[messaging] Separation of concerns, usability, and partial verification

Tom Ritter tom at ritter.vg
Wed Mar 12 23:37:19 PDT 2014

I think something very important for protocols we design going forward
is persistence of key material, ala ZRTP's.  The encrypted
conversation I am having with Alice now is authenticated via the last
encrypted conversation I had with Alice.  You can trick me by MITMing
all my conversations with Alice, but as long as I get one conversation
in that you don't - we'll be able to figure it out.

My happiness with this design evolved in a good deal from discussions
with some of the folks on this list at various events.  It's the
implementation I'm a wee bit nervous about, as this system needs to be
handle things like 'restarting after a lost device' and the ratcheting
issues of what happens when we aren't in the same state because I
think you received messages from me but you didn't.


On 12 March 2014 08:18, Ximin Luo <infinity0 at pwned.gg> wrote:
> This is a brief outline on the architecture of an independent/central "verification" program. This could be part of a keyring, or a contact manager, or even a combined contacts/keys manager that some UX folks were suggesting at the CTS IV. It would let a user:
> 1. store cryptographic material to authenticate their contact, either a public-key fpr, or a shared secret for kex, or whatever.
> 2. store/set the *verification status* of the material. this could be:
>   - full (physical), i.e. via a physical communication of fingerprint or shared-secret
>   - delegate, i.e. sent via a friend (the friend must themselves be verified). (one idea for mpOTR/groupchat is to have the initiator send all public keys to everyone else.)
>   - saw-multiple, i.e. saw the contact use/communicate the key via these insecure but independent channels/mediums (e.g. via email, phone, IM from several accounts)
> 3. read some subjective comment/advice about how "safe" the current situation is
> 4. set the *overall policy* for letting other applications treat a contact as "valid". e.g. require-full, require-friend-trust, allow-but-warn-if-new (i.e. a form of TOFU)
> 5. perform physical verification via technical means, such as scanning a QR code
> 6. sync this state to other trusted devices
> The point being that identity/key verification is a logistics and usability issue, and not really a cryptographic issue.
> It is semi-relevant to the other thread - we want to support not only fpr verification, but other methods of verification too, including "soft" verification (inc. the effortless TOFU) that is easier but not secure against Nation-State-Adversaries.
> Advantages:
> - user can set strict/loose verification policy based on their own preference, in one single place, that affects all applications
> - saves application writers from having to think about these issues
> - unified UI for verification
> - synced between different devices. most crypto-application writers will not bother with this, it is too hard and a separate concern from their program.
> Disadvantages:
> - the component's verification-status data-model may not cover everything that a certain application needs. this can be fixed with time, however, and it will eventually benefit all applications, not just a single one.
> - most developers of contact managers aren't security-trained. you would hope that developers of keyrings are a bit better, but we still see things like http://gaganpreet.in/blog/2013/07/24/kwallet-security-analysis/ mistaking EBC for CBC
> Thoughts?
> X
> --
> git://github.com/infinity0/pubkeys.git
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