code: public static byte[] convertToKeyByteArray(String yourGooglePrivateKeyString)
  	{
  		yourGooglePrivateKeyString = yourGooglePrivateKeyString.replace('-', '+');
  		yourGooglePrivateKeyString = yourGooglePrivateKeyString.replace('_', '/');
  		return Base64.getDecoder().decode(yourGooglePrivateKeyString);
  	}
docs: Converts the given private key as String to an base 64 encoded byte array.
  
  @param yourGooglePrivateKeyString
  the google private key as String
  @return the base 64 encoded byte array.