Add RFC5764 - SRTP key generation during DTLS handshake

include/mbedtls/config.h

@@ -1194,6 +1194,17 @@
  */
 #define MBEDTLS_SSL_DTLS_HELLO_VERIFY
 
+/**
+ * \def MBEDTLS_SSL_DTLS_SRTP
+ *
+ * Enable support for DTLS-SRTP, RFC5764
+ *
+ * Requires: MBEDTLS_SSL_PROTO_DTLS
+ *
+ * Comment this to disable support for DTLS-SRTP.
+ */
+#define MBEDTLS_SSL_DTLS_SRTP
+
 /**
  * \def MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE
  *

include/mbedtls/ssl.h

@@ -328,6 +328,8 @@
 
 #define MBEDTLS_TLS_EXT_SIG_ALG                     13
 
+#define MBEDTLS_TLS_EXT_USE_SRTP                    14
+
 #define MBEDTLS_TLS_EXT_ALPN                        16
 
 #define MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC            22 /* 0x16 */
@@ -339,6 +341,14 @@
 
 #define MBEDTLS_TLS_EXT_RENEGOTIATION_INFO      0xFF01
 
+/*
+ * use_srtp extension protection profiles values as defined in http://www.iana.org/assignments/srtp-protection/srtp-protection.xhtml
+ */
+#define MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_80_IANA_VALUE     0x0001
+#define MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_32_IANA_VALUE     0x0002
+#define MBEDTLS_SRTP_NULL_HMAC_SHA1_80_IANA_VALUE          0x0005
+#define MBEDTLS_SRTP_NULL_HMAC_SHA1_32_IANA_VALUE          0x0006
+
 /*
  * Size defines
  */
@@ -538,6 +548,21 @@ typedef struct mbedtls_ssl_key_cert mbedtls_ssl_key_cert;
 typedef struct mbedtls_ssl_flight_item mbedtls_ssl_flight_item;
 #endif
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP)
+/*
+ * List of SRTP profiles for DTLS-SRTP
+ */
+typedef enum
+{
+    MBEDTLS_SRTP_UNSET_PROFILE,
+    MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_80,
+    MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_32,
+    MBEDTLS_SRTP_NULL_HMAC_SHA1_80,
+    MBEDTLS_SRTP_NULL_HMAC_SHA1_32,
+}
+mbedtls_dtls_srtp_protection_profiles;
+#endif /* MBEDTLS_SSL_DTLS_SRTP */
+
 /*
  * This structure is used for storing current session data.
  */
@@ -678,6 +703,14 @@ struct mbedtls_ssl_config
     const char **alpn_list;         /*!< ordered list of protocols          */
 #endif
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP)
+    /*
+     * use_srtp extension
+     */
+    mbedtls_dtls_srtp_protection_profiles *dtls_srtp_profiles_list; /*!< ordered list of supported srtp profile */
+    size_t dtls_srtp_profiles_list_len; /*!< number of supported profiles */
+#endif /* MBEDTLS_SSL_DTLS_SRTP */
+
     /*
      * Numerical settings (int then char)
      */
@@ -877,6 +910,15 @@ struct mbedtls_ssl_context
     const char *alpn_chosen;    /*!<  negotiated protocol                   */
 #endif
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP)
+    /*
+     * use_srtp extension
+     */
+    mbedtls_dtls_srtp_protection_profiles chosen_dtls_srtp_profile; /*!< negotiated SRTP profile */
+    unsigned char *dtls_srtp_keys; /*!< master keys and master salt for SRTP generated during handshake */
+    size_t dtls_srtp_keys_len; /*!< length in bytes of master keys and master salt for SRTP generated during handshake */
+#endif /* MBEDTLS_SSL_DTLS_SRTP */
+
     /*
      * Information for DTLS hello verify
      */
@@ -1921,6 +1963,45 @@ int mbedtls_ssl_conf_alpn_protocols( mbedtls_ssl_config *conf, const char **prot
 const char *mbedtls_ssl_get_alpn_protocol( const mbedtls_ssl_context *ssl );
 #endif /* MBEDTLS_SSL_ALPN */
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP)
+/**
+ * \brief                   Set the supported DTLS-SRTP protection profiles.
+ *
+ * \param conf              SSL configuration
+ * \param profiles          List of supported protection profiles,
+ *                          in decreasing preference order.
+ * \param profiles_number   Number of supported profiles.
+ *
+ * \return         0 on success, or MBEDTLS_ERR_SSL_BAD_INPUT_DATA.
+ */
+int mbedtls_ssl_conf_dtls_srtp_protection_profiles( mbedtls_ssl_config *conf, const mbedtls_dtls_srtp_protection_profiles *profiles, size_t profiles_number);
+
+/**
+ * \brief          Get the negotiated DTLS-SRTP Protection Profile.
+ *                 This function should be called after the handshake is
+ *                 completed.
+ *
+ * \param ssl      SSL context
+ *
+ * \return         Protection Profile enum member, MBEDTLS_SRTP_UNSET_PROFILE if no protocol was negotiated.
+ */
+mbedtls_dtls_srtp_protection_profiles mbedtls_ssl_get_dtls_srtp_protection_profile( const mbedtls_ssl_context *ssl);
+
+/**
+ * \brief                  Get the generated DTLS-SRTP key material.
+ *                         This function should be called after the handshake is
+ *                         completed. It shall returns 80 bytes of key material generated according to RFC5764
+ *
+ * \param ssl              SSL context
+ * \param key              Buffer to hold the generated key material
+ * \param key_buffer_len   Length in bytes of the key buffer
+ * \param key_len          Actual length of data written in the key buffer
+ *
+ * \return         0 on succes, MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL if the key buffer is too small to hold the generated key
+ */
+int mbedtls_ssl_get_dtls_srtp_key_material( const mbedtls_ssl_context *ssl, unsigned char *key, const size_t key_buffer_len, size_t *key_len );
+#endif /* MBEDTLS_SSL_DTLS_SRTP */
+
 /**
  * \brief          Set the maximum supported version sent from the client side
  *                 and/or accepted at the server side

