Tuf: Make length and hashes optional in snapshot/timestamp metadata

[lukpueh]

Description of issue or feature request:

As per the specification (v1.0.1) length and hashes fields in timestamp and snapshot metadata are optional. The reference implementation should reflect this.

Current behavior:

• timestamp metadata always includes length and hashes of snapshot metadata (see reposoitory_lib.generate_timestamp_metadata)
• snapshot metadata always excludes length and hashes of (delegated) targets metadata (see repository_lib.generate_snapshot_metadata)

Expected behavior:
Add option to include/exclude these fields in the relevant functions in repository_lib. Since these functions are usually called via write/writeall the option should be made available there too. It's also conceivable to configure this via attributes on the Repository object.

[MVrachev]

I will work on that issue.

I have one question though: I am planning to add additional parameters to generate_timestamp_metadata and generate_snapshot_metadata which will be used to decide whenever hashes and length should be used, but my question is what should be their default value?

[lukpueh]

what should be their default value?

I suggest to choose them so that the default behavior of these functions matches their current behavior.

[joshuagl]

My initial feeling was that we should default to the current behaviour, but I wonder whether there's an argument to be made for _not_ providing default values. If we don't have default values, we force implementers to explicitly choose whether they want to include lengths and hashes, rather than inheriting a default.

Furthermore I think the specification suggesting the fields are optional implies that they would not be included by default, and thus would expect similar behaviour from a reference implementation.

Conversely, I think the current values probably are sensible defaults, particularly for i.e. the PEP 458 implementation. I'm curious to read what others think.

[MVrachev]

I feel like security by default should be implied here and use length and hashes fields by default.
That way if somebody decides to not use them then he/she should make it explicit by passing the use_length=False and use_hashes=False when calling the functions.

[MVrachev]

@joshuagl can you close this one? It's fixed by the above pr.

[joshuagl]

Fixed in #1031