TLS: Split tls_connection_prf() into two functions

Most protocols extracting keys from TLS use RFC 5705 exporters which is
commonly implemented in TLS libraries. This is the mechanism used by
EAP-TLS. (EAP-TLS actually predates RFC 5705, but RFC 5705 was defined
to be compatible with it.)

EAP-FAST, however, uses a legacy mechanism. It reuses the TLS internal
key block derivation and derives key material after the key block. This
is uncommon and a misuse of TLS internals, so not all TLS libraries
support this. Instead, we reimplement the PRF for the OpenSSL backend
and don't support it at all in the GnuTLS one.

Since these two are very different operations, split
tls_connection_prf() in two. tls_connection_export_key() implements the
standard RFC 5705 mechanism that we expect most TLS libraries to
support. tls_connection_get_eap_fast_key() implements the
EAP-FAST-specific legacy mechanism which may not be implemented on all
backends but is only used by EAP-FAST.

Signed-Off-By: David Benjamin <davidben@google.com>

src/crypto/tls.h

@@ -336,29 +336,36 @@ int __must_check tls_connection_get_random(void *tls_ctx,
 					 struct tls_random *data);
 
 /**
- * tls_connection_prf - Use TLS-PRF to derive keying material
+ * tls_connection_export_key - Derive keying material from a TLS connection
  * @tls_ctx: TLS context data from tls_init()
  * @conn: Connection context data from tls_connection_init()
  * @label: Label (e.g., description of the key) for PRF
- * @server_random_first: seed is 0 = client_random|server_random,
- * 1 = server_random|client_random
- * @skip_keyblock: Skip TLS key block from the beginning of PRF output
  * @out: Buffer for output data from TLS-PRF
  * @out_len: Length of the output buffer
  * Returns: 0 on success, -1 on failure
  *
- * tls_connection_prf() is required so that further keying material can be
- * derived from the master secret. Example implementation of this function is in
- * tls_prf_sha1_md5() when it is called with seed set to
- * client_random|server_random (or server_random|client_random). For TLSv1.2 and
- * newer, a different PRF is needed, though.
+ * Exports keying material using the mechanism described in RFC 5705.
  */
-int __must_check  tls_connection_prf(void *tls_ctx,
-				     struct tls_connection *conn,
-				     const char *label,
-				     int server_random_first,
-				     int skip_keyblock,
-				     u8 *out, size_t out_len);
+int __must_check tls_connection_export_key(void *tls_ctx,
+					   struct tls_connection *conn,
+					   const char *label,
+					   u8 *out, size_t out_len);
+
+/**
+ * tls_connection_get_eap_fast_key - Derive key material for EAP-FAST
+ * @tls_ctx: TLS context data from tls_init()
+ * @conn: Connection context data from tls_connection_init()
+ * @out: Buffer for output data from TLS-PRF
+ * @out_len: Length of the output buffer
+ * Returns: 0 on success, -1 on failure
+ *
+ * Exports key material after the normal TLS key block for use with
+ * EAP-FAST. Most callers will want tls_connection_export_key(), but EAP-FAST
+ * uses a different legacy mechanism.
+ */
+int __must_check tls_connection_get_eap_fast_key(void *tls_ctx,
+						 struct tls_connection *conn,
+						 u8 *out, size_t out_len);
 
 /**
  * tls_connection_handshake - Process TLS handshake (client side)

src/crypto/tls_gnutls.c

@@ -810,15 +810,22 @@ int tls_connection_get_random(void *ssl_ctx, struct tls_connection *conn,
 }
 
 
-int tls_connection_prf(void *tls_ctx, struct tls_connection *conn,
-		       const char *label, int server_random_first,
-		       int skip_keyblock, u8 *out, size_t out_len)
+int tls_connection_export_key(void *tls_ctx, struct tls_connection *conn,
+			      const char *label, u8 *out, size_t out_len)
 {
-	if (conn == NULL || conn->session == NULL || skip_keyblock)
+	if (conn == NULL || conn->session == NULL)
 		return -1;
 
 	return gnutls_prf(conn->session, os_strlen(label), label,
-			  server_random_first, 0, NULL, out_len, (char *) out);
+			  0 /* client_random first */, 0, NULL, out_len,
+			  (char *) out);
+}
+
+
+int tls_connection_get_eap_fast_key(void *tls_ctx, struct tls_connection *conn,
+				    u8 *out, size_t out_len)
+{
+	return -1;
 }
 
