scan an authorized_keys file and print key fingerprints

keyscan.rb

#!/usr/bin/env ruby
require 'tempfile'

KEYGEN = "/usr/bin/ssh-keygen"
KEYSFILE = "authorized_keys"

keys = []

File.open(KEYSFILE).each_line do |l|
  next if (l =~ /\A\s*\Z/) # blank line
  next if (l =~ /\A\s*#/) # comment line
  match = l.match(/\A(ssh-[rdsa]{3})\s+(\S+=)\s+(.*)\Z/) # XXX: won't parse keys with options
  raise "Couldn't parse: #{l.inspect}" unless match
  keys << {:type => match[1], :key => match[2], :comment => match[3], :raw => l}
end

keys.each do |key|
  f = Tempfile.open($0) 
  begin
    f << key[:raw]
    f.close

    res = `#{KEYGEN} -lf #{f.path}`
    raise "keygen failed on " + key[:key] + " (" + key[:comment] + ")" unless res
    (bits, fingerprint, path, type) = res.split

    puts key[:comment] + "\t" + fingerprint + "\t" + bits+"/"+type
  ensure
    f.close
    f.unlink
  end
end