Chore/update p2bk

[d4rp4t]

Fixes #26
Spec change: cashubtc/nuts#300 (review)

Summary by CodeRabbit

Release Notes

• Refactor

  • Streamlined core cryptographic operation signatures by removing redundant KeysetId parameters, reducing method complexity and developer integration overhead.

• Tests

  • Comprehensive test suite updates validating functionality with simplified API signatures and internal logic improvements.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Removes KeysetId parameter from P2BK (Pay-to-Public-Key) cryptographic computation APIs in alignment with NUT-28 specification updates. Updates method signatures for ComputeRi, BuildBlinded, and TrySignBlindPath to eliminate keyset-specific context from blinding factor calculations, with corresponding test updates and renamed test identifiers.

Changes

Cohort / File(s)	Summary
Crypto Computation API DotNut/Cashu.cs	Removed keysetId parameter from ComputeRi(byte[] Zx, int i) signature; updated hash computation logic to exclude keysetId from input material
P2PK Builder API DotNut/P2PKBuilder.cs	Updated two BuildBlinded overload signatures to remove KeysetId keysetId parameter; replaced ComputeRi(Zx, keysetIdBytes, i) with ComputeRi(Zx, i)
Blind Signature Flow DotNut/P2PKProofSecret.cs	Removed KeysetId from TrySignBlindPath calls; updated blind-witness path signing to use simplified ComputeRi(Zx, i) computation
Test Updates DotNut.Tests/UnitTest1.cs	Renamed test method from Nut26_Flow to Nut28_P2BK_Flow and test class identifier; updated all BuildBlinded and ComputeRi call sites to match new signatures; added null-safety via ToString()?.ToLowerInvariant() on assertions

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• PR #21: Directly modifies the same APIs and implementation points (ComputeRi, P2PKBuilder BuildBlinded overloads, P2PKProofSecret blind-witness flow).
• PR #24: Overlaps on P2PKBuilder and P2PKProofSecret APIs (BuildBlinded/TrySignBlindPath) and ComputeRi/keysetId parameter usage.

Suggested reviewers

• Kukks

Poem

🐰 A KeysetId hopped away,
BuildBlinded bloomed a simpler way,
ComputeRi sings with fewer notes,
NUT-28 in our cryptographic coats! ✨

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Title check	❓ Inconclusive	The title 'Chore/update p2bk' is vague and generic; it uses non-descriptive phrasing that does not clearly convey the main change (removing keyset id from P2BK blinding factor calculation).	Replace with a more specific title such as 'Remove keyset id from P2BK blinding factor calculation' or 'Update P2BK derivation per NUT-28 spec change'.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Linked Issues check	✅ Passed	The pull request successfully implements the core objective from issue #26: removing the keyset id parameter from ComputeRi and BuildBlinded methods used in P2BK derivation, aligning with the draft NUT-28 spec change.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing the P2BK spec change. Test renaming from Nut26 to Nut28 and API updates to ComputeRi/BuildBlinded are all within scope of removing keyset id from blinding factor calculations.

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✨ Finishing touches

• 📝 Generate docstrings

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