Hi Ethan


>> It is important to include the cipher-type into the symmetric cipher key to avoid weak-cipher-attacks.
> 
> the cipher-type here refers to the ECDH negotiation parameters?

No. Not to the ECDH negotiation.
BIP151 specifies a flexible symmetric key cipher type negotiation,
although, BIP151 only specifies chacha20-poly1305@openssh.com.

Lets assume someone adds another symmetric cipher type after BIP151 has
been deployed which has less strong security properties then
chacha20-poly1305.

If we don't include the ciphersuite-type in the key derivation HMAC, an
attacker/MITM could in theory force both nodes to use the weaker
symmetric cipher type.

</jonas>