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It appears that the triggers for bug compatibility modes in ssh_detect_bugs() are too lax. For instance, the following version string (presented by VShell):
SSH-2.0-2.0.1 (official) VShell
is mistakenly triggering the HMAC bug compatibility mode.
A workaround in 0.53 is to manually turn off detection of the bug (SSH / Bugs panel, "Miscomputes SSH2 HMAC keys").
A better solution would be to use the wildcard engine we already have for PS*P internally to match version strings. (This depends on our being able to reconstruct exactly which version strings we should be matching, of course!)
Update: an exception has been added to the bug compatibility modes for VShell, although I haven't tested it.