Re: XTS cipher mode for cluster file encryption

On 10/18/21 17:56, Stephen Frost wrote:
 >> ...
>> I've argued for storing the nonce, but I don't quite see why would we need
>> integrity guarantees?
>>
>> AFAICS the threat model the patch aims to address is an attacker who can
>> observe the data (e.g. a low-privileged OS user), but can't modify the
>> files. Which seems like a reasonable model for shared environments.
> 
> There are multiple threat models which we should be considering and
> that's why we may want to eventually add integrity.
> 

So what are these threat models? If we should be considering them it'd 
be nice to have a description, explaining what capabilities must the 
attacker have ...

My (perhaps naive) understanding is that the authentication / integrity 
provides (partial) protection against attackers that may modify instance 
data - modify files, etc. But I'd guess an attacker with such capability 
can do various other (simpler) things to extract data. Say, modify the 
config to load an extension that dumps keys from memory, or whatever.

So what's a plausible / practical threat model that would be mitigated 
by the authenticated encryption?

It'd be a bit silly to add complexity to allow AEAD, only to find out 
there are ways around it.


regards

-- 
Tomas Vondra
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