From afa8f71700bb056fb5e8cdbe02979d722dc34dfd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?=
 <manuel.pegourie-gonnard@arm.com>
Date: Tue, 28 May 2019 12:28:17 +0200
Subject: [PATCH] Add new config MBEDTLS_SSL_CONTEXT_SERIALIZATION

This is enabled by default as we generally enable things by default unless
there's a reason not to (experimental, deprecated, security risk).

We need a compile-time option because, even though the functions themselves
can be easily garbage-collected by the linker, implementing them will require
saving 64 bytes of Client/ServerHello.random values after the handshake, that
would otherwise not be needed, and people who don't need this feature
shouldn't have to pay the price of increased RAM usage.
---
 include/mbedtls/config.h    | 27 +++++++++++++++++++++++++++
 library/version_features.c  |  3 +++
 programs/ssl/query_config.c |  8 ++++++++
 3 files changed, 38 insertions(+)

diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 8e00fc477a..4e78493254 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1403,6 +1403,33 @@
  */
 //#define MBEDTLS_SSL_ASYNC_PRIVATE
 
+/**
+ * \def MBEDTLS_SSL_CONTEXT_SERIALIZATION
+ *
+ * Enable the APIs for serialization of a full SSL context:
+ * mbedtls_ssl_context_save() and mbedtls_ssl_context_load().
+ *
+ * This pair of functions allows one side of a connection to serialize the
+ * context associated with the connection, then free or re-use that context
+ * while the serialized state is persisted elsewhere, and finally deserialize
+ * that state to a live context for resuming read/write operations on the
+ * connection, in a way that's transparent to the peer, since from a protocol
+ * point of view, the state of the connection is unaffected.
+ *
+ * Note: this is distinct from TLS session resumption, which is part of the
+ * protocol and fully visible by the peer. TLS session resumption enables
+ * establishing new connections associated to a saved session with shorter,
+ * lighter handshakes, while context serialization is a local optimisation in
+ * handling a single, potentially long-lived connection.
+ *
+ * Enabling these APIs makes some SSL structures larger, as 64 extra bytes are
+ * saved after the handshake to allow for more efficient serialization, so if
+ * you don't need this feature you'll save RAM by disabling it.
+ *
+ * Comment to disable the context serialization APIs.
+ */
+#define MBEDTLS_SSL_CONTEXT_SERIALIZATION
+
 /**
  * \def MBEDTLS_SSL_DEBUG_ALL
  *
diff --git a/library/version_features.c b/library/version_features.c
index e83899d0a7..cc47dacc94 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -459,6 +459,9 @@ static const char * const features[] = {
 #if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
     "MBEDTLS_SSL_ASYNC_PRIVATE",
 #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
+#if defined(MBEDTLS_SSL_CONTEXT_SERIALIZATION)
+    "MBEDTLS_SSL_CONTEXT_SERIALIZATION",
+#endif /* MBEDTLS_SSL_CONTEXT_SERIALIZATION */
 #if defined(MBEDTLS_SSL_DEBUG_ALL)
     "MBEDTLS_SSL_DEBUG_ALL",
 #endif /* MBEDTLS_SSL_DEBUG_ALL */
diff --git a/programs/ssl/query_config.c b/programs/ssl/query_config.c
index c6d19bf093..361ec00004 100644
--- a/programs/ssl/query_config.c
+++ b/programs/ssl/query_config.c
@@ -1266,6 +1266,14 @@ int query_config( const char *config )
     }
 #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
 
+#if defined(MBEDTLS_SSL_CONTEXT_SERIALIZATION)
+    if( strcmp( "MBEDTLS_SSL_CONTEXT_SERIALIZATION", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_SSL_CONTEXT_SERIALIZATION );
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CONTEXT_SERIALIZATION */
+
 #if defined(MBEDTLS_SSL_DEBUG_ALL)
     if( strcmp( "MBEDTLS_SSL_DEBUG_ALL", config ) == 0 )
     {