ssh_key.rb

class SSHKey

  attr_reader :rsa

  def initialize(args={})
    source_file = args.fetch(:file) { fail ArgumentError, "Filename is required" }
    source = if source_file.respond_to?(:read)
               source_file.read()
             else
               File.open(source_file) do |file|
                 file.read()
               end
             end
    @rsa = load_ssh_rsa_key(source)
    @rsa ||= OpenSSL::PKey.read(source)
  end

  private

  def load_ssh_rsa_key(source)
    fail "DSA is not supported" if source =~ /^ssh-dsa/
    return nil unless source =~ /^ssh-rsa/

    keydata = decode_key(source)

    rsakey = OpenSSL::PKey::RSA.new
    
    skip_key_type_length = parse_data(keydata.slice!(0, 4))
    keydata.slice!(0, skip_key_type_length)

    exponent_length = parse_data(keydata.slice!(0, 4))
    rsakey.e = parse_data(keydata.slice!(0, exponent_length))

    modulus_length = parse_data(keydata.slice!(0, 4))
    rsakey.n = parse_data(keydata.slice!(0, modulus_length))

    @rsa = rsakey
  end

  def decode_key(source)
    base64 = source.chomp.split[1]
    keydata = base64.unpack("m").first
    keydata
  end

  def parse_data(data)
    data.bytes.inject(0) do |sum, byte|
      (sum << 8) + byte
    end
  end

end