auth_cram_md5

# File lib/net/smtp.rb, line 588
    def auth_cram_md5( user, secret )
      # CRAM-MD5: [RFC2195]
      res = nil
      critical {
        res = check_response(get_response('AUTH CRAM-MD5'), true)
        challenge = res.split(/ /)[1].unpack('m')[0]
        secret = Digest::MD5.digest(secret) if secret.size > 64

        isecret = secret + "\0" * (64 - secret.size)
        osecret = isecret.dup
        0.upto(63) do |i|
          isecret[i] ^= 0x36
          osecret[i] ^= 0x5c
        end
        tmp = Digest::MD5.digest(isecret + challenge)
        tmp = Digest::MD5.hexdigest(osecret + tmp)

        res = get_response(base64_encode(user + ' ' + tmp))
      }
      raise SMTPAuthenticationError, res unless /\A2../ === res
    end