Security
The many consumer VoIP solutions
do not support encryption yet, although having a secure phone
is much easier to implement with VoIP than traditional phone lines.
As a result, it is relatively easy to eavesdrop on VoIP calls
and even change their content. There are several open source solutions
that facilitate sniffing of VoIP conversations. A modicum of security
is afforded due to patented audio codecs that are not easily available
for open source applications, however such security through obscurity
has not proven effective in the long run in other fields. Some
vendors also use compression to make eavesdropping more difficult.
However, real security requires encryption and cryptographic authentication
which are not widely available at a consumer level. The existing
secure standard SRTP and the new ZRTP protocol is available on
Analog Telephone Adapters(ATAs) as well as various softphones.
It is possible to use IPsec to secure P2P VoIP by using opportunistic
encryption. Skype does not use SRTP, but uses encryption which
is transparent to the Skype provider.
The Voice VPN solution provides
secure voice for enterprise VoIP networks by applying IPSec encryption
to the digitized voice stream.