fix: ECDSA signature verification for XML DSig

[leifj]

Summary

Fix ECDSA signature verification by converting XML DSig raw r||s signatures to ASN.1 DER encoding before passing to Go's x509.Certificate.CheckSignature.

Problem

XML DSig (RFC 4050) encodes ECDSA signatures as raw r||s byte concatenation, while Go's x509.Certificate.CheckSignature expects ASN.1 DER encoding (SEQUENCE { INTEGER r, INTEGER s }). This mismatch caused valid ECDSA-signed documents (e.g., Greece EU TSL) to fail signature verification with a cryptographic error.

Changes

• Add isECDSAAlgorithm() helper to detect ECDSA signature algorithms (SHA1/SHA256/SHA384/SHA512)
• Add convertECDSARawToASN1() to convert raw r||s concatenation to DER SEQUENCE
• Apply the conversion in validateSignature() before calling CheckSignature when an ECDSA algorithm is detected
• Add comprehensive tests for both helpers:
  • P-256 and P-384 roundtrip tests (sign with ecdsa.Sign, convert to raw, convert back, verify)
  • Edge cases: empty input, odd-length input, zero values

Testing

All existing tests continue to pass. New tests in ecdsa_test.go cover the conversion logic.

[gemini-code-assist]

Summary of Changes

Hello, 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 addresses a critical issue where ECDSA-signed XML documents failed verification due to a mismatch in signature encoding formats. XML DSig uses a raw r||s concatenation for ECDSA signatures, while Go's x509.Certificate.CheckSignature expects an ASN.1 DER encoding. The changes introduce a conversion mechanism to correctly transform the raw signatures to DER format, ensuring proper validation of such documents.

Highlights

• ECDSA Algorithm Detection: Introduced isECDSAAlgorithm() to identify ECDSA signature algorithms (SHA1, SHA256, SHA384, SHA512).
• ECDSA Signature Conversion: Implemented convertECDSARawToASN1() to transform XML DSig's raw r||s ECDSA signatures into ASN.1 DER encoding.
• Signature Validation Logic: Modified validateSignature() to apply the ECDSA signature conversion before calling Go's x509.Certificate.CheckSignature when an ECDSA algorithm is detected.
• Comprehensive Testing: Added new tests in ecdsa_test.go covering P-256 and P-384 roundtrip conversions and edge cases for the new helper functions.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a bug in ECDSA signature verification by converting the signature format from raw r||s to ASN.1 DER. The added logic and tests are a good addition. I've provided a few suggestions to improve the new test code's correctness regarding cryptographic best practices and to enhance the readability of a new helper function.

[adamdecaf]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for ECDSA signatures in the signedxml package. It adds new functions isECDSAAlgorithm and convertECDSARawToASN1 to handle the conversion of raw r||s ECDSA signatures (as used in XML DSig) to the ASN.1 DER format required by Go's x509.CheckSignature function. Corresponding unit tests for these new functions, covering different elliptic curves and edge cases, have also been added in ecdsa_test.go. No feedback to provide.