[cryptography] Asynchronous forward secrecy encryption

ianG iang at iang.org
Wed Sep 18 03:23:03 EDT 2013


On 18/09/13 00:01 AM, Michael Rogers wrote:
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> Hi Marco,
>
> This is a problem we're working on as part of the Briar project. Our
> approach is pretty simple: establish a shared secret when you first
> communicate, periodically run that secret through a one-way function
> to get a new shared secret, and destroy the old one. Symmetric keys
> for encryption and authentication are derived from the current shared
> secret.


If I compromise your first shared secret, does that mean every shared 
secret thereafter is compromised?


> The rotation period depends on the latency of the underlying
> communication channel. For example, if you're communicating by email,
> you might rotate to a new shared secret once a week, to allow the
> other party to spend a week offline without losing any messages.


How do you coordinate between endpoints for the rotation?  Is it 
strictly time-based?  Or is there some sense of "searching the space" by 
hashing forward multiple rotations until the message decrypts?


> On
> the other hand if you're communicating by SD cards attached to
> migrating geese, you might rotate less often.


Ah, but does it consider the pâté attack?  ;)


> On 16/09/13 12:45, Marco Pozzato wrote:

>> I'm looking for an asynchronous messaging protocol with support
>> for forward secrecy: I found some ideas, some abstract paper but
>> nothing ready to be used.


Without some idea of the problem space you are trying to address, it's 
hard to do more than pluck ideas from existing protocols.



iang


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