Discussion: new terrapin resisting ciphers and macs (alternative to strict-kex) and -ctr mode question.

[Fabian Bäumer]

Hi there,

> So there could be a Chacha20-Poly1305v2 at openssh.com which uses AD data to chain the
> messages together, so it will be resistant against terrapin even without the strict-kex.
>
> Consequently the hmac-etmv2 at openssh.com mode could be deviced in a similar manner, to
> also include the transcript hash or similar things.
This would still require both, client and server, to receive an update 
to support these new algorithms. So I wonder what would be the benefit 
of having those over strict key exchange? Also, existing security proofs 
for ChaCha20-Poly1305 and Encrypt-then-MAC constructions in SSH would 
become meaningless then.

> Has anybody analyzed the aes-ctr / hmac-etm properties, whats making this resist and is it rigid?
> It looks like it is only the counter position which saves it?
Yes, we did. We describe this mode as "vulnerable, but not exploitable" 
in our paper. Terrapin works against the general Encrypt-then-MAC 
construction, although some cipher specifics may prevent exploitation in 
a real world scenario. In this case, dropping a message from the secure 
channel causes the AES-CTR keystream to become out of sync (similar 
issue with stream ciphers like arcfour, just for reference). As far as 
we are aware, there is no way for an attacker to realign the keystream 
to allow the session to continue. Note however, that the attack still 
passes MAC verification and that an exception is only thrown at the 
application layer (i.e. wrong message format of SSH_SERVICE_REQUEST / 
_ACCEPT).

> So is keeping hmac-etm iff aes-ctr is offered used still a safe option (or just "not broken"). Because luckily many
> have removed CBC already.
 From our current point of view, this combination can be used without 
risking any real world attack. However, this may change if someone finds 
a way to realign the keystreams. We suggest disabling Encrypt-then-MAC 
over CBC as a workaround to be on the safe side here. I'd also like to 
stress that this workaround is only meant as temporary stop-gap measure 
until patches are available.

> And while "enforce strict-key"
> could gain some of the trust back, the attack also shows, that those constructs are just very fragile.
For reference, there has been a similar request over on Ars Technica.

M. Sc. Fabian Bäumer

Chair for Network and Data Security
Ruhr University Bochum
Universitätsstr. 150, Building MC 4/145
44780 Bochum
Germany

Am 20.12.2023 um 11:57 schrieb Bernd Eckenfels:
> Hello,
>
>
> in addition to my last thread about a new config option to make strict-kex mandatory,
> I also wonder if a new mechanism for ciphers/macs can be introduced and is reliable
> by simple both sides using it.
>
> So there could be a Chacha20-Poly1305v2 at openssh.com which uses AD data to chain the
> messages together, so it will be resistant against terrapin even without the strict-kex.
>
> Consequently the hmac-etmv2 at openssh.com mode could be deviced in a similar manner, to
> also include the transcript hash or similar things.
>
> The impact of removing the only "alternative" cipher cc20p1305 because of terrapin hardening as well
> as falling back to the old eam-macs is really bad for ssh best practice. And while "enforce strict-key"
> could gain some of the trust back, the attack also shows, that those constructs are just very fragile.
>
> And while a redesign of the protocol might be a good option, its not the fastest to standardize and implement.
> Whereas openssh project has done it before both new ciphers.
>
> On that note - maybe combine it with PQ hybrid modes?
>
> I have a related question:
>
> Has anybody analyzed the aes-ctr / hmac-etm properties, whats making this resist and is it rigid?
> It looks like it is only the counter position which saves it?
>
> So is keeping hmac-etm iff aes-ctr is offered used still a safe option (or just "not broken"). Because luckily many
> have removed CBC already.
>
> _______________________________________________
> openssh-unix-dev mailing list
> openssh-unix-dev at mindrot.org
> https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 5977 bytes
Desc: Kryptografische S/MIME-Signatur
URL: <http://lists.mindrot.org/pipermail/openssh-unix-dev/attachments/20231220/2e4cbdf6/attachment-0001.p7s>