From 43f564f29d7665e278783ae8ae32161be1ac02db Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Fri, 22 Feb 2019 12:14:02 +0100
Subject: [PATCH] Define MBEDTLS_ECDH_LEGACY_CONTEXT in config.h

Define MBEDTLS_ECDH_LEGACY_CONTEXT in config.h instead of hard-coding
this in ecdh.h so that its absence can be tested. Document it as
experimental so that we reserve the right to change it in the future.
---
 include/mbedtls/check_config.h |  5 +++++
 include/mbedtls/config.h       | 31 ++++++++++++++++++++++++++++++-
 include/mbedtls/ecdh.h         | 12 ------------
 library/version_features.c     |  3 +++
 4 files changed, 38 insertions(+), 13 deletions(-)

diff --git a/include/mbedtls/check_config.h b/include/mbedtls/check_config.h
index 3d47899c7..d4e9e4e8c 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -124,6 +124,11 @@
 #error "MBEDTLS_ECP_RESTARTABLE defined, but it cannot coexist with an alternative ECP implementation"
 #endif
 
+#if defined(MBEDTLS_ECP_RESTARTABLE)           && \
+    ! defined(MBEDTLS_ECDH_LEGACY_CONTEXT)
+#error "MBEDTLS_ECP_RESTARTABLE defined, but not MBEDTLS_ECDH_LEGACY_CONTEXT"
+#endif
+
 #if defined(MBEDTLS_ECDSA_DETERMINISTIC) && !defined(MBEDTLS_HMAC_DRBG_C)
 #error "MBEDTLS_ECDSA_DETERMINISTIC defined, but not all prerequisites"
 #endif
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index e6abf24d5..664fc68dc 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -740,10 +740,39 @@
  *
  * \note  This option only works with the default software implementation of
  *        elliptic curve functionality. It is incompatible with
- *        MBEDTLS_ECP_ALT, MBEDTLS_ECDH_XXX_ALT and MBEDTLS_ECDSA_XXX_ALT.
+ *        MBEDTLS_ECP_ALT, MBEDTLS_ECDH_XXX_ALT, MBEDTLS_ECDSA_XXX_ALT
+ *        and MBEDTLS_ECDH_LEGACY_CONTEXT.
  */
 //#define MBEDTLS_ECP_RESTARTABLE
 
+/**
+ * \def MBEDTLS_ECDH_LEGACY_CONTEXT
+ *
+ * Use a backward compatible ECDH context.
+ *
+ * Mbed TLS supports two formats for ECDH contexts (#mbedtls_ecdh_context
+ * defined in `ecdh.h`). For most applications, the choice of format makes
+ * no difference, since all library functions can work with either format,
+ * except that the new format is incompatible with MBEDTLS_ECP_RESTARTABLE.
+
+ * The new format used when this option is disabled is smaller
+ * (56 bytes on a 32-bit platform). In future versions of the library, it
+ * will support alternative implementations of ECDH operations.
+ * The new format is incompatible with applications that access
+ * context fields directly and with restartable ECP operations.
+ *
+ * Define this macro if you enable MBEDTLS_ECP_RESTARTABLE or if you
+ * want to access ECDH context fields directly. Otherwise you should
+ * comment out this macro definition.
+ *
+ * This option has no effect if #MBEDTLS_ECDH_C is not enabled.
+ *
+ * \note This configuration option is experimental. Future versions of the
+ *       library may modify the way the ECDH context layout is configured
+ *       and may modify the layout of the new context type.
+ */
+#define MBEDTLS_ECDH_LEGACY_CONTEXT
+
 /**
  * \def MBEDTLS_ECDSA_DETERMINISTIC
  *
diff --git a/include/mbedtls/ecdh.h b/include/mbedtls/ecdh.h
index 4479a1d46..384c3dc07 100644
--- a/include/mbedtls/ecdh.h
+++ b/include/mbedtls/ecdh.h
@@ -42,18 +42,6 @@
 
 #include "ecp.h"
 
-/*
- * Use a backward compatible ECDH context.
- *
- * This flag is always enabled for now and future versions might add a
- * configuration option that conditionally undefines this flag.
- * The configuration option in question may have a different name.
- *
- * Features undefining this flag, must have a warning in their description in
- * config.h stating that the feature breaks backward compatibility.
- */
-#define MBEDTLS_ECDH_LEGACY_CONTEXT
-
 #ifdef __cplusplus
 extern "C" {
 #endif
diff --git a/library/version_features.c b/library/version_features.c
index 61094d4ed..bbc365b92 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -348,6 +348,9 @@ static const char *features[] = {
 #if defined(MBEDTLS_ECP_RESTARTABLE)
     "MBEDTLS_ECP_RESTARTABLE",
 #endif /* MBEDTLS_ECP_RESTARTABLE */
+#if defined(MBEDTLS_ECDH_LEGACY_CONTEXT)
+    "MBEDTLS_ECDH_LEGACY_CONTEXT",
+#endif /* MBEDTLS_ECDH_LEGACY_CONTEXT */
 #if defined(MBEDTLS_ECDSA_DETERMINISTIC)
     "MBEDTLS_ECDSA_DETERMINISTIC",
 #endif /* MBEDTLS_ECDSA_DETERMINISTIC */