def delete(key)
      raise BlankKeyError if key.blank?
      @backend.transaction{
        timestamp!(:last_modified)
        old_value = @backend[:user].delete(encrypt(key))
        raise KeyNotFoundError.new(key) unless old_value
        EntryMapper.from_json(decrypt(old_value))
      }
    end
def delete(key)
      raise BlankKeyError if key.blank?
      @backend.transaction{
        timestamp!(:last_modified)
        old_value = @backend[:user].delete(encrypt(key))
        raise KeyNotFoundError.new(key) unless old_value
        EntryMapper.from_json(decrypt(old_value))
      }
    end
Delete the value that is stored under key and return it