Fix Secure Session serialization format (#658)

secure_session_save() saves short serialized Secure Session state.

That is, it writes "is_client" as 4-byte value while its size in
SESSION_CTX_SERIZALIZED_SIZE is computed as sizeof(bool) which is 1 byte
on most platforms. This causes 3 least significant bytes of "in_seq"
being missing from the serialized data.

Correct size of Secure Session state is 60 bytes, we report only 57.

This issue is not detected by unit tests of JavaThemis -- the only
high-level wrapper supporting this interface -- which works only because
the allocated memory for the output array is slightly bigger than
requested 57 bytes, extra 3 bytes written past-the-end remain there
in RAM and the unit test generally works, if the data stays where it is.
However, sometimes garbage collection occurs at the right moment and the
issue manifests itself as a failing Secure Session test.

Now the output size is reported correctly and all Secure Session data is
written within the allocated bounds.

CHANGELOG.md

@@ -38,6 +38,11 @@ _Code:_
 
       provide Seal mode API that is safe to use with passphrases ([#577](https://github.com/cossacklabs/themis/pull/577)).
 
+  - **Secure Session**
+
+    - Fixed serialization issue in `secure_session_save()` and `secure_session_load()` methods
+      ([#658](https://github.com/cossacklabs/themis/pull/658).
+
   - **Breaking changes**
 
     - <a id="0.13.0-drop-0.9.6-compat">Secure Cell compatibility with Themis 0.9.6 is now disabled by default ([#614](https://github.com/cossacklabs/themis/pull/614)).
@@ -70,6 +75,8 @@ _Code:_
     ([#639](https://github.com/cossacklabs/themis/pull/639)).
   - Updated embedded BoringSSL to the latest version
     ([#643](https://github.com/cossacklabs/themis/pull/643)).
+  - Fixed broken `SecureSession#save` and `SecureSession#restore` methods
+    ([#658](https://github.com/cossacklabs/themis/pull/658).
 
   - **Breaking changes**
 
@@ -390,6 +397,8 @@ _Code:_
     ([#633](https://github.com/cossacklabs/themis/pull/633)).
   - Kotlin is now officially supported language for JavaThemis
     ([#637](https://github.com/cossacklabs/themis/pull/637).
+  - Fixed broken `SecureSession#save` and `SecureSession#restore` methods
+    ([#658](https://github.com/cossacklabs/themis/pull/658).
 
   - Secure Cell API updates:
 

src/themis/secure_session_serialize.c

@@ -25,9 +25,35 @@
 
 #define THEMIS_SESSION_CONTEXT_TAG "TSSC"
 
-#define SESSION_CTX_SERIZALIZED_SIZE(ctx)                                                     \
-    (sizeof((ctx)->session_id) + sizeof((ctx)->is_client) + sizeof((ctx)->session_master_key) \
-     + sizeof((ctx)->out_seq) + sizeof((ctx)->in_seq))
+/*
+ * Data layout of serialized Secure Session state looks like this:
+ *
+ *     0        1        2        3        4        5        6        7
+ * +--------+--------+--------+--------+--------+--------+--------+--------+
+ * |   'T'  |   'S'  |   'S'  |   'C'  |               size                | Soter Container header
+ * +--------+--------+--------+--------+--------+--------+--------+--------+
+ * |                CRC                |
+ * +--------+--------+--------+--------+
+ *
+ * +--------+--------+--------+--------+--------+--------+--------+--------+
+ * |             session ID            |             is_client             | Secure Session context
+ * +--------+--------+--------+--------+--------+--------+--------+--------+
+ * |                       Secure Session master key                       |
+ * + - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+
+ * |                                                                       |
+ * + - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+
+ * |                                                                       |
+ * + - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+ - - - -+
+ * |                                                                       |
+ * +--------+--------+--------+--------+--------+--------+--------+--------+
+ * |             out seqnum            |             in seqnum             |
+ * +--------+--------+--------+--------+--------+--------+--------+--------+
+ *
+ * All values are unsigned and encoded as big-endian.
+ */
+
+#define SESSION_CTX_SERIALIZED_SIZE \
+    (2 * sizeof(uint32_t) + SESSION_MASTER_KEY_LENGTH + 2 * sizeof(uint32_t))
 
 themis_status_t secure_session_save(const secure_session_t* session_ctx, void* out, size_t* out_length)
 {
@@ -45,15 +71,14 @@ themis_status_t secure_session_save(const secure_session_t* session_ctx, void* o
 
     /* | session_id | is_client | master_key | out_seq | in_seq | */
 
-    if ((!out)
-        || (*out_length < (sizeof(soter_container_hdr_t) + SESSION_CTX_SERIZALIZED_SIZE(session_ctx)))) {
-        *out_length = (sizeof(soter_container_hdr_t) + SESSION_CTX_SERIZALIZED_SIZE(session_ctx));
+    if ((!out) || (*out_length < (sizeof(soter_container_hdr_t) + SESSION_CTX_SERIALIZED_SIZE))) {
+        *out_length = (sizeof(soter_container_hdr_t) + SESSION_CTX_SERIALIZED_SIZE);
         return THEMIS_BUFFER_TOO_SMALL;
     }
 
-    *out_length = (sizeof(soter_container_hdr_t) + SESSION_CTX_SERIZALIZED_SIZE(session_ctx));
+    *out_length = (sizeof(soter_container_hdr_t) + SESSION_CTX_SERIALIZED_SIZE);
 
-    soter_container_set_data_size(hdr, SESSION_CTX_SERIZALIZED_SIZE(session_ctx));
+    soter_container_set_data_size(hdr, SESSION_CTX_SERIALIZED_SIZE);
     memcpy(hdr->tag, THEMIS_SESSION_CONTEXT_TAG, SOTER_CONTAINER_TAG_LENGTH);
 
     curr = (uint32_t*)soter_container_data(hdr);