Added note about function modifying its input

[sergey3bv]

@Roasbeef
@gijswijs
Addressing #73

[gemini-code-assist]

Summary of Changes

Hello @sergey3bv, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request improves the clarity and safety of cryptographic functions by adding explicit documentation about their side effects. Specifically, it warns users that chacha20polyEncrypt and chacha20polyDecrypt modify their input byte slices, preventing potential unexpected behavior and making the code easier to understand and use correctly.

Highlights

• Documentation Update: Added explicit warning comments to the chacha20polyEncrypt function in crypto.go, indicating that it modifies its plainTxt input parameter.
• Documentation Update: Added explicit warning comments to the chacha20polyDecrypt function in crypto.go, indicating that it modifies its cipherTxt input parameter.

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[gemini-code-assist]

Code Review

This pull request adds warnings to chacha20polyEncrypt and chacha20polyDecrypt to indicate that they modify their input arguments. While documenting this side effect is a good step, my review suggests a more robust solution: refactoring these functions to be side-effect-free by allocating new memory for the output instead of modifying the input slices in-place. This change would improve code safety and maintainability by preventing potential bugs related to unexpected argument modification.

[sergey3bv]

Hey, @Roasbeef, @gijswijs, could you please review? I can implement Gemini's suggestion if needed

[sergey3bv]

Hey, @gijswijs, @Roasbeef, could you please take a look?

[gijswijs]

I would keep the warning comments on both funcs for consistency and make zero behavioral changes, so basically ignore Gemini here.

The current input[:0] trick reuses the existing backing array, avoiding a heap allocation. Although that doesn't really matter for chacha20polyEncrypt which is triggered by the node owner, it can definitely add up for chacha20polyDecrypt, such that the allocation cost becomes non-negligible. Changing the behavior for one and not the other, is also not desired.

[ellemouton]

thanks for the explanation @gijswijs 🙏 i'd just update the comment to include that detail about heap allocation please @sergey3bv

[sergey3bv]

i'd just update the comment to include that detail about heap allocation please

Got it, I will update the comments to feature heap allocation

[lightninglabs-deploy]

@ellemouton: review reminder
@sergey3bv, remember to re-request review from reviewers when ready