Punycode (RFC 3492): encodes a string of Unicode code points as ASCII letters, digits, and hyphens, for the A-labels of internationalized domain names (xn-- followed by the encoding).

iex> Sovite.IDNA.Punycode.encode("bücher")
{:ok, "bcher-kva"}
iex> Sovite.IDNA.Punycode.decode("bcher-kva")
{:ok, "bücher"}

Both directions work on whole labels and never raise: invalid input returns :error. Case is kept as given; the IDNA rules decide what is valid.