def truncated_key(key)
      digest = digest_class.hexdigest(key)
      "#{key[0, prefix_length(digest)]}#{TRUNCATED_KEY_SEPARATOR}#{digest}"
    end
#
Produces a truncated key, if the raw key is longer than the maximum allowed
length.  The truncated key is produced by generating a hex digest
of the key, and appending that to a truncated section of the key.
#