This is a Ruby library for parsing SSH public keys and certificates.
gem install ssh_data
require "ssh_data"
key_data = File.read("~/.ssh/id_rsa.pub")
key = SSHData::PublicKey.parse_openssh(key_data)
#=> <SSHData::PublicKey::RSA>
cert_data = = File.read("~/.ssh/id_rsa-cert.pub")
cert = SSHData::Certificate.parse_openssh(cert_data)
#=> <SSHData::PublicKey::Certificate>
cert.key_id
#=> "mastahyeti"
cert.public_key
#=> <SSHData::PublicKey::RSA>