Body hashes (RFC 6376 §3.4.3, §3.4.4, §3.7), computed while the body streams by, for any number of signatures at once.

body = Sovite.DKIM.Body.new([{:relaxed, :sha256, nil}, {:simple, :sha256, 100}])
body = Sovite.DKIM.Body.update(body, chunk)
%{{:relaxed, :sha256, nil} => {hash, length}} = Sovite.DKIM.Body.finish(body)

A spec is the body canonicalization, the hash, and the l= length limit (nil for the whole body). Each canonicalization is done once, however many specs use it. finish/1 returns each hash with the length of the whole canonicalized body, so a verifier can tell an l= larger than the body.

The body is the data after the empty line that ends the header section, with CRLF line endings.