# Layer 3 — Mode 1: `FOF_CLOSED` Posts

**Scope**: New `PostVisibility::FoFClosed` variant. Both post body AND comments are gated to the FoF graph. Body is encrypted under CEK; readership emerges from keyring intersection with `wrap_slots`.

The wrap-slot structure, `pub_post_set`, per-`V_x` signing keypair, and CDN-level verification are all defined in [Layer 2](layer-2-mode2-fof-comments.md) (the canonical form). Layer 3 inherits Layer 2's structures unchanged — the Mode 1 vs Mode 2 distinction reduces to: **body encrypted under CEK (Mode 1) vs body plaintext (Mode 2)**. Wrap-slot read-part still carries the CEK in both modes; in Mode 2 the CEK is used only to derive `CEK_comments`, in Mode 1 it is used for both body and comments.

The legacy `pub_post`/`priv_post` field names that appear in some sections below are retained for readability; semantically read them as the canonical per-V_x `(pub_x, priv_x)` from Layer 2.

---

## Goal

- Author creates a post with `visibility = FoFClosed`.
- Body ciphertext in the CDN; only FoF-reachable readers can decrypt.
- `wrap_slots` carry both the post CEK and `priv_post`, wrapped under each `V_x` the author holds.
- Readers trial-decrypt slots whose 2-byte prefilter tag matches a held key.
- Comments use Layer 2's `group_sig` / `vouch_mac` mechanism — unchanged.
- Observers cannot enumerate recipients from the header; slot count padded to power-of-2 buckets.

---

## Lead decisions

- **New variant, not extended Encrypted.** `PostVisibility::FoFClosed` is its own variant. Existing `Encrypted{recipients}` wraps per-recipient NodeIds — visible on the wire. FoF wraps anonymously under symmetric keys — no NodeIds.
- **One wrap slot per unique `V_x`.** Dedup at the `V_x` byte level — if multiple personas hold the same `V_x`, include one slot. Friends-only post: one slot under `V_me`. FoF post: own `V_me` + every distinct `V_x` the author holds. Custom: subset chosen by author (deferred to v2 power-user UI; v1 ships three presets only: Public / Friends-only / Friends-of-Friends).
- **Bucketed slot-count padding.** Deterministic bucket boundaries throughout — observers learn the bucket, not the position within it. Buckets: 8, 16, 32, 64, 128, 256 (power-of-2 from a minimum of 8 up to 256), then 384, 512, 640, 768, … (linear +128 steps above 256). Author publishes `next_bucket(real_count)` slots with random dummies filling the gap. Minimum bucket of 8 means a brand-new persona's first post still publishes 8 slots — no "this persona has no vouchees" signal. Power-of-2 sub-256 keeps small-author overhead bounded; +128 steps above 256 avoid the 2× waste of pure power-of-2 at scale. Dummy slots are byte-identical to real ones, AEAD-fails on any `V_x`. Dummy entries also added to `pub_post_set` 1:1 (see Layer 2).
- **Bucketed body-size padding.** Same shape applied to body ciphertext bytes: power-of-2 buckets up to 256KB (1KB, 2KB, 4KB, …, 256KB), then 256KB-step buckets above (512KB, 768KB, 1024KB, …). 256KB above is the future storage chunk size — once large enough to chunk, padding aligns to chunk boundaries naturally.
- **Each slot carries both CEK and priv_x (Layer 2 dual-derivation).** Layer 2's `WrapSlot` dual-derivation (read → CEK, sign → priv_x) is the canonical form. Mode 1 simply also uses the CEK to encrypt the body, where Mode 2 leaves the body plaintext.
- **Prefilter tag is `HMAC(V_x, post_id)[:2B]`.** Readers precompute a 2-byte tag for each key in their keyring and skip slots that don't match. Cuts trial-decrypt cost by ~2^16 on average.
- **Order of slots is randomized.** No positional leak about which slot corresponds to which voucher. Re-shuffled on every header revision (including access-grant appends from Layer 2 — TBD whether append-only ordering is acceptable, or whether the entire set is re-shuffled at each grant).

---

## Data model

### Extend `PostVisibility`

```rust
pub enum PostVisibility {
    Public,
    Encrypted { recipients: Vec<NodeId> },
    GroupEncrypted { group_id: GroupId, epoch: u32, wrapped_cek: Vec<u8> },
    FoFClosed {                              // NEW
        pub_post: [u8; 32],
        wrap_slots: Vec<WrapSlot>,
    },
}
```

### `WrapSlot` byte layout

TBD — OPUS. Target shape:

```
WrapSlot {
    prefilter_tag:  [u8; 2],   // HMAC(V_x, post_id)[:2B]
    nonce:          [u8; 12],  // AEAD nonce (unique per slot)
    ciphertext:     [u8; N],   // AEAD(V_x, nonce, plaintext) — plaintext is CEK || priv_post
    tag:            [u8; 16],  // AEAD auth tag
}
```

Plaintext inside the slot:

```
SlotPlaintext {
    cek:       [u8; 32],       // ChaCha20-Poly1305 key for body
    priv_post: [u8; 32],       // ed25519 seed for per-post keypair
}
```

TBD — OPUS:
- AEAD choice (ChaCha20-Poly1305 matches existing usage — confirm).
- Whether to include `post_id` as AAD to bind slot to post and prevent slot-reuse across posts.
- Padding scheme for dummy slots (same size + random bytes that fail AEAD on any key).

