Single Packet Authorization
Increasing Security in SSH
Leandro Almeida
lcavalcanti.almeida@gmail.com
III ENSOL Liberdade no Extremo
João Pessoa­PB 19,20 e 21 de Junho de 2009

Who is this guy?
●
● Degree in Computer Network
● Post­graduate in Information
Security
● Security Analist

AGENDA
● SSH
● Firewall
● Port Knocking
● Single Packet Authorization
● FWKNOP
● Video
● Questions

Who here uses SSH?

Do you think the SSH
secure?

● CERT® Advisory CA­2002­18 OpenSSH
Vulnerabilities in Challenge Response Handling
● USN­649­1: OpenSSH vulnerabilities
● OpenSSH Security Advisory: cbc.adv ­ Plaintext
Recovery Attack Against SSH CPNI­957037
● CPNI Vulnerability Advisory SSH – CPNI­957037
● openssh vulnerability CVE­2008­0166,
http://www.ubuntu.com/usn/usn­612­1
● SSH is an application and have flaws

When someone comes and says...
If you are not safe places a
Firewall

Search / Design a solution to
your problem

Otherwise an attacker can
succeed!

There is a light at the
end of the tunnel

● Port Knocking
● Literally “door knocking”
● The technique is built on a
sequence of packages pre­
determined
● If the sequence is wrong, nothing
(SSH access) will be released
● Use the fields reserved for the
TCP/UDP
● Does not use encryption

1º Moment: Blue
2º Moment: Red
3º Moment: green

Problems...

The encryption can not be
used

Packets may arrive out of order,
which breaks a string
An attacker may be sending
packets to random ports, breaking
the sequence
Susceptible to attack by replay

And now? Who can save
us...

Single Packet Authorization

It is a technique based a Port Knocking
● The SPA inherits the strengths and
addresses the major flaws of Port Knocking
The application that implements the SPA is
FWKNOP (FireWall KNock OPerator)
The FWKNOP is Free Software maintained by
Michael Rash
http://cipherdyne.org/fwknop/

Only one packet is sent
Correcting the problem of delivery
out of order
Uses the field­related data of the package
Correcting the problem of encryption
● Creates a temporary rule in the firewall,
allowing access only to client
There is not the possibility of using the
same package in a range of pre­determined
time (default 60s)
– Correction of attacks on Replay

Ability to encrypt packets with keys
Symmetrical (Rijndael)
– Asymmetric (GPG + ElGamal)
Makes the deciphering of the packages
to verify
IP address of the packet with
the IP address of the encrypted
● Addition of a block of random content
generated for each packet, thus
allowing the encryption single

Packet SPA

Scenario testing

1º Moment: Without SPA

2º Moment: With SPA

SSH Access Released o/

Thanks!
Leandro Almeida
Blog:leandro­cavalcanti.blogspot.com
Email:lcavalcanti.almeida@gmail.com

References
● http://www.cipherdyne.org/fwknop/
● http://www.linuxjournal.com/article
/9565
● http://www.linux.com/archive/featur
e/135100
● http://www.jsena.info/downloads/pal
estras/JansenSena_FISL9_Single_Pack
et_Authorization.pdf
●