Interop problem with old dropbear and new openssh

[Graham Cobb]

Hi,

I have a router running an old version of OpenWRT with an old version of 
dropbear (Dropbear sshd v0.44test3).  It has been working for many years and I 
ssh in from my desktop systems (running Debian Testing) with no problem.

However, recently I upgraded one of my desktops and I can no longer connect to 
the router.  Dropbear on the router is exiting with:

exit before auth: bad buf_getwriteptr

Looking at the output from ssh -vv, the problem occurs right after

debug1: sending SSH2_MSG_KEXDH_INIT
debug1: expecting SSH2_MSG_KEXDH_REPLY

The desktop is running "SSH-2.0-OpenSSH_5.9p1 Debian-1" (it failed with 5.8 as 
well).  However, another desktop still running "SSH-2.0-OpenSSH_5.5p1 
Debian-6" still works fine.  The config files are identical and I can't find 
anything in the changelog between those versions which looks like it would 
affect the content of that message.

Has anyone seen this, or does anyone have any suggestions for possible 
workrounds?

Graham