You might take a look at `Gem::TUF::PublicKey` and `Gem::TUF::Verifier` from the TUF branch on rubygems itself:

https://github.com/square/rubygems/blob/tuf/lib/rubygems/tuf/public_key.rb
https://github.com/square/rubygems/blob/tuf/lib/rubygems/tuf/verifier.rb

Unfortunately we never got the two codebases integrated (among other things rubygems.org doesn't work on the latest rubygems :cry:)