PuTTY bug rsa-non8mult-verify-fail

summary: RSA signature validation fails with non-8-bit-multiple keys
class: bug: This is clearly an actual problem we want fixed.
difficulty: fun: Just needs tuits, and not many of them.
priority: high: This should be fixed in the next release.
present-in: 0.53b 2004-02-04
fixed-in: 2004-02-07 (0.54) (0.55) (0.56) (0.57) (0.58) (0.59) (0.60) (0.61) (0.62)

Charles Wilcox reported a signature validation bug with a large SSH-2
RSA host key - PuTTY reported `Server's host key did not match the
signature supplied'. We reproduced this with a 2500-bit RSA host key.

SGT: This _appears_ to be due to me computing the byte count of the
key by dividing the bit count by 8 and rounding _down_ rather than up.
Therefore, I can't see how this code could ever have worked on any
SSH-2 RSA key whose length was not a multiple of 8 bits; and therefore
I'm staggered that we haven't noticed it before! OpenSSH's keygen
appears to be scrupulous about ensuring the returned key length is
exactly what you asked for rather than one bit less, but even so I'm
astonished that _all_ keygen implementations for servers we've ever
interoperated with have avoided tripping this bug...

JTN: verified that this does indeed occur with 1023-bit and 513-bit
SSH-2 RSA host keys.

Audit trail for this bug.

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(last revision of this bug record was at 2005-03-10 16:36:42 +0000)