VoIP isn't just for saving money - it can be used to add security too, according to One Day Mobile, which said that its Babylon nG voice encryption software for mobile phones could have made recent phone tapping scandals impossible.

The software is a SIP client for Windows Mobile or Symbian smartphones and voice-enabled PDAs, which adds 256-bit AES encryption to the VoIP data. One Day Mobile's Joe Francis said that it operates along the lines of Skype or instant messaging, with each phone having a list of available contacts.

The software requires a central gateway server, and initially only mobile-to-mobile calls are possible, but Francis said that a desktop PC client is on the way. Users have the option to run their own gateway or subscribe to One Day Mobile's.

He added that the strength of the encryption means that anyone wanting to buy the software must have their identity verified. "We wouldn't want to be seen handing it out to anyone," he said. "It's aimed at PLCs, government bodies and so on."

One Day Mobile will localise, test and distribute the software in the UK - its developer is a German company, Safe-Com - but will sell it through specialist resellers such as Formjet, rather than through its own online retail channel, Francis said.

A potential problem is that Babylon nG runs over the packet data channel - and GPRS in particular is noted for its latency. Francis claimed that this has improved of late though, so while his engineers were getting delays of 2.8 seconds on GPRS VoIP in early testing, that's now down to 0.8 seconds. He added that latency hasn't been an issue on 3G and EDGE, and that if the mobile device supports WiFi it could run Babylon nG over that instead.

"We could improve the speed by moving to 128-bit encryption instead of 256-bit, but we don't want to do that," he said.

Users may also need to ensure that their tariff permits VoIP, as some T-Mobile ones don't, for example - although as Francis points out, if the data is encrypted, how is the network going to identify it?

A few other companies already offer secure VoIP to mobile handsets, for example earlier this year Allied Telesis (Allied Telesyn as-was) released a lightweight VPN for mobile devices, which also supports SIP-based telephony and includes 128-bit encryption.

Allied's David Ward warned that voice-only encryption was the wrong way to go - instead, you should encrypt the whole data channel, he said.

"My clients love IP telephony because they don't have to run a separate security network for voice, they just run it within the data envelope," he added. "It's narrow-band thinking to think only of voice - you need multiple modes of communication."

There is also the possibility that Safe-Com has jumped the gun on VoIP security. The Internet Engineering Task Force has produced an RFC for Secure SIP, which should eventually lead to a standard.

"We've got to have a common, standard, no-options protocol," said Kurt Jacobs, channel business development director at IP PBX developer Sphere Communications. He added that the problem is the load that encryption places on a smartphone's processor at a time when handset makers are trying to drive cost out of their phones.