CID update to RFC 9146

The DTLS 1.2 CID specification has been published as RFC 9146. This PR updates the implementation to match the RFC content.

Signed-off-by: Hannes Tschofenig <hannes.tschofenig@arm.com>

include/mbedtls/check_config.h

@@ -839,6 +839,12 @@
 #error "MBEDTLS_SSL_CID_OUT_LEN_MAX too large (max 255)"
 #endif
 
+#if !defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)            &&                 \
+    defined(MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT)
+#error "MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT defined, but not all prerequsites"
+#endif
+
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC) &&   \
     !defined(MBEDTLS_SSL_PROTO_TLS1_2)
 #error "MBEDTLS_SSL_ENCRYPT_THEN_MAC defined, but not all prerequisites"

include/mbedtls/mbedtls_config.h

@@ -1320,9 +1320,7 @@
 /**
  * \def MBEDTLS_SSL_DTLS_CONNECTION_ID
  *
- * Enable support for the DTLS Connection ID extension
+ * Enable support for the DTLS Connection ID extension,
- * (version draft-ietf-tls-dtls-connection-id-05,
- * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05)
  * which allows to identify DTLS connections across changes
  * in the underlying transport.
  *
@@ -1331,10 +1329,6 @@
  * `mbedtls_ssl_conf_cid()`. See the corresponding documentation for
  * more information.
  *
- * \warning The Connection ID extension is still in draft state.
- *          We make no stability promises for the availability
- *          or the shape of the API controlled by this option.
- *
  * The maximum lengths of outgoing and incoming CIDs can be configured
  * through the options
  * - MBEDTLS_SSL_CID_OUT_LEN_MAX
@@ -1344,7 +1338,23 @@
  *
  * Uncomment to enable the Connection ID extension.
  */
-//#define MBEDTLS_SSL_DTLS_CONNECTION_ID
+#define MBEDTLS_SSL_DTLS_CONNECTION_ID
+
+
+/**
+ * \def MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT
+ *
+ * Defines whether RFC 9146 (default) or the legacy version
+ * (version draft-ietf-tls-dtls-connection-id-05,
+ * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05)
+ * is used.
+ *
+ * Set the value to 0 for the standard version, and
+ * 1 for the legacy draft version.
+ *
+ * Requires: MBEDTLS_SSL_DTLS_CONNECTION_ID
+ */
+#define MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT 0
 
 /**
  * \def MBEDTLS_SSL_ASYNC_PRIVATE
@@ -3539,17 +3549,6 @@
 //#define MBEDTLS_PSK_MAX_LEN               32 /**< Max size of TLS pre-shared keys, in bytes (default 256 bits) */
 //#define MBEDTLS_SSL_COOKIE_TIMEOUT        60 /**< Default expiration delay of DTLS cookies, in seconds if HAVE_TIME, or in number of cookies issued */
 
-/** \def MBEDTLS_TLS_EXT_CID
- *
- * At the time of writing, the CID extension has not been assigned its
- * final value. Set this configuration option to make Mbed TLS use a
- * different value.
- *
- * A future minor revision of Mbed TLS may change the default value of
- * this option to match evolving standards and usage.
- */
-//#define MBEDTLS_TLS_EXT_CID                        254
-
 /**
  * Complete list of ciphersuites to use, in order of preference.
  *

include/mbedtls/ssl.h

@@ -401,7 +401,13 @@
 #define MBEDTLS_SSL_CID_TLS1_3_PADDING_GRANULARITY 16
 #endif
 
-/** \} name SECTION: Module settings */
+/*
+ * Default to standard CID mode
+ */
+#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID) && \
+    !defined(MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT)
+#define MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT 0
+#endif
 
 /*
  * Length of the verify data for secure renegotiation
@@ -571,15 +577,10 @@
 #define MBEDTLS_TLS_EXT_SIG_ALG_CERT                50 /* RFC 8446 TLS 1.3 */
 #define MBEDTLS_TLS_EXT_KEY_SHARE                   51 /* RFC 8446 TLS 1.3 */
 
-/* The value of the CID extension is still TBD as of
+#if MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT == 0
- * draft-ietf-tls-dtls-connection-id-05
+#define MBEDTLS_TLS_EXT_CID                         54 /* RFC 9146 DTLS 1.2 CID */
- * (https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05).
+#else
- *
+#define MBEDTLS_TLS_EXT_CID                        254 /* Pre-RFC 9146 DTLS 1.2 CID */
- * A future minor revision of Mbed TLS may change the default value of
- * this option to match evolving standards and usage.
- */
-#if !defined(MBEDTLS_TLS_EXT_CID)
-#define MBEDTLS_TLS_EXT_CID                        254 /* TBD */
 #endif
 
