Encrypted email search indexing

[meskio@sindominio.net (Ruben Pollan)]

Quoting rysiek (2015-12-05 15:57:10)
> Dnia czwartek, 3 grudnia 2015 12:06:43 stef pisze:
> > this makes little sense. pgp is for encryption in transit, not at rest. you
> > should decrypt and reencrypt your mail. and you can also index it easily
> > when doing so.
> 
> It *kinda sorta* make sense if we're talking about an e-mail provider that 
> wants the user to be able to search even when they're using their webmail and 
> have no access to the private key.
> 
> But on the other hand that in itself makes no sense to me. ;)

As long as you store emails in the server and you retrieve them on demand your 
provider will be able to guess the content of your encrypted emails. I'll 
explain it.

Imagine that you have the index in perfectly secure way, or locally stored or an 
ideal nifty way in the provider where you can do queries and the provider can 
not guess the content of the queries.

Let's first assume that you have your emails stored as they arrive, your 
encrypted email is stored encrypted and your decrypted email is stored 
decrypted. After each query you retrieve the resulted emails, so the server sees 
which decrypted emails are related to which encrypted ones and in the long term 
can infer the content of the encrypted ones as well.

Let's imagine then that you store all the emails encrypted. Then your provider 
could send to you crafted emails with the kind of content she cares about to 
discover, so it can notice each time you retrieve one of this crafted emails and 
what other emails are related to that.

You could minimize this attack by not only fetching the emails that you care 
about, but fetch way more. But at the end or you have your whole set of emails 
locally or the server will be able to infer data about the encrypted emails.

-- 
Ruben Pollan  | http://meskio.net/
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