MPI(0) in Ed448 signature
This is from my experience for GnuPG to implement Ed448 signature.
In the specification, the S-part is defined as 0.
Unfortunately, GnuPG 2.3 and libgcrypt are not good to handle MPI(0), and during my development, I encountered SEGV. I need to push a change for libgcrypt: https://dev.gnupg.org/rC34d55589b7d91a239435d77e3d45cf0deeba59b6 (Here, a->sign is length (==0), we needed to conditionalize)
That's because there are no cases to use MPI(0), until Ed448 signature.
For implementers' attention, I write this.