[messaging] Do quantum attacks/algos also lead to compromise of PFS?

[str4d]

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Taylor R Campbell wrote:
> Date: Sat, 24 Jan 2015 13:07:29 -0800 From: Tao Effect 
> <contact at taoeffect.com>
> 
> So, I understand that QM algos can pretty much dismantle all 
> popular asymmetric encryption algos with enough q-bits, but I 
> haven't thought hard enough to see if they also can be used to 
> compromise communications that used DH to do PFS underneath the 
> initial handshake.
> 
> Yes.  Shor's algorithm can compute finite field and elliptic curve
>  discrete logs, so an attacker who saved a transcript of g^a, g^b 
> over the wire today can, if/when quantum computers become 
> available, compute a, b, and g^ab and retroactively decrypt the 
> rest of the encrypted transcript.

That's not quite the same as breaking PFS though. PFS is the premise
that knowing the key to one session/message gives you no information
about the plaintext of any other session/message. QM algos like Shor's
algorithm speed up the decryption process, but you would still need to
break each session/message individually.

In other words, QM algos don't break PFS, because the requirement it
imposes (to decrypt all sessions/messages individually) is not a
significant barrier when the PFS protocol uses QM-vulnerable crypto
primitives. Building a PFS protocol using QM-resistant crypto would
restore the barrier.

str4d

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