OSCrypt for POSIX uses libsecret to store a randomised encryption key.

components/os_crypt/key_storage_mock.cc

Index: components/os_crypt/key_storage_mock.cc
diff --git a/components/os_crypt/key_storage_mock.cc b/components/os_crypt/key_storage_mock.cc
new file mode 100644
index 0000000000000000000000000000000000000000..b485b60f4c4ea888e391ff9f843bc17ed6f7789e
--- /dev/null
+++ b/components/os_crypt/key_storage_mock.cc
@@ -0,0 +1,26 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/os_crypt/key_storage_mock.h"
+
+#include "base/base64.h"
+#include "base/rand_util.h"
+
+std::string KeyStorageMock::GetKey() {
+  if (key_.empty())
+    base::Base64Encode(base::RandBytesAsString(16), &key_);
+  return key_;
+}
+
+bool KeyStorageMock::Init() {
+  return true;
+}
+
+void KeyStorageMock::ResetTo(base::StringPiece new_key) {
+  key_ = new_key.as_string();
+}
+
+std::string* KeyStorageMock::GetKeyPtr() {
+  return &key_;
+}