 

src/crypto/tls_internal.c

@@ -394,9 +394,9 @@ static int tls_get_keyblock_size(struct tls_connection *conn)
 }
 
 
-int tls_connection_prf(void *tls_ctx, struct tls_connection *conn,
-		       const char *label, int server_random_first,
-		       int skip_keyblock, u8 *out, size_t out_len)
+static int tls_connection_prf(void *tls_ctx, struct tls_connection *conn,
+			      const char *label, int server_random_first,
+			      int skip_keyblock, u8 *out, size_t out_len)
 {
 	int ret = -1, skip = 0;
 	u8 *tmp_out = NULL;
@@ -434,6 +434,21 @@ int tls_connection_prf(void *tls_ctx, struct tls_connection *conn,
 }
 
 
+int tls_connection_export_key(void *tls_ctx, struct tls_connection *conn,
+			      const char *label, u8 *out, size_t out_len)
+{
+	return tls_connection_prf(tls_ctx, conn, label, 0, 0, out, out_len);
+}
+
+
+int tls_connection_get_eap_fast_key(void *tls_ctx, struct tls_connection *conn,
+				    u8 *out, size_t out_len)
+{
+	return tls_connection_prf(tls_ctx, conn, "key expansion", 1, 1, out,
+				  out_len);
+}
+
+
 struct wpabuf * tls_connection_handshake(void *tls_ctx,
 					 struct tls_connection *conn,
 					 const struct wpabuf *in_data,

src/crypto/tls_none.c

@@ -86,9 +86,15 @@ int tls_connection_get_random(void *tls_ctx, struct tls_connection *conn,
 }
 
 
-int tls_connection_prf(void *tls_ctx, struct tls_connection *conn,
-		       const char *label, int server_random_first,
-		       int skip_keyblock, u8 *out, size_t out_len)
+int tls_connection_export_key(void *tls_ctx, struct tls_connection *conn,
+			      const char *label, u8 *out, size_t out_len)
+{
+	return -1;
+}
+
+
+int tls_connection_get_eap_fast_key(void *tls_ctx, struct tls_connection *conn,
+				    u8 *out, size_t out_len)
 {
 	return -1;
 }

src/crypto/tls_openssl.c

@@ -3146,9 +3146,19 @@ static int openssl_get_keyblock_size(SSL *ssl)
 #endif /* CONFIG_FIPS */
 
 
-static int openssl_tls_prf(struct tls_connection *conn,
-			   const char *label, int server_random_first,
-			   int skip_keyblock, u8 *out, size_t out_len)
+int tls_connection_export_key(void *tls_ctx, struct tls_connection *conn,
+			      const char *label, u8 *out, size_t out_len)
+{
+	if (!conn ||
+	    SSL_export_keying_material(conn->ssl, out, out_len, label,
+				       os_strlen(label), NULL, 0, 0) != 1)
+		return -1;
+	return 0;
+}
+
+
+int tls_connection_get_eap_fast_key(void *tls_ctx, struct tls_connection *conn,
+				    u8 *out, size_t out_len)
 {
 #ifdef CONFIG_FIPS
 	wpa_printf(MSG_ERROR, "OpenSSL: TLS keys cannot be exported in FIPS "
@@ -3169,9 +3179,9 @@ static int openssl_tls_prf(struct tls_connection *conn,
 	const char *ver;
 
 	/*
-	 * TLS library did not support key generation, so get the needed TLS
-	 * session parameters and use an internal implementation of TLS PRF to
-	 * derive the key.
+	 * TLS library did not support EAP-FAST key generation, so get the
+	 * needed TLS session parameters and use an internal implementation of
+	 * TLS PRF to derive the key.
 	 */
 
 	if (conn == NULL)
@@ -3184,15 +3194,13 @@ static int openssl_tls_prf(struct tls_connection *conn,
 	if (!ver || !sess)
 		return -1;
 
