RFC7518 Section 6.2 Byte Count Conformance

[justin-lovell]

This pull request improves the handling of EC (Elliptic Curve) key serialization to ensure that integer values are always encoded to the full byte size required by the curve, preventing potential truncation of leading zeros and ensuring compliance with RFC 7518. It also adds comprehensive tests to verify this behavior for different curve types.

Tests Implemented

Introduced new tests in test_ec_key.py to verify that exported EC keys for the P-256, P-384, and P-521 curves always emit base64url-encoded values of the expected full length, regardless of the leading zeros in the underlying integer values.

[Copilot]

Pull request overview

Improves EC JWK serialization to ensure curve integers are encoded at the full, fixed byte length required by RFC 7518 §6.2 (preserving leading zero octets), and adds tests to validate the exported base64url lengths for common NIST curves.

Changes:

• Extend int_to_base64 to support fixed-width integer encoding via an optional byte_count.
• Update EC key export to always encode x, y, and d using the curve’s fixed byte size.
• Add tests asserting fixed base64url string lengths for P-256, P-384, and P-521 exports.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/joserfc/util.py	Adds optional fixed-width support to integer→base64url encoding used by JWK exporters.
src/joserfc/_rfc7518/ec_key.py	Exports EC key parameters using fixed byte counts to avoid truncating leading zeros.
tests/jwk/test_ec_key.py	Adds tests ensuring EC JWK exports always emit base64url values of expected fixed length.

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[justin-lovell]

I've updated the code according to the feedback provided by Copilot. Python isn't used in my day job, so if there are other improvements please point it out.

The non-conformance to the RFC7518 specification was discovered when inspecting application logs failed to emit valid ephemeral key parameters in ECDH-ES+A256KW payloads.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.86%. Comparing base (8aeb191) to head (f881c6a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/joserfc/util.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
- Coverage   99.93%   99.86%   -0.07%     
==========================================
  Files          47       47              
  Lines        2923     2929       +6     
  Branches      337      339       +2     
==========================================
+ Hits         2921     2925       +4     
- Misses          2        3       +1     
- Partials        0        1       +1     

Flag	Coverage Δ
unittests	99.86% <77.77%> (-0.07%)	⬇️

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