library/ssl_cli.c

@@ -661,6 +661,79 @@ static void ssl_write_alpn_ext( mbedtls_ssl_context *ssl,
 }
 #endif /* MBEDTLS_SSL_ALPN */
 
+#if defined (MBEDTLS_SSL_DTLS_SRTP)
+static void ssl_write_use_srtp_ext( mbedtls_ssl_context *ssl,
+                                unsigned char *buf, size_t *olen )
+{
+    unsigned char *p = buf;
+    size_t protection_profiles_index = 0;
+
+    *olen = 0;
+
+    if( (ssl->conf->dtls_srtp_profiles_list == NULL)  || (ssl->conf->dtls_srtp_profiles_list_len == 0) )
+    {
+        return;
+    }
+
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, adding use_srtp extension" ) );
+
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_USE_SRTP >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_USE_SRTP      ) & 0xFF );
+
+    /* RFC5764 section 4.1.1
+     * uint8 SRTPProtectionProfile[2];
+     *
+     * struct {
+     *   SRTPProtectionProfiles SRTPProtectionProfiles;
+     *   opaque srtp_mki<0..255>;
+     * } UseSRTPData;
+
+     * SRTPProtectionProfile SRTPProtectionProfiles<2..2^16-1>;
+     *
+     * Note: srtp_mki is not supported
+     */
+
+    /* Extension length = 2bytes for profiles lenght, ssl->conf->dtls_srtp_profiles_list_len*2 (each profile is 2 bytes length ) + 1 byte for the non implemented srtp_mki vector length (always 0) */
+    *p++ = (unsigned char)( ( ( 2 + 2*(ssl->conf->dtls_srtp_profiles_list_len) + 1 ) >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( ( 2 + 2*(ssl->conf->dtls_srtp_profiles_list_len) + 1 )      ) & 0xFF );
+
+
+    /* protection profile length: 2*(ssl->conf->dtls_srtp_profiles_list_len) */
+    *p++ = (unsigned char)( ( ( 2*(ssl->conf->dtls_srtp_profiles_list_len) ) >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( 2*(ssl->conf->dtls_srtp_profiles_list_len) ) & 0xFF );
+
+    for( protection_profiles_index=0; protection_profiles_index < ssl->conf->dtls_srtp_profiles_list_len; protection_profiles_index++ )
+    {
+        switch (ssl->conf->dtls_srtp_profiles_list[protection_profiles_index]) {
+            case MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_80:
+                *p++ = ( ( ( MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_80_IANA_VALUE ) >> 8 ) & 0xFF);
+                *p++ = ( ( MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_80_IANA_VALUE ) & 0xFF);
+                break;
+            case MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_32:
+                *p++ = ( ( ( MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_32_IANA_VALUE ) >> 8 ) & 0xFF);
+                *p++ = ( ( MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_32_IANA_VALUE ) & 0xFF);
+                break;
+            case MBEDTLS_SRTP_NULL_HMAC_SHA1_80:
+                *p++ = ( ( ( MBEDTLS_SRTP_NULL_HMAC_SHA1_80_IANA_VALUE ) >> 8 ) & 0xFF);
+                *p++ = ( ( MBEDTLS_SRTP_NULL_HMAC_SHA1_80_IANA_VALUE ) & 0xFF);
+                break;
+            case MBEDTLS_SRTP_NULL_HMAC_SHA1_32:
+                *p++ = ( ( ( MBEDTLS_SRTP_NULL_HMAC_SHA1_32_IANA_VALUE ) >> 8 ) & 0xFF);
+                *p++ = ( ( MBEDTLS_SRTP_NULL_HMAC_SHA1_32_IANA_VALUE ) & 0xFF);
+                break;
+            default:
+                /* Note: we shall never arrive here as protection profiles is checked by ssl_set_dtls_srtp_protection_profiles function */
+                MBEDTLS_SSL_DEBUG_MSG( 1, ( "client hello, ignore illegal DTLS-SRTP protection profile %d",  ssl->conf->dtls_srtp_profiles_list[protection_profiles_index]) );
+                break;
+        }
+    }
+
+    *p++ = 0x00;  /* non implemented srtp_mki vector length is always 0 */
+    /* total extension length: extension type (2 bytes) + extension length (2 bytes) + protection profile length (2 bytes) + 2*nb protection profiles + srtp_mki vector length(1 byte)*/
+    *olen = 2 + 2 + 2 + 2*(ssl->conf->dtls_srtp_profiles_list_len) + 1;
+}
+#endif /* MBEDTLS_SSL_DTLS_SRTP */
+
 /*
  * Generate random bytes for ClientHello
  */
@@ -1011,6 +1084,11 @@ static int ssl_write_client_hello( mbedtls_ssl_context *ssl )
     ext_len += olen;
 #endif
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP)
