OPIE considered insecure

[Daniel Roethlisberger]

Lyndon Nerenberg <lyndon at orthanc.ca> 2009-02-09:
> >Right, but that's not the problem they're trying to solve.
> >They're trying to solve the problem of logging in _from_ an
> >untrusted machine, to a trusted machine.
> 
> Okay, I got it backawrds.
> 
> >So, an alternative might be to carry around a USB key with a
> >one-time private key, different from your normal private keys,
> >and have the public key command-squashed on the server to
> >remove itself from authorized_keys before running the shell.
> 
> That's what I do -- multiple throw-away keys on a USB stick,
> for emergencies. However if you're that paranoid you better be
> carrying around your own set of ssh binaries on that stick as
> well.

My use case is primarily to log in from highly untrusted and
malware infested systems.  OPIE has been a usable solution to
that problem.  I'm primarily worried about keyloggers and USB
memory stick content dumpers.  OPIE fits that bill quite well.

> >You could generate several, each with a different passphrase
> >(assuming that you could manage to remember that many
> >passphrases and which keys they go with), and get a similar
> >effect to printing out a card with the next ten OPIE
> >passwords.
> 
> It's not that hard to come up with a scheme that lets you map
> from an identifier tagged to the private key to the
> corresponding password (in your head). It's a pain at the
> start, but once you've used a given scheme for a while it
> becomes second nature.
> 
> Akso, note that you can get similar behaviour using K5 with
> one-off instances of your principal (e.g.
> lyndon.a6d5mps at EXAMPLE.ORG). The advantage here is that there
> are no key files involved (but you still want to carry a
> trusted kinit binary with you). The downside is that most sites
> don't have K5/GSSAPI enabled. And of those that do, a
> significant percentage of the implementations still don't to
> dynamic realm discovery, therefore you need a pre-existing
> arrangement to map your realm to the appropriate KDCs.

I prefer OPIE also because it does not need anything fancy on the
client side beyond a standard SSH2 client.

-- 
Daniel Roethlisberger
http://daniel.roe.ch/