Implement optional CA list suppression in Certificate Request

According to RFC5246 the server can indicate the known Certificate
Authorities or can constrain the aurhorisation space by sending a
certificate list. This part of the message is optional and if omitted,
the client may send any certificate in the response.

The previous behaviour of mbed TLS was to always send the name of all the
CAs that are configured as root CAs. In certain cases this might cause
usability and privacy issues for example:
- If the list of the CA names is longer than the peers input buffer then
  the handshake will fail
- If the configured CAs belong to third parties, this message gives away
  information on the relations to these third parties

Therefore we introduce an option to suppress the CA list in the
Certificate Request message.

Providing this feature as a runtime option comes with a little cost in
code size and advantages in maintenance and flexibility.

include/mbedtls/ssl.h

@@ -185,6 +185,9 @@
 #define MBEDTLS_SSL_PRESET_DEFAULT              0
 #define MBEDTLS_SSL_PRESET_SUITEB               2
 
+#define MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED       1
+#define MBEDTLS_SSL_CERT_REQ_CA_LIST_DISABLED      0
+
 /*
  * Default range for DTLS retransmission timer value, in milliseconds.
  * RFC 6347 4.2.4.1 says from 1 second to 60 seconds.
@@ -749,6 +752,10 @@ struct mbedtls_ssl_config
 #if defined(MBEDTLS_SSL_FALLBACK_SCSV) && defined(MBEDTLS_SSL_CLI_C)
     unsigned int fallback : 1;      /*!< is this a fallback?                */
 #endif
+#if defined(MBEDTLS_SSL_SRV_C)
+    unsigned int cert_req_ca_list : 1;  /*!< enable sending CA list in
+                                          Certificate Request messages?     */
+#endif
 };
 
 
@@ -2031,6 +2038,20 @@ void mbedtls_ssl_conf_extended_master_secret( mbedtls_ssl_config *conf, char ems
 void mbedtls_ssl_conf_arc4_support( mbedtls_ssl_config *conf, char arc4 );
 #endif /* MBEDTLS_ARC4_C */
 
+#if defined(MBEDTLS_SSL_SRV_C)
+/**
+ * \brief          Whether to send a list of acceptable CAs in
+ *                 CertificateRequest messages.
+ *                 (Default: do send)
+ *
+ * \param conf     SSL configuration
+ * \param cert_req_ca_list   MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED or
+ *                          MBEDTLS_SSL_CERT_REQ_CA_LIST_DISABLED
+ */
+void mbedtls_ssl_conf_cert_req_ca_list( mbedtls_ssl_config *conf,
+                                          char cert_req_ca_list );
+#endif /* MBEDTLS_SSL_SRV_C */
+
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
 /**
  * \brief          Set the maximum fragment length to emit and/or negotiate

library/ssl_srv.c

@@ -2588,35 +2588,40 @@ static int ssl_write_certificate_request( mbedtls_ssl_context *ssl )
      * opaque DistinguishedName<1..2^16-1>;
      */
     p += 2;
-#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
-    if( ssl->handshake->sni_ca_chain != NULL )
-        crt = ssl->handshake->sni_ca_chain;
-    else
-#endif
-        crt = ssl->conf->ca_chain;
 
     total_dn_size = 0;
-    while( crt != NULL && crt->version != 0 )
+
+    if( ssl->conf->cert_req_ca_list ==  MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED )
     {
-        dn_size = crt->subject_raw.len;
+#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
+        if( ssl->handshake->sni_ca_chain != NULL )
+            crt = ssl->handshake->sni_ca_chain;
+        else
+#endif
+            crt = ssl->conf->ca_chain;
 
-        if( end < p ||
+        while( crt != NULL && crt->version != 0 )
-            (size_t)( end - p ) < dn_size ||
-            (size_t)( end - p ) < 2 + dn_size )
         {
-            MBEDTLS_SSL_DEBUG_MSG( 1, ( "skipping CAs: buffer too short" ) );
+            dn_size = crt->subject_raw.len;
-            break;
+
+            if( end < p ||
+                (size_t)( end - p ) < dn_size ||
+                (size_t)( end - p ) < 2 + dn_size )
+            {
+                MBEDTLS_SSL_DEBUG_MSG( 1, ( "skipping CAs: buffer too short" ) );
+                break;
+            }
+
+            *p++ = (unsigned char)( dn_size >> 8 );
+            *p++ = (unsigned char)( dn_size      );
+            memcpy( p, crt->subject_raw.p, dn_size );
+            p += dn_size;
+
+            MBEDTLS_SSL_DEBUG_BUF( 3, "requested DN", p - dn_size, dn_size );
+
+            total_dn_size += 2 + dn_size;
+            crt = crt->next;
         }
-
-        *p++ = (unsigned char)( dn_size >> 8 );
-        *p++ = (unsigned char)( dn_size      );
-        memcpy( p, crt->subject_raw.p, dn_size );
-        p += dn_size;
-
-        MBEDTLS_SSL_DEBUG_BUF( 3, "requested DN", p - dn_size, dn_size );
-
-        total_dn_size += 2 + dn_size;
-        crt = crt->next;
     }
 
     ssl->out_msglen  = p - buf;

library/ssl_tls.c

@@ -6050,6 +6050,14 @@ void mbedtls_ssl_conf_fallback( mbedtls_ssl_config *conf, char fallback )
 }
 #endif
 
+#if defined(MBEDTLS_SSL_SRV_C)
+void mbedtls_ssl_conf_cert_req_ca_list( mbedtls_ssl_config *conf,
+                                          char cert_req_ca_list )
+{
+    conf->cert_req_ca_list = cert_req_ca_list;
+}
+#endif
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
 void mbedtls_ssl_conf_encrypt_then_mac( mbedtls_ssl_config *conf, char etm )
 {
@@ -7232,6 +7240,10 @@ int mbedtls_ssl_config_defaults( mbedtls_ssl_config *conf,
     conf->anti_replay = MBEDTLS_SSL_ANTI_REPLAY_ENABLED;
 #endif
 
+#if defined(MBEDTLS_SSL_SRV_C)
+    conf->cert_req_ca_list = MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED;
+#endif
+
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
     conf->hs_timeout_min = MBEDTLS_SSL_DTLS_TIMEOUT_DFL_MIN;
     conf->hs_timeout_max = MBEDTLS_SSL_DTLS_TIMEOUT_DFL_MAX;