Page 233 - Asterisk™: The Future of Telephony

Encrypting Audio with Secure RTP
If you can sniff the packets coming out of an Asterisk system, you can extract the audio
from the RTP streams. This data can be fed offline to a speech processing system, which
can listen for keywords such as “credit card number” or “PIN”, and present that data
to someone who has an interest in it. The stream can also be evaluated to see if there
are DTMF tones embedded in it, which is dangerous because many services ask for
password and credit card information input via the dialpad. In business, strategic in-
formation could also be gleaned from being able to capture and evaluate audio.
Using Secure RTP can combat this problem by encrypting the RTP streams; however,
Asterisk does not support SRTP as of this writing. Work is under way to provide SRTP
support (a patch exists in the trunk release, but it is not known as of this writing whether
this will be back-ported to 1.4).

Spoofing
In the traditional telephone network, it is very difficult to successfully adopt someone
else’s identity. Your activities can (and will) be traced back to you, and the authorities
will quickly put an end to the fun. In the world of IP, it is much easier to remain
anonymous. As such, it is no stretch to imagine that hordes of enterprising criminals
will only be too happy to make calls to your credit card company or bank, pretending
to be you. If a trusted mechanism is not discovered to combat spoofing, we will quickly
learn that we cannot trust VoIP calls.

What Can Be Done?

The first thing to keep in mind when considering security on a VoIP system is that VoIP
is based on network protocols, and needs be evaluated from that perspective. This is
not to say that traditional telecom security should be ignored, but we need to pay
attention to the underlying network.

Basic network security
One of the most effective things that can be done is to secure access to the voice network.
The use of firewalls and VLANs are examples of how this can be achieved. By default,
the voice network should be accessible only to those things that have a need. For ex-
ample, if you do not have any softphones in use, do not allow client PCs access to the
voice network.
Segregating voice and data traffic. Unless there is a need to have voice and data on the same
network, there may be some value in keeping them separate (this can have other benefits
as well, such as simplifying QoS configurations). It is not unheard of to build the in-
ternal voice network on a totally separate LAN, using existing CAT3 cabling and
terminating on inexpensive network switches. It can be less expensive too.

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