-	if (skip_keyblock) {
-		skip = openssl_get_keyblock_size(ssl);
-		if (skip < 0)
-			return -1;
-		tmp_out = os_malloc(skip + out_len);
-		if (!tmp_out)
-			return -1;
-		_out = tmp_out;
-	}
+	skip = openssl_get_keyblock_size(ssl);
+	if (skip < 0)
+		return -1;
+	tmp_out = os_malloc(skip + out_len);
+	if (!tmp_out)
+		return -1;
+	_out = tmp_out;
 
 	rnd = os_malloc(2 * SSL3_RANDOM_SIZE);
 	if (!rnd) {
@@ -3205,29 +3213,22 @@ static int openssl_tls_prf(struct tls_connection *conn,
 	master_key_len = SSL_SESSION_get_master_key(sess, master_key,
 						    sizeof(master_key));
 
-	if (server_random_first) {
-		os_memcpy(rnd, server_random, SSL3_RANDOM_SIZE);
-		os_memcpy(rnd + SSL3_RANDOM_SIZE, client_random,
-			  SSL3_RANDOM_SIZE);
-	} else {
-		os_memcpy(rnd, client_random, SSL3_RANDOM_SIZE);
-		os_memcpy(rnd + SSL3_RANDOM_SIZE, server_random,
-			  SSL3_RANDOM_SIZE);
-	}
+	os_memcpy(rnd, server_random, SSL3_RANDOM_SIZE);
+	os_memcpy(rnd + SSL3_RANDOM_SIZE, client_random, SSL3_RANDOM_SIZE);
 
 	if (os_strcmp(ver, "TLSv1.2") == 0) {
 		tls_prf_sha256(master_key, master_key_len,
-			       label, rnd, 2 * SSL3_RANDOM_SIZE,
+			       "key expansion", rnd, 2 * SSL3_RANDOM_SIZE,
 			       _out, skip + out_len);
 		ret = 0;
 	} else if (tls_prf_sha1_md5(master_key, master_key_len,
-				    label, rnd, 2 * SSL3_RANDOM_SIZE,
+				    "key expansion", rnd, 2 * SSL3_RANDOM_SIZE,
 				    _out, skip + out_len) == 0) {
 		ret = 0;
 	}
 	os_memset(master_key, 0, sizeof(master_key));
 	os_free(rnd);
-	if (ret == 0 && skip_keyblock)
+	if (ret == 0)
 		os_memcpy(out, _out + skip, out_len);
 	bin_clear_free(tmp_out, skip);
 
@@ -3236,26 +3237,6 @@ static int openssl_tls_prf(struct tls_connection *conn,
 }
 
 
-int tls_connection_prf(void *tls_ctx, struct tls_connection *conn,
-		       const char *label, int server_random_first,
-		       int skip_keyblock, u8 *out, size_t out_len)
-{
-	if (conn == NULL)
-		return -1;
-	if (server_random_first || skip_keyblock)
-		return openssl_tls_prf(conn, label,
-				       server_random_first, skip_keyblock,
-				       out, out_len);
-	if (SSL_export_keying_material(conn->ssl, out, out_len, label,
-				       os_strlen(label), NULL, 0, 0) == 1) {
-		wpa_printf(MSG_DEBUG, "OpenSSL: Using internal PRF");
-		return 0;
-	}
-	return openssl_tls_prf(conn, label, server_random_first,
-			       skip_keyblock, out, out_len);
-}
-
-
 static struct wpabuf *
 openssl_handshake(struct tls_connection *conn, const struct wpabuf *in_data,
 		  int server)

src/eap_common/eap_fast_common.c

@@ -93,8 +93,7 @@ void eap_fast_derive_master_secret(const u8 *pac_key, const u8 *server_random,
 }
 
 
-u8 * eap_fast_derive_key(void *ssl_ctx, struct tls_connection *conn,
-			 const char *label, size_t len)
+u8 * eap_fast_derive_key(void *ssl_ctx, struct tls_connection *conn, size_t len)
 {
 	u8 *out;
 
@@ -102,7 +101,7 @@ u8 * eap_fast_derive_key(void *ssl_ctx, struct tls_connection *conn,
 	if (out == NULL)
 		return NULL;
 
