(reader, &sig_scheme))
+		{
+			lookup = tls_signature_scheme_to_key_type(sig_scheme);
+			if (v == lookup)
+			{
+				array_insert(key_types, ARRAY_TAIL, &lookup);
+				break;
+			}
+		}
+		reader->destroy(reader);
+	}
+	enumerator->destroy(enumerator);
+	return key_types;
+}
+
 /**
  * Find a cipher suite and a server key
  */
 static bool select_suite_and_key(private_tls_server_t *this,
 								 tls_cipher_suite_t *suites, int count)
 {
+	array_t *key_types;
+	tls_version_t version_min, version_max;
 	private_key_t *key;
 	key_type_t type;
 
-	key = lib->credmgr->get_private(lib->credmgr, KEY_ANY, this->server,
-									this->server_auth);
+	version_min = this->tls->get_version_min(this->tls);
+	version_max = this->tls->get_version_max(this->tls);
+	key_types = create_common_key_types(this->hashsig, version_min, version_max);
+	if (!array_count(key_types))
+	{
+		DBG1(DBG_TLS, "no common signature algorithms found");
+		array_destroy(key_types);
+		return FALSE;
+	}
+	while (array_remove(key_types, ARRAY_HEAD, &type))
+	{
+		key = lib->credmgr->get_private(lib->credmgr, type, this->server,
+										this->server_auth);
+		if (key)
+		{
+			break;
+		}
+	}
 	if (!key)
 	{
 		DBG1(DBG_TLS, "no usable TLS server certificate found for '%Y'",
 			 this->server);
+		array_destroy(key_types);
 		return FALSE;
 	}
 
-	if (this->tls->get_version_max(this->tls) >= TLS_1_3)
+	if (version_max >= TLS_1_3)
 	{
-		/* currently no support to derive key based on client supported
-		 * signature schemes */
 		this->suite = this->crypto->select_cipher_suite(this->crypto, suites,
 												  		count, KEY_ANY);
-		if (!this->suite)
-		{
-			DBG1(DBG_TLS, "received cipher suites unacceptable");
-			return FALSE;
-		}
 	}
 	else
 	{
 		this->suite = this->crypto->select_cipher_suite(this->crypto, suites,
-												  		count,
-												  		key->get_type(key));
-		if (!this->suite)
-		{	/* no match for this key, try to find another type */
-			if (key->get_type(key) == KEY_ECDSA)
-			{
-				type = KEY_RSA;
-			}
-			else
-			{
-				type = KEY_ECDSA;
-			}
-			key->destroy(key);
-
-			this->suite = this->crypto->select_cipher_suite(this->crypto, suites,
-															count, type);
-			if (!this->suite)
-			{
-				DBG1(DBG_TLS, "received cipher suites unacceptable");
-				return FALSE;
-			}
+														count, type);
+		while (!this->suite && array_remove(key_types, ARRAY_HEAD, &type))
+		{	/* find a key and cipher suite for one of the remaining key types */
+			DESTROY_IF(key);
 			this->server_auth->destroy(this->server_auth);
 			this->server_auth = auth_cfg_create();
 			key = lib->credmgr->get_private(lib->credmgr, type, this->server,
 											this->server_auth);
-			if (!key)
+			if (key)
 			{
-				DBG1(DBG_TLS, "received cipher suites unacceptable");
-				return FALSE;
+				this->suite = this->crypto->select_cipher_suite(this->crypto,
+																suites, count,
+																type);
 			}
 		}
 	}
+	array_destroy(key_types);
+	if (!this->suite || !key)
+	{
+		DBG1(DBG_TLS, "received cipher suites or signature schemes unacceptable");
+		return FALSE;
+	}
+	DBG1(DBG_TLS, "using key of type %N", key_type_names, key->get_type(key));
 	this->private = key;
 	return TRUE;
 }