@Override
  public KmsClient withCredentials(String credentials) throws GeneralSecurityException {
    try {
      credentialFileContents = Files.readAllBytes(Paths.get(credentials));
      return this;
    } catch (IOException e) {
      throw new GeneralSecurityException(e);
    }
  }
A client for testing.

<p>The client supports all key uris which start with "tinkey-test-kms-client://" followed by a
JSON-encoded AEAD keyset. It will use this Aead when "getAead" is called. As credentials, it must
be given a file which starts whose contents are "VALID CREDENTIALS".
tools/tinkey/src/main/java/com/google/crypto/tink/tinkey/TinkeyTestKmsClient.java