fix(security): protect owned encryption secrets

[polaz]

Summary

• Zeroize owned recipient KEKs and decryption-resolver keys on drop, including independent clones; leave borrowed caller material unchanged without creating a copy.
• Compare recovered key bytes and recovery validity using constant-time primitives; retain handle identity for non-exportable keys.
• Add secret-ownership regressions and a real cryptographic signature-wrapping test across compiled XML backends.
• Document security invariants, their regression coverage, and caller ownership responsibilities.

Validation

• All-feature workspace suite with isolated SoftHSM and pinned libxmlsec1 oracle: 4,398 passed, zero skipped.
• Workspace all-feature build, all-target Clippy with warnings denied, formatting, and doctests passed.
• Host and bare-metal alloc-only checks passed.
• Dedicated roxmltree-only and xmloxide-only public suites: 20 passed each; C14N-only all-target check passed.
• Capability and behavior ledger checks passed.

Closes #213

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-11T14:40:47.924572Z	ff0bf08	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[coderabbitai]

Warning

Review limit reached

You've used all free OSS reviews for now. Wait for the free limit to reset to keep reviewing this public repository.

Next included review available in 38 minutes.

Check out review usage here.

View limit details

Limit details: You’ve used the included review currently available.

Learn how review limits work.

Review configuration:

⚙️ Run configuration

• Configuration used: Repository: structured-world/xml-sec/.coderabbit.yaml
• Review profile: CHILL
• Plan: Advanced
• Run ID: 13be67e0-76d2-48da-9d32-af08ca248f35

---

📥 Commits

Reviewing files that changed from the base of the PR and between be757dc and ff0bf08.

---

📒 Files selected for processing (6)

• docs/security-invariants.md
• src/provider.rs
• src/xmlenc/decrypt.rs
• src/xmlenc/types.rs
• tests/secret_ownership.rs
• tests/verification_evidence.rs

---

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

---

_{Comment @coderabbitai help to get the list of available commands.}