From c403b264e83baefbcfd0fcf2b6d6e03fd1613994 Mon Sep 17 00:00:00 2001
From: Hanno Becker <hanno.becker@arm.com>
Date: Fri, 26 Apr 2019 13:56:39 +0100
Subject: [PATCH] Implement parsing of CID extension in ClientHello

---
 library/ssl_srv.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)

diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 72d3c79bd..60f2538ee 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -433,6 +433,78 @@ static int ssl_parse_max_fragment_length_ext( mbedtls_ssl_context *ssl,
 }
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
+#if defined(MBEDTLS_SSL_CID)
+static int ssl_parse_cid_ext( mbedtls_ssl_context *ssl,
+                              const unsigned char *buf,
+                              size_t len )
+{
+    size_t peer_cid_len;
+
+    /* CID extension only makes sense in DTLS */
+    if( ssl->conf->transport != MBEDTLS_SSL_TRANSPORT_DATAGRAM )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    /*
+     * Quoting
+     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04:
+     *
+     *   struct {
+     *      opaque cid<0..2^8-1>;
+     *   } ConnectionId;
+    */
+
+    if( len < 1 )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    peer_cid_len = *buf++;
+    len--;
+
+    if( len != peer_cid_len )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    /* Ignore CID if the user has disabled its use. */
+    if( ssl->negotiate_cid == MBEDTLS_SSL_CID_DISABLED )
+    {
+        /* Leave ssl->handshake->cid_in_use in its default
+         * value of MBEDTLS_SSL_CID_DISABLED. */
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "Client sent CID extension, but CID disabled" ) );
+        return( 0 );
+    }
+
+    if( peer_cid_len > MBEDTLS_SSL_CID_OUT_LEN_MAX )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client hello message" ) );
+        mbedtls_ssl_send_alert_message( ssl, MBEDTLS_SSL_ALERT_LEVEL_FATAL,
+                                        MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER );
+        return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO );
+    }
+
+    ssl->handshake->peer_cid_len = (uint8_t) peer_cid_len;
+    memcpy( ssl->handshake->peer_cid, buf, peer_cid_len );
+
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "Use of CID extension negotiated" ) );
+    MBEDTLS_SSL_DEBUG_BUF( 3, "Client CID", buf, peer_cid_len );
+
+    ssl->handshake->cid_in_use = MBEDTLS_SSL_CID_ENABLED;
+    return( 0 );
+}
+#endif /* MBEDTLS_SSL_CID */
+
 #if defined(MBEDTLS_SSL_TRUNCATED_HMAC)
 static int ssl_parse_truncated_hmac_ext( mbedtls_ssl_context *ssl,
                                          const unsigned char *buf,
@@ -1783,6 +1855,16 @@ read_record_header:
                 break;
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
 
+#if defined(MBEDTLS_SSL_CID)
+            case MBEDTLS_TLS_EXT_CID:
+                MBEDTLS_SSL_DEBUG_MSG( 3, ( "found CID extension" ) );
+
+                ret = ssl_parse_cid_ext( ssl, ext + 4, ext_size );
+                if( ret != 0 )
+                    return( ret );
+                break;
+#endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
             case MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC:
                 MBEDTLS_SSL_DEBUG_MSG( 3, ( "found encrypt then mac extension" ) );