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

components/os_crypt/key_storage_mock.h

Index: components/os_crypt/key_storage_mock.h
diff --git a/components/os_crypt/key_storage_mock.h b/components/os_crypt/key_storage_mock.h
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+++ b/components/os_crypt/key_storage_mock.h
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+// 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.
+
+#ifndef COMPONENTS_OS_CRYPT_KEY_STORAGE_MOCK_H_
+#define COMPONENTS_OS_CRYPT_KEY_STORAGE_MOCK_H_
+
+#include <string>
+
+#include "base/macros.h"
+#include "base/strings/string_piece.h"
+
+#include "components/os_crypt/key_storage_linux.h"
+
+// Holds and serves a password from memory.
+class KeyStorageMock : public KeyStorageLinux {
+ public:
+  KeyStorageMock() = default;
+  ~KeyStorageMock() override = default;
+
+  // KeyStorageLinux
+  std::string GetKey() override;
+
+  // Set the password that |KeyStorageMock| holds
+  void ResetTo(base::StringPiece new_key);
+
+  // Get a pointer to the stored password. |KeyStorageMock| owns the pointer
+  std::string* GetKeyPtr();
+
+ protected:
+  // KeyStorageLinux
+  bool Init() override;
+
+ private:
+  std::string key_;
+
+  DISALLOW_COPY_AND_ASSIGN(KeyStorageMock);
+};
+
+#endif  // COMPONENTS_OS_CRYPT_KEY_STORAGE_MOCK_H_