Zfone and ZRTP :: encryption for voip protocols

[Damien Miller]

On Wed, 15 Mar 2006, Ed Gerck wrote:

> cybergio wrote:
> > 
> > Zfone :: http://www.philzimmermann.com/EN/zfone/index.html
> 
> "...it achieves security without reliance on a PKI, key certification,
> trust models, certificate authorities, or key management..."
> 
> Good. But, uf course, there's a trust model and you need to rely on it.

Points to them for making it explicit.

> "...allows the detection of man-in-the-middle (MiTM) attacks by
> displaying a short authentication string for the users to read and
> compare over the phone."
> 
> Depends on the trust model. May not work.

This is incomplete. The paragraph goes on to say:

> we still get fairly decent authentication against a MiTM attack, based
> on a form of key continuity. It does this by caching some key material
> to use in the next call, to be mixed in with the next call's DH shared
> secret, giving it key continuity properties analogous to SSH.

The SSH trust model has certainly proved itself as useful, and is
probably perfectly appropriate for semi-adhoc telephony where voice
nuance offers an additional means of detecting phonies (pun!).

The screenshot on that page seems to indicate only three [a-z0-9]
characters form the "key fingerprint". My first impression was that
this was insufficient, but it is probably a good tradeoff. It is
short enough that people will actually use it, and an attacker might
only get a couple of tries of getting it wrong (in a 2^15 bit space)
before a human would be very suspicious.

-d


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