MSc Thesis
Sebastien Jeanquier
info () securethoughts ! net
GPG Key ID: 0xBE4D6CE8
Supervisor: Dr. Alex Dent
Information Security Group
Royal Holloway College, University of London
September 9, 2006

Executive Summary
This thesis will analyse the network security concept of Port Knocking and
its younger brother Single Packet Authorization and assess their suitability as
‘Firewall Authentication’ mechanisms for opening network ports or performing
certain actions on servers using these mechanisms.
The introduction provides a short history of network security and why this
concept has come about at the start of this century. It will also cover the
basics of networking and cryptography required to understand the fundamental
workings of port knocking systems and the threats and attacks pertinent to
them. An overview of both port knocking and single packet authorization and
the security aspects involved, including the debated topic of security through
obscurity, will enable a clearer understanding of port knocking in actual use
and the analysis of implementations of both forms of firewall authentication
schemes.
The aim of this thesis is to analyse the security offered by both systems
and assess which threats exist in theory and in the real world, and outline
the practicalities of using port knocking as part of defence in depth. Finally,
this thesis attempts to mention certain possible improvements to port knocking
schemes, as well as an overview of alternate uses of port knocking in other
aspects of information security.
The two primary implementations that will be analysed are Martin Krzy-
winski’s Port Knocking Perl Prototype and Michael Rash’s single packet autho-
rization Firewall Knock Operator (fwknop). In actual use, it was found that the
Perl Prototype may be more restrictive due to the long ‘knocks’ required when
encryption is used, and anti-replay features require that state be maintained on
both the server and client. The extremely low transmission rate and delivery-
order issues involved with port knocking make it less suitable where more data
may be required for a secure and practical knock. On the other hand, the sin-
gle packet authorization implementation, fwknop, uses single UDP packets to
transmit authorization data, much in the fashion described in ISO/IEC 9798-
2 on entity authentication, but loses the ‘knocking’ aspect of port knocking,
which is a novel and unique delivery mechanism. In its default configuration,
fwknop is quite vulnerable to dictionary attacks, simply due to the way in which
passphrases are turned into cryptographic keys. A will present a simple tool,
fwknop da, designed to illustrate how a live attacker could intercept fwknop
authorization packets and crack them.