Helper to decode and check for deterministic encoding with minicbor.

[stevenj]

Summary

We need a way to validate that decoded data is deterministicly encoded.

Description

We need an optional flag when decoding cbor thats supposed to be deterministically encoded.

• A Context variable to the encoder should allow the encoder to fail if the data is not deterministically encoded, to log warnings, or to silently accept.

Tasks:

• For complex types which are encoded as stings, or bytes, arrays or maps, there is nothing to do except that variable encoding may not be used.
• For simple types, integers, etc. MiniCbor has a function to give you the length of a value in cbor, to check if the value was properly encoded, all that needs to happen is:
  • Get the start of the value in the byte stream before its decoded,
  • Get the end after its decoded
  • Make sure that length matches minicbors stated length for that value.
  • If it does, it was deterministically encoded.
• For maps, keys must be in the correct order. See: https://datatracker.ietf.org/doc/html/rfc8949#name-length-first-map-key-orderi See comment below for suggested solution.

The Deterministic Map decoder helper should be made in a general way so it can be re-used for different data structures.

[stevenj]

A way to decode maps which would be acceptable is a "Is this the next key in an arbitrary deterministic sequence of keys" which is general (doesn't care what the keys are, only they order they appeared), so it would fail if you gave it a key which was not properly next. And then a specific decoder which is just a match over the key. This might be a lot easier to do, given the keys and values are not homogonous.
Something like:

Let Key, Raw Value = Next Map Entry.
if not next_key(Key) then
   error handling
match key {
  content-type: decode content type and update struct 
  type: decode raw value as type and update struct
  ...
  _ : unknown key handling
  }

Which eliminates the need for some kind of mutable array, and lets each decoder of values handle its own logic.