fix(core): cap the blinded recipient scan in open_blob (#419)

[beardthelion]

Summary

Closes #419. open_blob trial-decrypts every entry in the envelope's blinded recipient list, one X25519 exchange each, and the envelope author controls the count. The scan itself is by design (the AEAD tag authenticates the reader's own entry), so the bound has to be a cap: a header carrying more than MAX_RECIPIENTS = 256 entries is rejected right after decode, before any crypto. Real envelopes carry a handful of authorized readers, so 256 is far above legitimate use.

Test plan

• cargo test -p gitlawb-core — 100/100 pass.
• New over_cap_recipient_list_is_rejected_before_the_scan reframes a valid envelope with 257 junk recipient entries and asserts the cap error; an at-cap (256) envelope reaches the scan and fails as "not a recipient" rather than tripping the cap.
• Revert check: with the cap removed, the 257-entry envelope runs the full scan and returns the plain not-a-recipient error, so the test is load-bearing.
• cargo test --workspace green; cargo clippy --workspace --all-targets -- -D warnings clean via the pre-push gate.

Notes

• Overlap: fix(core): reject small-order Ed25519 keys at DID resolution and before X25519 #314 edits the my_x setup a few lines below this insertion and appends tests at the same mod anchor; the changes compose, and whichever lands second rebases.
• Seal side untouched: seal_blob takes the caller's own recipient set.

Summary by CodeRabbit

• Bug Fixes
  • Added a safety limit for recipients when sealing data.
  • Oversized recipient lists are rejected before encryption or serialization.

[coderabbitai]

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• crates/gitlawb-core/src/encrypt.rs

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• crates/gitlawb-core/src/encrypt.rs

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📝 Walkthrough

Walkthrough

The encryption module adds a shared 256-recipient limit. seal_blob and open_blob reject larger lists before encryption or recipient scanning. Tests cover oversized lists and the boundary value.

Changes

Envelope recipient cap

Layer / File(s)	Summary
Recipient limit and validation crates/gitlawb-core/src/encrypt.rs	Defines MAX_RECIPIENTS as 256. seal_blob rejects oversized recipient lists before encryption. open_blob rejects them before recipient scanning and key exchanges. Tests cover rejection above the limit and processing at the limit.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 9a8dd

The recipient cap is applied on both sealing and opening paths with boundary coverage; no current merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and clearly identifies the primary change: capping the blinded recipient scan in open_blob for issue #419.
Description check	✅ Passed	The description provides the motivation, behavior change, test coverage, verification commands, and scope notes. It does not use every template heading or complete the requested checklist, but the req…
Linked Issues check	✅ Passed	Issue #419 requires a bound on the attacker-controlled blinded recipient scan in open_blob. The reviewed encrypt.rs decodes the header, rejects lists over MAX_RECIPIENTS = 256, and performs this…
Out of Scope Changes check	✅ Passed	The current-head change adds the sealing-side cap and its test. This change directly preserves the opening-path cap invariant because seal_blob otherwise could create envelopes that open_blob cann…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 1 files.

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• Commit unit tests in branch fix/issue-419-recipient-scan-cap

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Inline comments:
In `@crates/gitlawb-core/src/encrypt.rs`:
- Line 138: Update seal_blob to reject recipient lists whose length exceeds
MAX_RECIPIENTS before encryption or serialization, matching open_blob’s
validation, and add a test confirming oversized recipient sets are rejected.

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Reviewing files that changed from the base of the PR and between bfc44f9 and ecee615.

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• crates/gitlawb-core/src/encrypt.rs

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[greptile-apps]

Greptile Summary

This PR introduces a 256-entry limit on blinded recipient lists before open_blob begins its X25519 trial-decryption scan and adds boundary coverage for 256 and 257 entries.

• The scan itself is now bounded.
• The sealing path remains uncapped and can therefore generate envelopes that the new opening path refuses to decrypt.

Confidence Score: 4/5

The PR is not safe to merge until sealing and opening enforce compatible recipient-count constraints.

The new reader-side cap bounds malicious scans, but an unrestricted production sealing path can still create an over-cap envelope that every legitimate recipient is then prevented from opening.

Files Needing Attention: crates/gitlawb-core/src/encrypt.rs

Important Files Changed

Filename	Overview
crates/gitlawb-core/src/encrypt.rs	Adds the recipient scan cap and boundary test, but leaves seal_blob able to create envelopes exceeding the new reader-side limit.

_{Reviews (1): Last reviewed commit: "fix(core): cap the blinded recipient sca..." | Re-trigger Greptile}

[greptile-apps]

Writer Exceeds Reader Limit

seal_blob accepts every recipient from the unrestricted visibility-rule reader set, so a rule with more than 256 readers produces an envelope that this new check rejects before scanning—even for a valid recipient. The resulting blob cannot be decrypted by any authorized reader. Enforce the same limit before sealing, or otherwise prevent the writer from producing envelopes that readers reject.

[jatmn]

LGTM