fix(base64): Reject too-short encodings

[gibson042]

Fixes #1990

Description

Test invalid encodings, and reject input that is too short.

Security Considerations

None known.

Scaling Considerations

n/a

Documentation Considerations

Should this have a CHANGELOG entry?

Testing Considerations

New and improved!

Upgrade Considerations

n/a

[kriskowal]

With this change, do we accept all strings we previously generated? That is, do we generate the appropriate padding in all cases?

[gibson042]

Yes, the generation code already correctly outputs padding.

[gibson042]

@kriskowal This is good without a CHANGELOG.md entry, right?

[kriskowal]

@kriskowal This is good without a CHANGELOG.md entry, right?

This is fine without a NEWS.md entry (CHANGELOG.md gets generated from commit messages at time of release.)

I prefer (and do not insist) on redundantly capturing messages to upgraders in NEWS.md if the dependant package needs to make some changes to compensate for an upgrade. Like, if a method is deprecated, there should be a recommendation to use a different equivalent behavior (or stop that). Likewise for removal of deprecated methods.