---

## Encryption (author, post creation)

1. Generate per-post ephemeral ed25519 keypair `(priv_post, pub_post)`.
2. Generate random 32B CEK.
3. Encrypt body: `body_ct = ChaCha20-Poly1305(CEK, nonce, body || aad=post_id)`.
4. For each `V_x` in selected set (own `V_me` + held vouches for FoF, or custom subset):
   - `prefilter_tag = HMAC(V_x, post_id)[:2B]`
   - `slot_ct = AEAD(V_x, slot_nonce, CEK || priv_post, aad=post_id)`
   - Append `WrapSlot { prefilter_tag, slot_nonce, slot_ct, tag }` to `wrap_slots`
5. Pad `wrap_slots` to next power of 2 with dummy slots.
6. Randomize slot order.
7. Sign post header with author identity key as normal.

---

## Decryption (reader side)

1. Receive post. Parse header → `pub_post`, `wrap_slots`.
2. For each `V_x` in reader's keyring (all personas):
   - Compute `candidate_tag = HMAC(V_x, post_id)[:2B]`.
   - For each slot with matching `prefilter_tag`, attempt `AEAD-open(V_x, slot.nonce, slot.ct)`.
   - On success: extract `CEK`, `priv_post`. Decrypt body with CEK. Record which `(persona, V_x)` won (Layer 5 cache).
3. If no slot unwraps across all personas: mark post as unreadable (Layer 5 retry table).

Expected cost without prefilter: `keyring_size × slot_count` AEAD attempts. With 2B prefilter: `keyring_size × slot_count / 65536` AEAD attempts on average. For 500×500 = 250K attempts → ~4 attempts average. Prefilter is load-bearing for feasibility; see Layer 5.

---

## Receive-path integration

Extend `control::receive_post` with a new gate branch for `FoFClosed`:

- Verify author identity sig over header (as existing).
- Verify `wrap_slots` well-formedness: each slot has correct field sizes; slot count is power of 2.
- Accept without being able to decrypt. Non-FoF nodes still store and propagate the ciphertext as part of normal CDN replication. Decryption is a READ-side concern.

---

## Ciphertext propagation

Ciphertext `FoFClosed` posts ride the same CDN propagation as other encrypted posts. No change to `BlobHeaderDiff` / `file_holders`. Any node can hold and forward the ciphertext; only FoF-graph nodes can decrypt.

---

## Open questions

- **Prefilter false-positive cost.** 1/65536 false positive per slot. With 500 slots × reader iterating 500 keys, expected ~3.8 false-positive AEAD attempts per post. Acceptable.
- **Prefilter collision on legitimate hits.** Two different `V_x` could produce the same `prefilter_tag` for the same `post_id`. Reader just tries both. No correctness issue.
- **Slot-reuse across posts.** If the same `V_x` is used across many posts, an attacker can observe prefilter tags recur. Since `post_id` is in the HMAC input, tags differ per post. No leak.

## Resolved (2026-04-24)

- **Slot count padding**: bucketed throughout. Power-of-2 buckets from 8 to 256 (minimum bucket is 8 — singleton/tiny-set posts pad up to 8 to avoid leaking "new persona with no vouchees"), then linear +128 buckets (384, 512, 640, …). Body-size padding follows the same shape with 256KB as the power-of-2 ceiling and 256KB linear steps above.
- **Access-grant ordering**: **append at tail** (Scott's call). New entries land at the end of `pub_post_set` + `wrap_slots`. `pub_x_index` values in already-stored comments stay valid. Small positional-recency leak (tail = recent grants) is the accepted cost.
- **Custom mode UI**: deferred. v1 ships only the three presets (Public / Friends-only / FoF). Power-user custom-subset UI is v2.
- **Slot deduplication**: dedup at the `V_x` byte level. One slot per unique key.
- **Body length padding**: yes — pad to next power of 2 up to 256KB, then 256KB chunks above.

---

## Ship criteria for Layer 3

- `PostVisibility::FoFClosed` exists end-to-end.
- Author creation path generates per-post keypairs, wraps CEK+priv_x under each unique `V_x` (deduped), and pads to the next slot bucket: power-of-2 up to 256, then +128 steps above.
- Body-size padded to next body bucket: power-of-2 up to 256KB, then 256KB steps above.
- Reader decryption path iterates personas × keyring with prefilter tag.
- `receive_post` accepts FoFClosed ciphertext without decrypting.
- UI surface: post composer has three presets — Public / Friends-only / Friends-of-Friends. Custom subset is v2.
- Integration test: A posts FoFClosed. B (direct vouchee) reads. C (FoF via B) reads. D (unrelated) gets ciphertext, cannot decrypt.
- Performance: decryption completes within budget at 500-key keyring × 500-slot posts (see Layer 5 for the optimization work that makes this budget feasible).