class Net::SMTP::AuthCramMD5

def auth(user, secret)
  challenge = continue('AUTH CRAM-MD5')
  crammed = cram_md5_response(secret, challenge.unpack1('m'))
  finish(base64_encode("#{user} #{crammed}"))
end

def cram_md5_response(secret, challenge)
  tmp = digest_class::MD5.digest(cram_secret(secret, IMASK) + challenge)
  digest_class::MD5.hexdigest(cram_secret(secret, OMASK) + tmp)
end

def cram_secret(secret, mask)
  secret = digest_class::MD5.digest(secret) if secret.size > CRAM_BUFSIZE
  buf = secret.ljust(CRAM_BUFSIZE, "\0")
  0.upto(buf.size - 1) do |i|
    buf[i] = (buf[i].ord ^ mask).chr
  end
  buf
end

def digest_class
  @digest_class ||= if defined?(OpenSSL::Digest)
                      OpenSSL::Digest
                    elsif defined?(::Digest)
                      ::Digest
                    else
                      raise '"openssl" or "digest" library is required'
                    end
end