 #define MBEDTLS_TLS_EXT_ECJPAKE_KKPP               256 /* experimental */
@@ -2003,8 +2004,9 @@ void mbedtls_ssl_set_bio( mbedtls_ssl_context *ssl,
  * \brief             Configure the use of the Connection ID (CID)
  *                    extension in the next handshake.
  *
- *                    Reference: draft-ietf-tls-dtls-connection-id-05
+ *                    Reference: RFC 9146 (or draft-ietf-tls-dtls-connection-id-05
  *                    https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05
+ *                    for legacy version)
  *
  *                    The DTLS CID extension allows the reliable association of
  *                    DTLS records to DTLS connections across changes in the

library/ssl_msg.c

@@ -388,30 +388,80 @@ static int ssl_parse_inner_plaintext( unsigned char const *content,
 }
 #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID || MBEDTLS_SSL_PROTO_TLS1_3 */
 
-/* `add_data` must have size 13 Bytes if the CID extension is disabled,
+/* The size of the `add_data` structure depends on various
- * and 13 + 1 + CID-length Bytes if the CID extension is enabled. */
+ * factors, namely
+ *
+ * 1) CID functionality disabled
+ *
+ * additional_data =
+ *    8:                    seq_num +
+ *    1:                       type +
+ *    2:                    version +
+ *    2:  length of inner plaintext +
+ *
+ * size = 13 bytes
+ *
+ * 2) CID functionality based on RFC 9146 enabled
+ *
+ * size = 8 + 1 + 1 + 1 + 2 + 2 + 6 + 2 + CID-length
+ *      = 23 + CID-length
+ *
+ * 3) CID functionality based on legacy CID version
+    according to draft-ietf-tls-dtls-connection-id-05
+ *  https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05
+ *
+ * size = 13 + 1 + CID-length
+ *
+ * More information about the CID usage:
+ *
+ * Per Section 5.3 of draft-ietf-tls-dtls-connection-id-05 the
+ * size of the additional data structure is calculated as:
+ *
+ * additional_data =
+ *    8:                    seq_num +
+ *    1:                  tls12_cid +
+ *    2:     DTLSCipherText.version +
+ *    n:                        cid +
+ *    1:                 cid_length +
+ *    2: length_of_DTLSInnerPlaintext
+ *
+ * Per RFC 9146 the size of the add_data structure is calculated as:
+ *
+ * additional_data =
+ *    8:        seq_num_placeholder +
+ *    1:                  tls12_cid +
+ *    1:                 cid_length +
+ *    1:                  tls12_cid +
+ *    2:     DTLSCiphertext.version +
+ *    2:                      epoch +
+ *    6:            sequence_number +
+ *    n:                        cid +
+ *    2: length_of_DTLSInnerPlaintext
+ *
+ */
 static void ssl_extract_add_data_from_record( unsigned char* add_data,
                                               size_t *add_data_len,
                                               mbedtls_record *rec,
                                               mbedtls_ssl_protocol_version
-                                                tls_version,
+                                              tls_version,
                                               size_t taglen )
 {
-    /* Quoting RFC 5246 (TLS 1.2):
+    /* Several types of ciphers have been defined for use with TLS and DTLS,
+     * and the MAC calculations for those ciphers differ slightly. Further
+     * variants were added when the CID functionality was added with RFC 9146.
+     * This implementations also considers the use of a legacy version of the
+     * CID specification published in draft-ietf-tls-dtls-connection-id-05,
+     * which is used in deployments.
+     *
+     * We will distinguish between the non-CID and the CID cases below.
+     *
+     * --- Non-CID cases ---
+     *
+     * Quoting RFC 5246 (TLS 1.2):
      *
      *    additional_data = seq_num + TLSCompressed.type +
      *                      TLSCompressed.version + TLSCompressed.length;
      *
-     * For the CID extension, this is extended as follows
-     * (quoting draft-ietf-tls-dtls-connection-id-05,
-     *  https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05):
-     *
-     *       additional_data = seq_num + DTLSPlaintext.type +
-     *                         DTLSPlaintext.version +
-     *                         cid +
-     *                         cid_length +
-     *                         length_of_DTLSInnerPlaintext;
-     *
      * For TLS 1.3, the record sequence number is dropped from the AAD
      * and encoded within the nonce of the AEAD operation instead.
      * Moreover, the additional data involves the length of the TLS
@@ -427,11 +477,72 @@ static void ssl_extract_add_data_from_record( unsigned char* add_data,
      *
      *     TLSCiphertext.length = TLSInnerPlaintext.length + taglen.
      *
-     */
+     * --- CID cases ---
+     *
+     * RFC 9146 uses a common pattern when constructing the data
+     * passed into a MAC / AEAD cipher.
+     *
+     * Data concatenation for MACs used with block ciphers with
+     * Encrypt-then-MAC Processing (with CID):
+     *
+     *  data = seq_num_placeholder +
+     *         tls12_cid +
+     *         cid_length +
+     *         tls12_cid +
+     *         DTLSCiphertext.version +
+     *         epoch +
+     *         sequence_number +
+     *         cid +
+     *         DTLSCiphertext.length +
+     *         IV +
+     *         ENC(content + padding + padding_length)
+     *
+     * Data concatenation for MACs used with block ciphers (with CID):
+     *
+     *  data =  seq_num_placeholder +
+     *          tls12_cid +
+     *          cid_length +
+     *          tls12_cid +
+     *          DTLSCiphertext.version +
+     *          epoch +
+     *          sequence_number +
+     *          cid +
+     *          length_of_DTLSInnerPlaintext +
+     *          DTLSInnerPlaintext.content +
+     *          DTLSInnerPlaintext.real_type +
+     *          DTLSInnerPlaintext.zeros
+     *
+     * AEAD ciphers use the following additional data calculation (with CIDs):
+     *
+     *     additional_data = seq_num_placeholder +
+     *                tls12_cid +
+     *                cid_length +
+     *                tls12_cid +
+     *                DTLSCiphertext.version +
+     *                epoch +
+     *                sequence_number +
+     *                cid +
+     *                length_of_DTLSInnerPlaintext
+     *
+     * Section 5.3 of draft-ietf-tls-dtls-connection-id-05 (for legacy CID use)
+     * defines the additional data calculation as follows:
+     *
+     *     additional_data = seq_num +
+     *                tls12_cid +
+     *                DTLSCipherText.version +
+     *                cid +
+     *                cid_length +
+     *                length_of_DTLSInnerPlaintext
+    */
 