+    ssl_write_use_srtp_ext( ssl, p + 2 + ext_len, &olen );
+    ext_len += olen;
+#endif
+
 #if defined(MBEDTLS_SSL_SESSION_TICKETS)
     ssl_write_session_ticket_ext( ssl, p + 2 + ext_len, &olen );
     ext_len += olen;
@@ -1313,6 +1391,81 @@ static int ssl_parse_alpn_ext( mbedtls_ssl_context *ssl,
 }
 #endif /* MBEDTLS_SSL_ALPN */
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP)
+static int ssl_parse_use_srtp_ext( mbedtls_ssl_context *ssl,
+                               const unsigned char *buf, size_t len )
+{
+    mbedtls_dtls_srtp_protection_profiles server_protection = MBEDTLS_SRTP_UNSET_PROFILE;
+    size_t i;
+    uint16_t server_protection_profile_value = 0;
+
+    /* If use_srtp is not configured, just ignore the extension */
+    if( ( ssl->conf->dtls_srtp_profiles_list == NULL ) || ( ssl->conf->dtls_srtp_profiles_list_len == 0 ) )
+        return( 0 );
+
+    /* RFC5764 section 4.1.1
+     * uint8 SRTPProtectionProfile[2];
+     *
+     * struct {
+     *   SRTPProtectionProfiles SRTPProtectionProfiles;
+     *   opaque srtp_mki<0..255>;
+     * } UseSRTPData;
+
+     * SRTPProtectionProfile SRTPProtectionProfiles<2..2^16-1>;
+     *
+     * Note: srtp_mki is not supported
+     */
+
+    /* Length is 5 : one protection profile(2 bytes) + length(2 bytes) and potential srtp_mki which won't be parsed */
+    if( len < 5 )
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+
+    /*
+     * get the server protection profile
+     */
+    if (((uint16_t)(buf[0]<<8 | buf[1])) != 0x0002) { /* protection profile length must be 0x0002 as we must have only one protection profile in server Hello */
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+    } else {
+        server_protection_profile_value = buf[2]<<8 | buf[3];
+    }
+
+    /*
+     * Check we have the server profile in our list
+     */
+    for( i=0; i < ssl->conf->dtls_srtp_profiles_list_len; i++)
+    {
+        switch ( server_protection_profile_value ) {
+            case MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_80_IANA_VALUE:
+                server_protection = MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_80;
+                break;
+            case MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_32_IANA_VALUE:
+                server_protection = MBEDTLS_SRTP_AES128_CM_HMAC_SHA1_32;
+                break;
+            case MBEDTLS_SRTP_NULL_HMAC_SHA1_80_IANA_VALUE:
+                server_protection = MBEDTLS_SRTP_NULL_HMAC_SHA1_80;
+                break;
+            case MBEDTLS_SRTP_NULL_HMAC_SHA1_32_IANA_VALUE:
+                server_protection = MBEDTLS_SRTP_NULL_HMAC_SHA1_32;
+                break;
+            default:
+                server_protection = MBEDTLS_SRTP_UNSET_PROFILE;
+                break;
+        }
+
+        if (server_protection == ssl->conf->dtls_srtp_profiles_list[i]) {
+            ssl->chosen_dtls_srtp_profile = ssl->conf->dtls_srtp_profiles_list[i];
+            return 0;
+        }
+    }
+
+    /* If we get there, no match was found : server problem, it shall never answer with incompatible profile */
+    ssl->chosen_dtls_srtp_profile = MBEDTLS_SRTP_UNSET_PROFILE;
+    mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                            MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE );
+    return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO );
+}
+#endif /* MBEDTLS_SSL_DTLS_SRTP */
+
 /*
  * Parse HelloVerifyRequest.  Only called after verifying the HS type.
  */
@@ -1800,6 +1953,16 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
             break;
 #endif /* MBEDTLS_SSL_ALPN */
 
+#if defined(MBEDTLS_SSL_DTLS_SRTP)
+        case MBEDTLS_TLS_EXT_USE_SRTP:
+            MBEDTLS_SSL_DEBUG_MSG( 3, ( "found use_srtp extension" ) );
+
+            if( ( ret = ssl_parse_use_srtp_ext( ssl, ext + 4, ext_size ) ) != 0 )
+                return( ret );
+
+            break;
+#endif /* MBEDTLS_SSL_DTLS_SRTP */
+
         default:
             MBEDTLS_SSL_DEBUG_MSG( 3, ( "unknown extension found: %d (ignoring)",
                            ext_id ) );