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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.