     unsigned char *cur = add_data;
     size_t ad_len_field = rec->data_len;
 
+#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID) && \
+    MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT == 0
+    const unsigned char seq_num_placeholder[] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
+#endif
+
 #if defined(MBEDTLS_SSL_PROTO_TLS1_3)
     if( tls_version == MBEDTLS_SSL_VERSION_TLS1_3 )
     {
@@ -445,25 +556,78 @@ static void ssl_extract_add_data_from_record( unsigned char* add_data,
     {
         ((void) tls_version);
         ((void) taglen);
-        memcpy( cur, rec->ctr, sizeof( rec->ctr ) );
+
-        cur += sizeof( rec->ctr );
+#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
+
+#if MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT == 0
+        if( rec->cid_len != 0 )
+        {
+            // seq_num_placeholder
+            memcpy( cur, seq_num_placeholder, sizeof(seq_num_placeholder) );
+            cur += sizeof( seq_num_placeholder );
+
+            // tls12_cid type
+            *cur = rec->type;
+            cur++;
+
+            // cid_length
+            *cur = rec->cid_len;
+            cur++;
+        }
+        else
+        {
+            // epoch + sequence number
+            memcpy( cur, rec->ctr, sizeof( rec->ctr ) );
+            cur += sizeof( rec->ctr );
+        }
+#endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT == 0 */
+#else
+        // epoch + sequence number
+        memcpy(cur, rec->ctr, sizeof(rec->ctr));
+        cur += sizeof(rec->ctr);
+#endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
     }
 
+    // type
     *cur = rec->type;
     cur++;
 
+    // version
     memcpy( cur, rec->ver, sizeof( rec->ver ) );
     cur += sizeof( rec->ver );
 
-#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
+#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID) && \
-    if( rec->cid_len != 0 )
+    MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT == 1
+
+    if (rec->cid_len != 0)
     {
-        memcpy( cur, rec->cid, rec->cid_len );
+        // CID
+        memcpy(cur, rec->cid, rec->cid_len);
         cur += rec->cid_len;
 
+        // cid_length
         *cur = rec->cid_len;
         cur++;
 
