@spec header(String.t(), algorithm()) :: String.t()

Canonicalizes one header field, given as its raw text with the final CRLF.

iex> Sovite.DKIM.Canon.header("SubJect :  Hello \r\n  world  \r\n", :relaxed)
"subject:Hello world\r\n"

@spec select([{String.t() | nil, String.t()}], [String.t()]) :: [String.t()]

Selects the fields named in names (a signature's h= tag), as §5.4.2 says: each occurrence of a name takes the next instance from the bottom of the header; a name with no instance left adds nothing.

fields are as Sovite.Message.Headers.parse/1 returns them.

@spec signed_data([String.t()], String.t(), algorithm()) :: iodata()

The data that is hashed and signed: the selected fields, canonicalized, then the signature field itself with its b= value emptied and no final CRLF.