add shared secret derivation and test for ECDH.

dh.go

@@ -0,0 +1,70 @@
+// Copyright (C) 2017. See AUTHORS.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package openssl
+
+// #include "shim.h"
+import "C"
+import (
+	"errors"
+	"unsafe"
+)
+
+// DeriveSharedSecret derives a shared secret using a private key and a peer's
+// public key.
+// The specific algorithm that is used depends on the types of the
+// keys, but it is most commonly a variant of Diffie-Hellman.
+func DeriveSharedSecret(private PrivateKey, public PublicKey) ([]byte, error) {
+	// Create context for the shared secret derivation
+	dhCtx := C.EVP_PKEY_CTX_new(private.evpPKey(), nil)
+	if dhCtx == nil {
+		return nil, errors.New("failed creating shared secret derivation context")
+	}
+	defer C.EVP_PKEY_CTX_free(dhCtx)
+
+	// Initialize the context
+	rc := C.EVP_PKEY_derive_init(dhCtx)
+	if rc != 1 {
+		return nil, errors.New("failed initializing shared secret derivation context")
+	}
+
+	// Provide the peer's public key
+	rc = C.EVP_PKEY_derive_set_peer(dhCtx, public.evpPKey())
+	if rc != 1 {
+		return nil, errors.New("failed adding peer public key to context")
+	}
+
+	// Determine how large of a buffer we need for the shared secret
+	var buffLen C.size_t
+	rc = C.EVP_PKEY_derive(dhCtx, nil, &buffLen)
+	if rc != 1 {
+		return nil, errors.New("failed determining shared secret length")
+	}
+
+	// Allocate a buffer
+	buffer := C.X_OPENSSL_malloc(buffLen)
+	if buffer == nil {
+		return nil, errors.New("failed allocating buffer for shared secret")
+	}
+	defer C.X_OPENSSL_free(buffer)
+
+	// Derive the shared secret
+	rc = C.EVP_PKEY_derive(dhCtx, (*C.uchar)(buffer), &buffLen)
+	if rc != 1 {
+		return nil, errors.New("failed deriving the shared secret")
+	}
+
+	secret := C.GoBytes(unsafe.Pointer(buffer), C.int(buffLen))
+	return secret, nil
+}

dh_test.go

@@ -0,0 +1,46 @@
+// Copyright (C) 2017. See AUTHORS.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package openssl
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestECDH(t *testing.T) {
+	t.Parallel()
+
+	myKey, err := GenerateECKey(Prime256v1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	peerKey, err := GenerateECKey(Prime256v1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mySecret, err := DeriveSharedSecret(myKey, peerKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	theirSecret, err := DeriveSharedSecret(peerKey, myKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if bytes.Compare(mySecret, theirSecret) != 0 {
+		t.Fatal("shared secrets are different")
+	}
+}

shim.c

@@ -312,6 +312,10 @@ int X_shim_init() {
 	return 0;
 }
 
+void * X_OPENSSL_malloc(size_t size) {
+	return OPENSSL_malloc(size);
+}
+
 void X_OPENSSL_free(void *ref) {
 	OPENSSL_free(ref);
 }

shim.h

@@ -42,6 +42,7 @@ extern int X_shim_init();
 
 /* Library methods */
 extern void X_OPENSSL_free(void *ref);
+extern void *X_OPENSSL_malloc(size_t size);
 
 /* SSL methods */
 extern long X_SSL_set_options(SSL* ssl, long options);