email.md

Hi,

Based on the code, docs, and an email on this list (http://bit.ly/oSLiox) I’ve started implementing my own certificate verification code, but my experience with OpenSSL is very limited and I’ve run into a few snags so I’d like some feedback on this code.

The code (and a prettier version of this email) is at: https://gist.github.com/1151454

The snags are:

• Adding a certificate to a OpenSSL::X509::Store which is already in the chain will raise an exception. This is something that can happen quite easily, because:

  • some servers send the same certificate multiple times (cough gmail cough).
  • servers may also send the root certificate, which is already in the store because I initialize it with a list of the root CA certificates.
  • it's impossible to ask the store if a certificate is already in the chain.

  So am I doing this completely wrong? Right now I assume I have the root certificate and ignore duplicates by keeping a reference to the last seen certificate, but without these assumptions/checks the only way I see to do it is by rescueing the exception :'(

• Is there an existing way to get the hostname that the Connection might have been initialized with? As far as I could tell you can only get the actual address (ip/port)

Cheers, Eloy

exception-based-example.rb

        def ssl_verify_peer(cert_string)
          @last_seen_cert = OpenSSL::X509::Certificate.new(cert_string)
          if ca_store.verify(@last_seen_cert)
            begin
              ca_store.add_cert(@last_seen_cert)
            rescue OpenSSL::X509::StoreError => e
              # This is so lame, but there appears to be no way to check this in a simple way.
              #
              # TODO It _might_ be possible to do this with the C api.
              raise e unless e.message == 'cert already in hash table'
            end
            true
          else
            fail OpenSSL::OpenSSLError.new("unable to verify the server certificate of `#{@account.host}'")
            false
          end
        end

gistfile1.rb

require 'rubygems'
require 'eventmachine'

require 'openssl'

class OpenSSL::X509::Certificate
  def ==(other)
    other.respond_to?(:to_pem) && to_pem == other.to_pem
  end

  # A serial *must* be unique for each certificate. Self-signed certificates,
  # and thus root CA certificates, have the same `issuer' as `subject'.
  def top_level?
    serial == serial && issuer.to_s == subject.to_s
  end
  alias_method :root?, :top_level?
  alias_method :self_signed?, :top_level?
end

# Verifies that the peer certificate is a valid chained certificate. That is,
# it's signed by a root CA or a CA signed by a root CA.
#
# This module will also perform hostname verification against the server’s
# certificate, but _only_ if an instance variable called +@hostname+ exists.
module SSLCertificateVerification
  class << self
    # In PEM format.
    #
    # Eg: http://curl.haxx.se/docs/caextract.html
    attr_accessor :ca_cert_file
  end

  def ca_store
    unless @ca_store
      if file = SSLCertificateVerification.ca_cert_file
        @ca_store = OpenSSL::X509::Store.new
        @ca_store.add_file(file)
      else
        fail "you must specify a file with root CA certificates as `SSLCertificateVerification.ca_cert_file'"
      end
    end
    @ca_store
  end

  # It's important that we try to not add a certificate to the store that's
  # already in the store, because OpenSSL::X509::Store will raise an exception.
  def ssl_verify_peer(cert_string)
    cert = OpenSSL::X509::Certificate.new(cert_string)
    # Some servers send the same certificate multiple times. I'm not even joking... (gmail.com)
    return true if cert == @last_seen_cert
    @last_seen_cert = cert

    if ca_store.verify(@last_seen_cert)
      # A server may send the root certifiacte, which we already have and thus
      # should not be added to the store again.
      ca_store.add_cert(@last_seen_cert) unless @last_seen_cert.root?
      true
    else
      fail "unable to verify the server certificate of `#{@hostname}'"
      false
    end
  end

  def ssl_handshake_completed
    if @hostname
      unless OpenSSL::SSL.verify_certificate_identity(@last_seen_cert, @hostname)
        fail "the hostname `HOSTNAME' does not match the server certificate"
      end
    else
      warn "Skipping hostname verification because `@hostname' is not available."
    end
  end
end

module Handler
  include SSLCertificateVerification

  attr_accessor :hostname

  def self.connect(hostname, port)
    conn = EventMachine.connect(hostname, port, self)
    conn.hostname = hostname
    conn
  end

  def post_init
    start_tls(:verify_peer => true)
  end

  def receive_data(data)
    super
    puts data
    unless @sent
      send_data "x CAPABILITY\r\n"
      @sent = true
    end
  end
end

SSLCertificateVerification.ca_cert_file = File.expand_path('../cacert.pem', __FILE__)

EventMachine.run do
  Handler.connect("imap.gmail.com", 993)
  #Handler.connect("imap.mail.yahoo.com", 993)
  #Handler.connect("mx.easymail.ca", 993)
end