# M4a task 01: SHA-1 in `proto`

**Branch:** `m4a` (run `git switch m4a`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `proto: sha1, for the WebSocket handshake check`

## Goal

`gatewayd` speaks WebSocket to Mattermost. The server proves it understood the handshake by
sending back the SHA-1 of our key (RFC 6455, section 4.2.2), so we need SHA-1. It lives in
`proto`, which has no dependencies of its own. It is used for that check and nothing else: it must
never protect anything.

## Files

- Copy: `crates/proto/tests/sha1.rs`, and the skeleton `crates/proto/src/sha1.rs`
- Modify: `crates/proto/src/lib.rs` (`pub mod sha1;`), `docs/implementer-log.md`

## Interface (in the skeleton)

```rust
pub fn sha1(data: &[u8]) -> [u8; 20];          // written: new, update, finish

pub struct Sha1 { state: [u32; 5], block: [u8; 64], filled: usize, length: u64 }
impl Sha1 {
    pub fn new() -> Sha1;                        // written: the five initial words
    pub fn update(&mut self, data: &[u8]);       // todo
    pub fn finish(self) -> [u8; 20];             // todo
    fn compress(&mut self, block: &[u8; 64]);    // todo
}
```

`length` counts **bits** fed so far. Each `todo!()` has its steps above it. No indexing that can go
out of bounds: `w[i]` on the fixed `[u32; 80]` inside `compress` is fine; slicing `data` or `block`
by a computed range is not (use `get`, `get_mut`, `split_at`). Use `as_chunks::<4>()` to read
big-endian words.

## Steps

- [ ] **1. Copy.** `git switch m4a`, then
  `cp docs/plans/M4a/files/crates/proto/tests/sha1.rs crates/proto/tests/` and
  `cp docs/plans/M4a/files/crates/proto/src/sha1.rs crates/proto/src/`. Add `pub mod sha1;` to
  `crates/proto/src/lib.rs`.
- [ ] **2. See it fail.** `cargo test -p proto --test sha1`. Expected: it compiles, and 4 tests
  fail with `not yet implemented`.
- [ ] **3. Fill the three functions**, one at a time: `compress`, then `update`, then `finish`. Run
  `cargo check -p proto` after each.
- [ ] **4. See it pass.** `cargo test -p proto --test sha1`. Expected: 4 passed. The tests include
  FIPS 180 and RFC 3174 vectors, a million `a`s, the lengths around the 64-byte block where the
  padding changes shape (checked with `sha1sum`), and the RFC 6455 handshake example; every case is
  also fed in pieces.
- [ ] **5. Run the gate.** `cargo fmt --all`, then `make gate`. Expected last line: `gate: ok`.
- [ ] **6. Log and commit.** `git add crates/proto docs/implementer-log.md Cargo.lock && git commit`

## Done when

- `cargo test -p proto --test sha1` passes; `make gate` prints `gate: ok`.

## Stop and report if

- A test vector fails and you cannot find why after checking `compress` against the comment twice.