+        // length of inner plaintext
+        MBEDTLS_PUT_UINT16_BE(ad_len_field, cur, 0);
+        cur += 2;
+    }
+    else
+#elif defined(MBEDTLS_SSL_DTLS_CONNECTION_ID) && \
+    MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT == 0
+
+    if( rec->cid_len != 0 )
+    {
+        // epoch + sequence number
+        memcpy(cur, rec->ctr, sizeof(rec->ctr));
+        cur += sizeof(rec->ctr);
+
+        // CID
+        memcpy( cur, rec->cid, rec->cid_len );
+        cur += rec->cid_len;
+
+        // length of inner plaintext
         MBEDTLS_PUT_UINT16_BE( ad_len_field, cur, 0 );
         cur += 2;
     }
@@ -538,7 +702,14 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
     mbedtls_ssl_mode_t ssl_mode;
     int auth_done = 0;
     unsigned char * data;
-    unsigned char add_data[13 + 1 + MBEDTLS_SSL_CID_OUT_LEN_MAX ];
+    /* For an explanation of the additional data length see
+    * the descrpition of ssl_extract_add_data_from_record().
+    */
+#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
+    unsigned char add_data[23 + MBEDTLS_SSL_CID_OUT_LEN_MAX];
+#else
+    unsigned char add_data[13];
+#endif
     size_t add_data_len;
     size_t post_avail;
 
@@ -1021,13 +1192,7 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
             size_t sign_mac_length = 0;
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
-            /*
+            /* MAC(MAC_write_key, add_data, IV, ENC(content + padding + padding_length))
-             * MAC(MAC_write_key, seq_num +
-             *     TLSCipherText.type +
-             *     TLSCipherText.version +
-             *     length_of( (IV +) ENC(...) ) +
-             *     IV +
-             *     ENC(content + padding + padding_length));
              */
 
             if( post_avail < transform->maclen)
@@ -1133,7 +1298,14 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
     size_t padlen = 0, correct = 1;
 #endif
     unsigned char* data;
-    unsigned char add_data[13 + 1 + MBEDTLS_SSL_CID_IN_LEN_MAX ];
+    /* For an explanation of the additional data length see
+    * the descrpition of ssl_extract_add_data_from_record().
+    */
+#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
+    unsigned char add_data[23 + MBEDTLS_SSL_CID_IN_LEN_MAX];
+#else
+    unsigned char add_data[13];
+#endif
     size_t add_data_len;
 
 #if !defined(MBEDTLS_DEBUG_C)
@@ -3487,7 +3659,7 @@ static int ssl_parse_record_header( mbedtls_ssl_context const *ssl,
     {
         /* Shift pointers to account for record header including CID
          * struct {
-         *   ContentType special_type = tls12_cid;
+         *   ContentType outer_type = tls12_cid;
          *   ProtocolVersion version;
          *   uint16 epoch;
          *   uint48 sequence_number;

library/ssl_tls12_client.c

@@ -235,9 +235,6 @@ static int ssl_write_cid_ext( mbedtls_ssl_context *ssl,
     size_t ext_len;
 
     /*
-     * Quoting draft-ietf-tls-dtls-connection-id-05
-     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05
-     *
      *   struct {
      *      opaque cid<0..2^8-1>;
      *   } ConnectionId;

library/ssl_tls12_server.c

@@ -358,9 +358,6 @@ static int ssl_parse_cid_ext( mbedtls_ssl_context *ssl,
     }
 
     /*
-     * Quoting draft-ietf-tls-dtls-connection-id-05
-     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05
-     *
      *   struct {
      *      opaque cid<0..2^8-1>;
      *   } ConnectionId;
@@ -1748,9 +1745,6 @@ static void ssl_write_cid_ext( mbedtls_ssl_context *ssl,
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, adding CID extension" ) );
 
     /*
-     * Quoting draft-ietf-tls-dtls-connection-id-05
-     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05
-     *
      *   struct {
      *      opaque cid<0..2^8-1>;
      *   } ConnectionId;

tests/scripts/all.sh

@@ -2449,6 +2449,25 @@ component_test_variable_ssl_in_out_buffer_len_CID () {
     tests/compat.sh
 }
 
