This is a harness to help with fuzzing KEX.

To use it, you first set it to count packets in each direction:

./kexfuzz -K diffie-hellman-group1-sha1 -k host_ed25519_key -c
S2C: 29
C2S: 31

Then get it to record a particular packet (in this case the 4th
packet from client->server):

./kexfuzz -K diffie-hellman-group1-sha1 -k host_ed25519_key \
    -d -D C2S -i 3 -f packet_3

Fuzz the packet somehow:

dd if=/dev/urandom of=packet_3 bs=32 count=1 # Just for example

Then re-run the key exchange substituting the modified packet in
its original sequence:

./kexfuzz -K diffie-hellman-group1-sha1 -k host_ed25519_key \
    -r -D C2S -i 3 -f packet_3

A comprehensive KEX fuzz run would fuzz every packet in both
directions for each key exchange type and every hostkey type.
This will take some time.

Limitations: kexfuzz can't change the ordering of packets at
present. It is limited to replacing individual packets with
fuzzed variants with the same type. It really should allow
insertion, deletion on replacement of packets too.