Can't use signed pubkey #128

I'm trying to sign my krypton public key to SSH into a machine which trusts that certificate authority key. I can do so with a non krypton key (just a normal rsa key with the private key stored on my computer), but krypton doesn't seem to be able to authenticate the request when I try to sign my id_krypton.pub key.

I signed it like this:-
ssh-keygen -s ../temp/ca-key -I david -n root -V +1w -z 1 id_krypton.pub

Krypton on iOS reports:-
Request failed
The incoming request was invalid/
UnsupportedSSHDigestAlgorithm(). Please try again.

ssh -vvv reports
debug2: sign_and_send_pubkey: using private key "/Users/david/.ssh/id_krypton" for certificate
debug3: sign_and_send_pubkey: signing using rsa-sha2-512-cert-v01@openssh.com

It seems that rsa-sha2-512-cert-v01@openssh.com is not one of the supported digests (see extension DigestType in Krypton/Keypair+SSHFormat.swift)

What would be required to support this? Would the phone need a copy of the CA public key?