+component_test_variable_ssl_in_out_buffer_len_CID_legacy () {
+    msg "build: MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH and MBEDTLS_SSL_DTLS_CONNECTION_ID (legacy) enabled (ASan build)"
+    scripts/config.py set MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH
+    scripts/config.py set MBEDTLS_SSL_DTLS_CONNECTION_ID
+    scripts/config.py set MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT 1
+
+    CC=gcc cmake -D CMAKE_BUILD_TYPE:String=Asan .
+    make
+
+    msg "test: MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH and MBEDTLS_SSL_DTLS_CONNECTION_ID"
+    make test
+
+    msg "test: ssl-opt.sh, MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH and MBEDTLS_SSL_DTLS_CONNECTION_ID enabled"
+    tests/ssl-opt.sh
+
+    msg "test: compat.sh, MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH and MBEDTLS_SSL_DTLS_CONNECTION_ID enabled"
+    tests/compat.sh
+}
+
 component_test_ssl_alloc_buffer_and_mfl () {
     msg "build: default config with memory buffer allocator and MFL extension"
     scripts/config.py set MBEDTLS_MEMORY_BUFFER_ALLOC_C

tests/ssl-opt.sh

@@ -440,6 +440,14 @@ requires_max_content_len() {
     requires_config_value_at_least "MBEDTLS_SSL_OUT_CONTENT_LEN" $1
 }
 
+CID_MODE=$( get_config_value_or_default "MBEDTLS_SSL_DTLS_CONNECTION_ID_COMPAT" )
+
+requires_cid_compat() {
+    if [ "$CID_MODE" = "0" ]; then
+        SKIP_NEXT="YES"
+    fi
+}
+
 # skip next test if GnuTLS isn't available
 requires_gnutls() {
     if [ -z "${GNUTLS_AVAILABLE:-}" ]; then
@@ -2386,6 +2394,17 @@ run_test    "Context serialization, client serializes, with CID" \
             -S "Deserializing connection..."
 
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
+requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
+requires_config_enabled MBEDTLS_SSL_DTLS_CONNECTION_ID
+requires_cid_compat
+run_test    "Context serialization, client serializes, with CID (legacy)" \
+            "$P_SRV dtls=1 serialize=0 exchanges=2 cid=1 cid_val=dead" \
+            "$P_CLI dtls=1 serialize=1 exchanges=2 cid=1 cid_val=beef" \
+            0 \
+            -c "Deserializing connection..." \
+            -S "Deserializing connection..."
+
+
 requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
 run_test    "Context serialization, server serializes, CCM" \
             "$P_SRV dtls=1 serialize=1 exchanges=2" \
@@ -2422,6 +2441,16 @@ run_test    "Context serialization, server serializes, with CID" \
             -C "Deserializing connection..." \
             -s "Deserializing connection..."
 
+requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
+requires_config_enabled MBEDTLS_SSL_DTLS_CONNECTION_ID
+requires_cid_compat
+run_test    "Context serialization, server serializes, with CID (legacy)" \
+            "$P_SRV dtls=1 serialize=1 exchanges=2 cid=1 cid_val=dead" \
+            "$P_CLI dtls=1 serialize=0 exchanges=2 cid=1 cid_val=beef" \
+            0 \
+            -C "Deserializing connection..." \
+            -s "Deserializing connection..."
+
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
 requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
 run_test    "Context serialization, both serialize, CCM" \
@@ -2460,6 +2489,17 @@ run_test    "Context serialization, both serialize, with CID" \
             -s "Deserializing connection..."
 
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
+requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
+requires_config_enabled MBEDTLS_SSL_DTLS_CONNECTION_ID
+requires_cid_compat
+run_test    "Context serialization, both serialize, with CID (legacy)" \
+            "$P_SRV dtls=1 serialize=1 exchanges=2 cid=1 cid_val=dead" \
+            "$P_CLI dtls=1 serialize=1 exchanges=2 cid=1 cid_val=beef" \
+            0 \
+            -c "Deserializing connection..." \
+            -s "Deserializing connection..."
+
+
 requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
 run_test    "Context serialization, re-init, client serializes, CCM" \
             "$P_SRV dtls=1 serialize=0 exchanges=2" \
@@ -2497,6 +2537,16 @@ run_test    "Context serialization, re-init, client serializes, with CID" \
             -S "Deserializing connection..."
 
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
+requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
+requires_config_enabled MBEDTLS_SSL_DTLS_CONNECTION_ID
+requires_cid_compat
+run_test    "Context serialization, re-init, client serializes, with CID (legacy)" \
+            "$P_SRV dtls=1 serialize=0 exchanges=2 cid=1 cid_val=dead" \
+            "$P_CLI dtls=1 serialize=2 exchanges=2 cid=1 cid_val=beef" \
+            0 \
+            -c "Deserializing connection..." \
+            -S "Deserializing connection..."
+
 requires_config_enabled MBEDTLS_SSL_CONTEXT_SERIALIZATION
 run_test    "Context serialization, re-init, server serializes, CCM" \
             "$P_SRV dtls=1 serialize=2 exchanges=2" \