RE: New version of rsa-sha2-256 draft: Back to PKCS#1 v1.5

denis bider <[email protected]> writes:

>I'm not sure we're referring to the same attack. The most critical SSH + CBC
>attack I can think of right now is this:
>
>http://isg.rhul.ac.uk/~kp/SandPfinal.pdf

Ah, OK, I was thinking of the original attack(s) that motivated the suggestion
to use CTR.

>This requires only a MITM position and allows recovery of up to 32 plaintext
>bits once per about ~200k intercepted connections. That's perfectly feasible
>if the connections are being automatically retried over some time without
>supervision (which is not unusual in deployment).

Hmm, I wouldn't necessarily call it a feasible attack, more a certificational
weakness (meaning the protocol doesn't meet its design requirements).

>Our defense against this is to not obviously leak info about whether the
>result of packet length decryption was something sensible or not. But it may
>be that most implementations don't do this.

Same here, I do a lot more rigorous checking than OpenSSH (as described in the
paper) does, checking lengths, packet types, and padding size, and the
disconnect notification is an empty string.  As you say though, the problem
isn't necessarily your code but the other side.  OTOH I think that an
implementation that does little checking and so is particularly vulnerable
probably won't know about using CTR mode either...

Peter.