MSP curve is hardcoded in constructor, ignoring IdemixMSPConfig.CurveId #85

[adecaro]

…urveId## Summary

• Curve and BCCSP construction moved from the constructor into Setup, driven by IdemixMSPConfig.CurveId.
• Setup now enforces that the config type matches the constructor used.
• NewIdemixMspWithLogger / NewIdemixMspAriesWithLogger added for custom-logger injection.

Motivation

IdemixMSPConfig has carried a curve_id string since idemixgen began writing it, but
Setup never read the field — the curve was hardcoded per constructor. This meant loading
a config generated with idemixgen --curve BLS12_381 through NewIdemixMsp would silently
parse keys against the wrong curve and fail with a cryptographic error rather than a clear
message. There was also no guard against mixing a dlog constructor with an Aries config.

Changes

msp/idemixmsp.go

Idemixmsp struct — two new fields:

• aries bool — set by the constructor to record which scheme was chosen.
• exportable bool — carries the (always-true) exportable flag into the deferred BCCSP build.

Constructors — NewIdemixMsp and NewIdemixMspAries no longer build a BCCSP eagerly.
They return a lightweight struct and delegate to the new WithLogger variants:

NewIdemixMsp(version)            → NewIdemixMspWithLogger(version, nil)
NewIdemixMspAries(version)       → NewIdemixMspAriesWithLogger(version, nil)
NewIdemixMspWithLogger(version, logger)       // dlog, custom logger
NewIdemixMspAriesWithLogger(version, logger)  // Aries, custom logger

Passing nil as the logger falls back to the default zap-based logger.

curveAndTranslator helper — maps curve_id strings to (*math.Curve, Translator),
mirroring the switch in tools/idemixgen/main.go. Supported values: FP256BN_AMCL,
BN254, FP256BN_AMCL_MIRACL, BLS12_377_GURVY, BLS12_381_GURVY, BLS12_381,
BLS12_381_BBS, BLS12_381_BBS_GURVY.

Setup — after unmarshalling the config:

1. Enforces type/constructor match (IDEMIX ↔ dlog, IDEMIX_ARIES ↔ Aries).
2. Resolves curve_id with per-scheme defaults and restrictions:
   • dlog: any valid curve; empty → FP256BN_AMCL.
   • Aries: only BLS12_381_BBS or BLS12_381_BBS_GURVY; empty → BLS12_381_BBS.
3. Builds the BCCSP (idemix.New or idemix.NewAries) with the resolved curve.
4. BCCSP construction failure is now a returned error instead of a panic.

msp/idemixmsp_test.go

Updated TestSetupBad: the error string check for a wrong config type is relaxed to
"setup error:" to match the now-more-descriptive message.

README.md

Added an "MSP Usage" section documenting the two constructors, the curve-selection
mechanism, the full curve_id table, and the enforcement rules.

Backward compatibility

All existing callers are unaffected:

• NewIdemixMsp / NewIdemixMspAries signatures are unchanged.
• Empty curve_id in a config falls back to the same curve that was previously hardcoded.

[ale-linux]

LGTM overall - just a couple of Qs

[ale-linux]

We only support these 2?

[adecaro]

if you explicitly called NewIdemixMspAries, then we enforce this because the constructor was enforcing that curve

[ale-linux]

is this expected? We have CLAUDE.md pointing to @AGENTS.md, and AGENTS.md whose first line is CLAUDE.md?

[ale-linux]

this is no longer correct right?

[adecaro]

correct

[ale-linux]

LGTM - please squash-merge and don't rebase :)