pub trait Foo
where
    SignatureSize<Self::SignatureCurve>: ArrayLength<u8>,
{
    type SignatureCurve: PrimeCurve;

    /// Decode and validate a private signing key from `key_der`.
    fn signer(&self, _key_der: &[u8]) -> Result<SecretKey<Self::SignatureCurve>, ()> {
        todo!()
    }
}