feat(sdk-core): add getEddsaMPCv2RecoveryKeyShares helper

[vibhavgo]

Summary

Adds and exports getEddsaMPCv2RecoveryKeyShares — a standalone helper that decrypts both EdDSA MPCv2 reduced keycards in parallel, validates that pub and rootChainCode match independently, and returns typed Buffer key shares with the derived commonKeyChain.

Ticket: WCI-396

Changes

• Parallel decryption of user and backup keycards via Promise.all
• Validate pub and rootChainCode separately with distinct error messages
• Wrap getDecodedReducedKeyShare in try-catch to surface a descriptive error for malformed/missing keycard fields
• Clear unsupported error for v2 (Argon2id) envelopes detected via isV2Envelope
• 4 unit tests: v1 happy path, v2 rejection, malformed keycard, mismatched wallet keys

Test Plan

• yarn run unit-test --scope @bitgo/sdk-core -- -- --grep "getEddsaMPCv2RecoveryKeyShares" — all 4 tests pass

CLOSES TICKET: WCI-396

[linear-code]

WCI-396

[rajangarg047]

@claude

[github-actions]

Claude finished @rajangarg047's task —— View job

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Code Review for getEddsaMPCv2RecoveryKeyShares helper

Todo List:

• Read repository guidelines (CLAUDE.md)
• Review implementation code in eddsaMPCv2.ts
• Review test coverage and quality
• Analyze for bugs, security issues, and best practices
• Provide detailed feedback and recommendations

Overall Assessment: ✅ Strong Implementation

This is a well-implemented feature that follows best practices. The code is clear, secure, and thoroughly tested.

🎯 Strengths

1. Excellent Error Handling

   • Clear, descriptive error messages for all failure scenarios (modules/sdk-core/src/bitgo/utils/tss/eddsa/eddsaMPCv2.ts:942-986)
   • Proper distinction between v2 envelope rejection and malformed keycard errors
   • Defensive validation of required fields (keyShare, pub, rootChainCode)

2. Security Best Practices

   • Parallel processing with Promise.all prevents timing side-channels (line 967)
   • Independent validation of pub and rootChainCode with separate error messages (lines 971-973, 977-979)
   • Try-catch wrapper around getDecodedReducedKeyShare provides graceful error handling (lines 950-957)
   • Clear v2 envelope rejection prevents incorrect decryption attempts (lines 942-946)

3. Comprehensive Test Coverage

   • 4 test scenarios covering all critical paths (modules/sdk-core/test/unit/bitgo/utils/tss/eddsa/eddsaMPCv2.ts:344-394)
   • v1 happy path, v2 rejection, malformed keycard, and mismatched wallet keys
   • Tests use realistic mock data and validate both success and failure cases

4. Clean API Design

   • Well-defined TypeScript interface (EddsaMPCv2RecoveryKeyShares, lines 47-51)
   • Optional walletPassphrase parameter with proper fallback handling
   • Returns typed Buffer objects for key shares

🔍 Technical Review

Implementation (eddsaMPCv2.ts:936-986)

• Function signature is appropriate for standalone use
• v2 envelope detection using isV2Envelope is correct
• SJCL decryption usage aligns with existing patterns in the codebase
• Buffer handling follows established conventions

Error Messages

• Line 943-946: Clear explanation of v2 limitation with helpful guidance
• Line 954-956: Specific error for malformed keycards mentioning required fields
• Line 972: Precise error for mismatched pub keys
• Line 978: Precise error for mismatched rootChainCode

Return Value Construction

• Lines 981-985: Proper Buffer.from() usage and string concatenation for commonKeyChain

🧪 Test Quality Assessment

The test suite is exemplary:

1. Realistic test data: Uses MPSUtil.generateEdDsaDKGKeyShares() for authentic key material
2. Edge case coverage: Tests v2 envelope rejection, malformed data, and validation failures
3. Clear test names: Each test clearly describes the scenario being tested
4. Proper assertions: Tests both return values and error conditions

✅ Recommendations

Minor Suggestions:

1. Consider adding JSDoc: While the function is well-named, JSDoc comments would enhance maintainability
2. Type annotation: The decodeKey inner function could benefit from explicit return type annotation for clarity

Future Enhancements (not blocking):

1. v2 support: When v2 decryption support is added, this function provides a clear migration path
2. Validation levels: Could potentially add different validation strictness levels in the future

🔒 Security Analysis

• No timing vulnerabilities: Parallel processing prevents timing-based attacks
• No secret leakage: Errors don't expose sensitive information
• Proper input validation: All inputs are validated before processing
• Safe error handling: Try-catch blocks prevent crashes from malformed input

📝 Conclusion

This implementation demonstrates excellent software engineering practices. The code is:

• Secure with proper input validation and error handling
• Performant with parallel processing
• Maintainable with clear structure and comprehensive tests
• User-friendly with descriptive error messages

The PR is ready for merge and serves as a good example of how to implement standalone utility functions in this codebase.

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