Track B · PR 4a/7 — Rust: tls/ module (parsers, no networking)

[bradh11]

Parent: #28 · Track B (wire-side) · No dependencies · Blocks PR 4b

Scope

Foundational Rust submodule for TLS 1.3 handshake parsing. No networking, no PyO3 export — purely deterministic byte parsing.

New files

```
rust_certinfo/src/tls/
├── mod.rs
├── groups.rs ← IANA Supported Groups table (contributor-facing data file)
├── records.rs ← TLS record framing (read + write)
└── handshake.rs ← ClientHello builder + ServerHello / HelloRetryRequest parser
```

In scope

• GroupKind enum (ClassicalEcdh, ClassicalFfdh, HybridPq, PurePq, Unknown)
• GroupInfo { id, name, kind } static slice
• Lookup function
• TLS record framing: read TLS records up to a bounded length, recognize content types
• ClientHello byte builder with realistic extension set (SNI, supported_versions, signature_algorithms, supported_groups, key_share, ALPN)
• ServerHello / HelloRetryRequest parser that extracts negotiated/selected group ID
• Re-use DerReader patterns where they fit; create a small TlsHandshakeReader if needed

Out of scope

• TCP sockets
• PyO3 exports
• TLS 1.2 ServerKeyExchange (Future / PR 7)

Tests

• Parse captured ServerHello bytes (capture beforehand via openssl s_client -msg against Cloudflare for TLS 1.3 + hybrid PQ, a stock nginx, a TLS-1.2-only host)
• Check fixtures in under rust_certinfo/tests/fixtures/
• ClientHello build → parse round-trip
• Malformed input: oversized length fields, truncated records, unknown extensions all handled without panic

Definition of Done

• make ci clean
• Coverage ≥ 95%
• CHANGELOG entry added ("foundation for upcoming PQ KEX validator")
• PR opened against develop from feat/tls-handshake-parsers
• No PyO3 export yet — not visible from Python
• tls/groups.rs has a clear "add new groups here" comment for contributors