PuTTY bug short-rsa2-key-crash

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summary: Short RSA2 key causes PuTTY crash
class: bug: This is clearly an actual problem we want fixed.
difficulty: tricky: Needs many tuits.
priority: medium: This should be fixed one day.
present-in: 0.53b 2003-11-03

"If I create a really short RSA2 key (I used 256 bits) it is generated and
accepted OK, but the PuTTY code (CVS head, 22:00 GMT yesterday) segfaults."

More detail: <20031101102009.GA29907@chiark.greenend.org.uk>
(includes suspected root cause)

(I've tested that DSA and RSA1 keys don't suffer from the same problem.)

More analysis:

> Looking at rsa2_sign(), if the RSA modulus of the key is too small, we
> can't fit in the padding, ASN1 goop and hash, but the loop is merrily
> attempting to cram it all in anyway, overrunning the malloced block and
> (apparently) causing fruity stack badness.

*nods* Yes, I managed to trace that much myself. The trouble is not
that I don't know the cause of the problem, but more that I don't
know how best to fix it.

[...] the plausible options are (a) reject the key when PuTTY attempts
to load it, or (b) return failure and a useful error message from
the sign operation. I'm undecided between those two; (a) is nice
from the point of view of detecting and reporting any error
condition at the earliest possible moment, but (b) feels a little
more robust since the check can be performed in the _same_ code that
will then go and use the buffer space - so rather than having to
keep two pieces of similar-looking code in sync, there's just one
piece of code which is likely to keep naturally in sync with itself.
Either one, however, requires some infrastructure change to support,
which is why I didn't fix it before the 0.54 release.

(Quite why anyone's using 256-bit keys _anyway_ is beyond me, but
these days no weird user behaviour is a surprise to me.)

BJH21: I've added an assert() to the code so that the explosion when trying
       to use a too-short key is more controlled. The signature of the
       assertion failure looks something like this:

         Assertion failed!

         Program: putty.exe
         File: ..\sshrsa.c
         Line: 793
         Expression: 1 <= nbytes - 20 - ASN1_LEN

       It occurs to me that PuTTYgen really should warn (and maybe fail)
       when asked to generate an ssh-rsa key that can't possibly be used.
Audit trail for this bug.

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(last revision of this bug record was at 2016-12-21 16:52:17 +0000)