Putting domain separators in the IDL is interesting but you can also avoid the problem by putting the domain separators in-band (e.g. in some kind of "type" field that is always present).
Tangentially, depending on what your input and data model look like, canonicalisation takes O(nlogn) time (i.e. the cost of sorting your fields).
Tangentially, depending on what your input and data model look like, canonicalisation takes O(nlogn) time (i.e. the cost of sorting your fields).
Here I describe an alternative approach that produces deterministic hashes without a distinct canonicalization step, using multiset hashing: https://www.da.vidbuchanan.co.uk/blog/signing-json.html