Re: [PR] Add PQC (ML-DSA and ML-KEM) support for XML Dig ital Signature and Enc… [santuario-xml-security-java]

Arpan0995 commented on PR #645:
URL: https://github.com/apache/santuario-xml-security-java/pull/645#issuecomment-5317670857

   I have been looking at post-quantum support across a number of Java crypto libraries and read through this PR with interest. Two choices in the KEM path stood out as the right ones: it takes the encapsulation length from the KEM API (Decapsulator.encapsulationSize()) rather than a hardcoded per-algorithm table, and it derives the key-wrap key through a KDF rather than using the raw shared secret.
   
   One gap I noticed while reading the tests. The new PQC tests (XMLSignatureMLDSATest, XMLEncryptionMLKEMTest, StaxMLDSASignatureTest, StaxMLKEMEncryptionTest) all exercise the happy path only, sign-then-verify and encrypt-then-decrypt. For a signature and encryption feature it would be worth adding a few negative cases that lock in the security-relevant behaviour, for example:
   
   - a tampered ML-DSA signature (a flipped byte of SignatureValue) is rejected by verification;
   - an ML-DSA signature checked against the wrong public key fails;
   - ML-KEM decryption with the wrong recipient private key fails cleanly rather than returning wrong plaintext;
   - a truncated or corrupted KEM encapsulation (shorter than encapsulationSize, or a flipped byte) is rejected with a clear exception.
   
   These are the cases that would catch a regression in the verify or decapsulate path later, and they are quick to add on top of the round-trip tests already there. Since you are planning to split this into separate Signature and Encryption PRs, the ML-DSA cases would go with the signature side and the ML-KEM cases with the encryption side. Happy to help with any of them if useful.


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