From 424fda5d7bbe86c351efd1d45a29b1e6b2a3980c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= <mpg@elzevir.fr>
Date: Mon, 11 Feb 2013 22:05:42 +0100
Subject: [PATCH] Add ecdh_calc_secret()

---
 include/polarssl/ecdh.h               | 13 +++++++++++++
 library/ecdh.c                        | 17 +++++++++++++++++
 tests/suites/test_suite_ecdh.function |  6 +++++-
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/include/polarssl/ecdh.h b/include/polarssl/ecdh.h
index 67d1df512..7f6f4cb28 100644
--- a/include/polarssl/ecdh.h
+++ b/include/polarssl/ecdh.h
@@ -149,6 +149,19 @@ int ecdh_make_public( ecdh_context *ctx, size_t *olen,
 int ecdh_read_public( ecdh_context *ctx,
                       const unsigned char *buf, size_t blen );
 
+/**
+ * \brief           Derive and export the shared secret
+ *
+ * \param ctx       ECDH context
+ * \param olen      number of bytes written
+ * \param buf       destination buffer
+ * \param blen      buffer length
+ *
+ * \return          0 if successful, or an POLARSSL_ERR_ECP_XXX error code
+ */
+int ecdh_calc_secret( ecdh_context *ctx, size_t *olen,
+                      unsigned char *buf, size_t blen );
+
 /**
  * \brief          Checkup routine
  *
diff --git a/library/ecdh.c b/library/ecdh.c
index dc585f47e..c04d34c42 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -186,6 +186,23 @@ int ecdh_read_public( ecdh_context *ctx,
     return ecp_tls_read_point( &ctx->grp, &ctx->Qp, &buf, blen );
 }
 
+/*
+ * Derive and export the shared secret
+ */
+int ecdh_calc_secret( ecdh_context *ctx, size_t *olen,
+                      unsigned char *buf, size_t blen )
+{
+    int ret;
+
+    if( ( ret = ecdh_compute_shared( &ctx->grp, &ctx->z, &ctx->Qp, &ctx->d ) )
+                != 0 )
+        return( ret );
+
+    *olen = mpi_size( &ctx->z );
+    return mpi_write_binary( &ctx->z, buf, blen );
+}
+
+
 #if defined(POLARSSL_SELF_TEST)
 
 /*
diff --git a/tests/suites/test_suite_ecdh.function b/tests/suites/test_suite_ecdh.function
index 6f2d39937..105f99ce0 100644
--- a/tests/suites/test_suite_ecdh.function
+++ b/tests/suites/test_suite_ecdh.function
@@ -99,11 +99,15 @@ ecdh_exchange:id
                                    &rnd_pseudo_rand, &rnd_info ) == 0 );
     TEST_ASSERT( ecdh_read_params( &cli, &vbuf, buf + len ) == 0 );
 
-    memset( buf, 0x00, sizeof( buf ) ); vbuf = buf;
+    memset( buf, 0x00, sizeof( buf ) );
     TEST_ASSERT( ecdh_make_public( &cli, &len, buf, 1000,
                                    &rnd_pseudo_rand, &rnd_info ) == 0 );
     TEST_ASSERT( ecdh_read_public( &srv, buf, len ) == 0 );
 
+    TEST_ASSERT( ecdh_calc_secret( &srv, &len, buf, 1000 ) == 0 );
+    TEST_ASSERT( ecdh_calc_secret( &cli, &len, buf, 1000 ) == 0 );
+    TEST_ASSERT( mpi_cmp_mpi( &srv.z, &cli.z ) == 0 );
+
     ecdh_free( &srv );
     ecdh_free( &cli );
 }