-	if (tls_connection_prf(ssl_ctx, conn, label, 1, 1, out, len)) {
+	if (tls_connection_get_eap_fast_key(ssl_ctx, conn, out, len)) {
 		os_free(out);
 		return NULL;
 	}

src/eap_common/eap_fast_common.h

@@ -98,7 +98,7 @@ struct wpabuf * eap_fast_tlv_eap_payload(struct wpabuf *buf);
 void eap_fast_derive_master_secret(const u8 *pac_key, const u8 *server_random,
 				   const u8 *client_random, u8 *master_secret);
 u8 * eap_fast_derive_key(void *ssl_ctx, struct tls_connection *conn,
-			 const char *label, size_t len);
+			 size_t len);
 int eap_fast_derive_eap_msk(const u8 *simck, u8 *msk);
 int eap_fast_derive_eap_emsk(const u8 *simck, u8 *emsk);
 int eap_fast_parse_tlv(struct eap_fast_tlv_parse *tlv,

src/eap_peer/eap_fast.c

@@ -275,7 +275,7 @@ static int eap_fast_derive_key_auth(struct eap_sm *sm,
 	 * Extra key material after TLS key_block: session_key_seed[40]
 	 */
 
-	sks = eap_fast_derive_key(sm->ssl_ctx, data->ssl.conn, "key expansion",
+	sks = eap_fast_derive_key(sm->ssl_ctx, data->ssl.conn,
 				  EAP_FAST_SKS_LEN);
 	if (sks == NULL) {
 		wpa_printf(MSG_DEBUG, "EAP-FAST: Failed to derive "
@@ -303,7 +303,6 @@ static int eap_fast_derive_key_provisioning(struct eap_sm *sm,
 	os_free(data->key_block_p);
 	data->key_block_p = (struct eap_fast_key_block_provisioning *)
 		eap_fast_derive_key(sm->ssl_ctx, data->ssl.conn,
-				    "key expansion",
 				    sizeof(*data->key_block_p));
 	if (data->key_block_p == NULL) {
 		wpa_printf(MSG_DEBUG, "EAP-FAST: Failed to derive key block");

src/eap_peer/eap_tls_common.c

@@ -328,8 +328,8 @@ u8 * eap_peer_tls_derive_key(struct eap_sm *sm, struct eap_ssl_data *data,
 	if (out == NULL)
 		return NULL;
 
-	if (tls_connection_prf(data->ssl_ctx, data->conn, label, 0, 0,
-			       out, len)) {
+	if (tls_connection_export_key(data->ssl_ctx, data->conn, label, out,
+				      len)) {
 		os_free(out);
 		return NULL;
 	}

src/eap_server/eap_server_fast.c

@@ -278,7 +278,7 @@ static void eap_fast_derive_key_auth(struct eap_sm *sm,
 	 * Extra key material after TLS key_block: session_key_seed[40]
 	 */
 
-	sks = eap_fast_derive_key(sm->ssl_ctx, data->ssl.conn, "key expansion",
+	sks = eap_fast_derive_key(sm->ssl_ctx, data->ssl.conn,
 				  EAP_FAST_SKS_LEN);
 	if (sks == NULL) {
 		wpa_printf(MSG_DEBUG, "EAP-FAST: Failed to derive "
@@ -305,7 +305,6 @@ static void eap_fast_derive_key_provisioning(struct eap_sm *sm,
 	os_free(data->key_block_p);
 	data->key_block_p = (struct eap_fast_key_block_provisioning *)
 		eap_fast_derive_key(sm->ssl_ctx, data->ssl.conn,
-				    "key expansion",
 				    sizeof(*data->key_block_p));
 	if (data->key_block_p == NULL) {
 		wpa_printf(MSG_DEBUG, "EAP-FAST: Failed to derive key block");

src/eap_server/eap_server_tls_common.c

@@ -115,8 +115,8 @@ u8 * eap_server_tls_derive_key(struct eap_sm *sm, struct eap_ssl_data *data,
 	if (out == NULL)
 		return NULL;
 
-	if (tls_connection_prf(sm->ssl_ctx, data->conn, label, 0, 0,
-			       out, len)) {
+	if (tls_connection_export_key(sm->ssl_ctx, data->conn, label, out,
+				      len)) {
 		os_free(out);
 		return NULL;
 	}