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

components/os_crypt/os_crypt.h

 // Copyright 2014 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_OS_CRYPT_H_
 #define COMPONENTS_OS_CRYPT_OS_CRYPT_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "base/strings/string16.h"
 #include "build/build_config.h"
 
+#if defined(USE_LIBSECRET)
+#include "components/os_crypt/key_storage_linux.h"
+#endif
+
 // The OSCrypt class gives access to simple encryption and decryption of
 // strings. Note that on Mac, access to the system Keychain is required and
 // these calls can block the current thread to collect user input.
 class OSCrypt {
  public:
   // Encrypt a string16. The output (second argument) is really an array of
   // bytes, but we're passing it back as a std::string.
   static bool EncryptString16(const base::string16& plaintext,
                               std::string* ciphertext);
 
@@ skipping 16 matching lines @@
 #if defined(OS_MACOSX)
   // For unit testing purposes we instruct the Encryptor to use a mock Keychain
   // on the Mac. The default is to use the real Keychain.
   static void UseMockKeychain(bool use_mock);
 #endif
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(OSCrypt);
 };
 
+#if defined(USE_LIBSECRET) && defined(UNIT_TEST)
+// For unit testing purposes, inject methods to be used.
+// |get_key_storage_mock| provides the desired |KeyStorage| implementation.
+// If the provider returns |nullptr|, a hardcoded password will be used.
+// |get_password_v11_mock| provides a password to derive the encryption key from
+// If |use| is true, the provided values will be injected. Otherwise, the
+// real implementation is restored.
+void UseMockKeyStorageForTesting(bool use,

[Lei Zhang, 2016/05/19 22:45:38]

You probably don't need |use| - you can indicate use = false with both function
pointers being nullptr.

[cfroussios, 2016/05/20 16:44:52]

Done.

+                                 KeyStorageLinux* (*get_key_storage_mock)(),
+                                 std::string* (*get_password_v11_mock)());
+#endif  // defined(USE_LIBSECRET) && defined(UNIT_TEST)
+
 #endif  // COMPONENTS_OS_CRYPT_OS_CRYPT_H_