Harden public key deserialization: add curve point validation

[dobby-coder]

A draft security advisory has been filed for a medium-severity finding in public key deserialization across multiple IBE/KEM schemes.

Advisory: https://github.com/encryption4all/ibe/security/advisories/GHSA-25fp-2fjj-g84w

The advisory describes a gap between the documented trust assumption (public keys are authenticated infrastructure) and what the API enforces at the boundary. The fix involves replacing unchecked curve point deserialization with checked variants or adding explicit subgroup membership verification.

Affected files (public-facing from_bytes implementations):

• src/ibe/cgw.rs
• src/kem/kiltz_vahlis_one.rs
• src/kem/cgw_kv.rs
• src/ibe/boyen_waters.rs

/dobby fix — switch from_compressed_unchecked to from_compressed (or add explicit subgroup checks) in all public-key from_bytes implementations listed in the advisory; ensure CI passes

[dobby-coder]

On it! I'm replacing unchecked curve point deserialization with checked variants across the IBE/KEM files in this repo and will open a draft PR once the fixes are in.

[dobby-coder]

PR #51 is gatekept and ready. One nit surfaced during review: the security-rationale comment in 4 files (and the PR body) said "BLS12-381 G1 cofactor is 76" — the real G1 cofactor is a 126-bit value, not 76. I fixed all four comments and the PR body in commit 42a9e89, re-ran cargo fmt and the full test suite (31/31 pass), and flipped the draft to ready-for-review. Since GitHub blocks APPROVE on self-authored PRs, the gatekeeper sign-off was submitted as a comment.