[VOIPSEC] ipsec vs. tls/srtp ?

Thu Feb 23 20:45:40 CST 2006

Hello Ari - Thank you for your useful insight on the
pros & cons of voip encryption.   The reason why we
are considering using external SIP security proxies
such as the Covergence platform in our enterprise VOIP
infrastructure is precisely to address some of the
encryption related problems that you describe.   We
feel that traditional network firewalls are not
suitable for dealing with encrypted SIP signaling and
media traffic nor are they suitable for protecting our
infrastructure against SIP layer DOS attacks.   

The reason why we like the Covergence security proxy
is that it would provide us with nat fix-up support
and midstream encryption and decryption capabilities
for TLS and SRTP.   This means that we can use it to
not only encrypt signaling and media streams from voip
elements that don’t support TLS and/or SRTP but we can
also use it to provide monitoring and intercept
capabilities for encrypted calls.   The monitoring and
intercept capabilities are highly important to us as
our enterprise is a regulated financial services
business.      If the list members are aware of any
other technology that can provide us with this
capability, please let me know but we have not been
able to find this SIP-focused capability in our
existing network firewalls and other IDS/IPS elements.

In summary, I’m not sure I agree with your statement
“But fortunately in 99% of the deployments, encryption
is not necessary nor desired.”   Maybe for cheap
residential voip this is true but in the enterprise
environment (especially regulated financial services
enterprises), encryption of not only SIP trunks
between offices as well as encryption of endpoint
traffic for certain employees (especially teleworkers)
is both desired and necessary.      Maybe you can make
the argument that media encryption isn’t necessary in
all cases (because the TDM media we have today in our
network is not encrypted) but anyone that deploys voip
without protecting the signaling with TLS is just
asking for bad things to happen.

Jin

--- Ari Takanen <art at codenomicon.com> wrote:

> Hello everyone interested in VoIP security,
> 
> On Wed, Feb 22, 2006 at 04:30:39PM -0500, Irwin
> Lazar wrote:
> > Can you separate out the signaling encryption from
> the media
> > encryption? That is, can one typically use SRTP
> for encrypting the
> > actual voice stream without encrypting the
> signaling stream?
> 
> I suppose you have received a dozen answers by now,
> but yes you are
> correct. The encryption is done separately for
> signalling (TLS,
> S/MIME) and for media (SRTP, ZRTP). With SRTP, the
> key management for
> media encryption is done within the signalling. This
> brings up
> additional encryption requirements for signaling to
> protect the key
> exchange for SRTP. But you can do SRTP without TLS
> as long as key
> management is done correctly. ZRTP tries to solve
> this problem.
> 
> > The reason I ask this is my assumption is that if
> the signaling
> > stream is encrypted, VoIP-aware firewalls are no
> longer viable since
> > the FW can't see inside the signaling session to
> know which ports to
> > open for the media session.
> 
> As you mention, the encryption does have its
> problems.
> 
> The encryption can (and must) be hop-to-hop for
> signalling
> purposes. This means you need a tls-enabled
> sip-aware firewall (sip
> proxy). Without any real deployments to compare
> against, I would guess
> that the same will apply for media. With hop-to-hop
> media encryption
> you need encryption enabled media-gateways (or
> SBC's).
> 
> Interestingly with peer-to-peer media encryption it
> is usually
> impossible for the perimeter defences to see and
> analyze the media
> stream. In real life there are extremely few
> firewalls (if any) that
> analyze the media stream, and with peer-to-peer SRTP
> that would be
> impossible. So you will never be certain what
> actually is travelling
> within the SRTP stream. SIP/SRTP combination can
> basically be used to
> open up the perimeter defences to any peer-to-peer
> communications. Imagine for example NFS over SRTP?
> That would be
> unacceptable in enterprise environment.
> 
> So planning an encrypted network architecture really
> requires
> understanding how the encryption works, and where it
> is on and where
> it is off. And the key management for this to
> succeed is not trivial
> (and I do not mean here the key exchange but the
> storage and
> distribution of the private keys and building of the
> key chains). From
> deployment perspective, it is essential to know who
> needs to access
> the encrypted data.
> 
> Note that encryption is not the same thing as
> security. Encryption and
> key management adds complexity to the network and
> therefore actually
> increases the vulnerability of the network.
> Encryption only reduces
> the confidentiality related threats. It does nothing
> to prevent from
> availability related threats. Actually it does the
> opposite and
> increases the probability of these DoS attacks. It
> also makes
> intrusion detection extremely difficult as the
> attacker will also use
> encryption to hide their attacks.
> 
> A necessary pitch for some TLS implementations:
> 
> OpenSSL has huge fingerprint, but quite secure
> nowadays:
> 
>
http://www.google.com/search?q=tls+codenomicon+openssl
> 
> I do not know if anyone is using Certicom stack in
> VoIP yet, but it is
> definitely one of the best from quality and
> robustness perspective:
> 
>
http://www.google.com/search?q=tls+codenomicon+certicom
> 
> But fortunately in 99% of the deployments,
> encryption is not necessary
> nor desired. Nobody really cares about encrypting
> email either, so how
> is VoIP different? And email is much easier to
> eavesdrop, and analyze,
> and contains much more security critical data than
> voice. Encryption
> is a feature, and a marketing tool for VoIP. For
> some environments it
> is also a requirement.
> 
> Best regards,
> 
> /Ari Takanen
>  Codenomicon CTO
> 
> PS: Remember to update your voip components
> regularly!
> 
> > 
> > Thoughts?
> > 
> > Irwin
> > 
> > -- 
> > Irwin Lazar, CISSP
> > Senior Analyst, Burton Group
> > ilazar at burtongroup.com
> > Phone: 703-742-9659
> > AIM/Gizmo/Google/MSN/Skype/Yahoo: imlazar
> > SightSpeed: ilazar at burtongroup.com
> 
> -- 
> -o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-
> Ari Takanen                       Codenomicon Ltd.
> ari.takanen at codenomicon.com       Kaitovayla 1
> tel: +358-40 50 67678             FIN-90570 Oulu
> http://www.codenomicon.com        Finland
> PGP: http://www.codenomicon.com/codenomicon-key.asc
> -o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-
> 

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