Add support for alternative ECJPAKE implementation

This commit allows users to provide alternative implementations of the
ECJPAKE interface through the configuration option MBEDTLS_ECJPAKE_ALT.
When set, the user must add `ecjpake_alt.h` declaring the same
interface as `ecjpake.h`, as well as add some compilation unit which
implements the functionality. This is in line with the preexisting
support for alternative implementations of other modules.

ChangeLog

@@ -34,6 +34,8 @@ Features
      The following functions from the ECDH module can be replaced
      with an alternative implementation:
      mbedtls_ecdh_gen_public() and mbedtls_ecdh_compute_shared().
+   * Add support for alternative implementation for ECJPAKE, controlled by
+     new configuration flag MBEDTLS_ECJPAKE_ALT.
 
 API Changes
    * Extend RSA interface by multiple functions allowing structure-

include/mbedtls/config.h

@@ -271,6 +271,7 @@
 //#define MBEDTLS_CMAC_ALT
 //#define MBEDTLS_DES_ALT
 //#define MBEDTLS_DHM_ALT
+//#define MBEDTLS_ECJPAKE_ALT
 //#define MBEDTLS_GCM_ALT
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT

include/mbedtls/ecjpake.h

@@ -44,6 +44,8 @@
 #include "ecp.h"
 #include "md.h"
 
+#if !defined(MBEDTLS_ECJPAKE_ALT)
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -223,17 +225,31 @@ int mbedtls_ecjpake_derive_secret( mbedtls_ecjpake_context *ctx,
  */
 void mbedtls_ecjpake_free( mbedtls_ecjpake_context *ctx );
 
+#ifdef __cplusplus
+}
+#endif
+
+#else  /* MBEDTLS_ECJPAKE_ALT */
+#include "ecjpake_alt.h"
+#endif /* MBEDTLS_ECJPAKE_ALT */
+
 #if defined(MBEDTLS_SELF_TEST)
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * \brief          Checkup routine
  *
  * \return         0 if successful, or 1 if a test failed
  */
 int mbedtls_ecjpake_self_test( int verbose );
-#endif
 
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #endif /* ecjpake.h */

library/ecjpake.c

@@ -36,6 +36,8 @@
 
 #include <string.h>
 
+#if !defined(MBEDTLS_ECJPAKE_ALT)
+
 /*
  * Convert a mbedtls_ecjpake_role to identifier string
  */
@@ -764,6 +766,7 @@ cleanup:
 #undef ID_MINE
 #undef ID_PEER
 
+#endif /* ! MBEDTLS_ECJPAKE_ALT */
 
 #if defined(MBEDTLS_SELF_TEST)
 

library/version_features.c

@@ -108,6 +108,9 @@ static const char *features[] = {
 #if defined(MBEDTLS_DHM_ALT)
     "MBEDTLS_DHM_ALT",
 #endif /* MBEDTLS_DHM_ALT */
+#if defined(MBEDTLS_ECJPAKE_ALT)
+    "MBEDTLS_ECJPAKE_ALT",
+#endif /* MBEDTLS_ECJPAKE_ALT */
 #if defined(MBEDTLS_GCM_ALT)
     "MBEDTLS_GCM_ALT",
 #endif /* MBEDTLS_GCM_ALT */