# FOG Architecture

Status: Draft 0.1

Date: 2026-08-08

## 1. Purpose

This document defines the normative component architecture, trust boundaries,
information flows, state ownership, deployment units, and dependency rules for
the FOG autonomous mix network.

It refines `FOG-THREAT-MODEL.md`. It does not select final cryptographic
algorithms, packet geometry, delay distributions, or active numeric message,
storage, and Composer suites. Those decisions belong to narrower protocol
specifications and MUST satisfy the boundaries defined here.

FOG is not implemented yet. Statements in this document are design
requirements, not claims about deployed software.

The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, and MAY describe
normative requirements in the sense of BCP 14 when they appear in uppercase.

## 2. Architectural Scenario

FOG is one product and one coordinated protocol suite with multiple
independently deployable security roles. It is not a single daemon, a shared
database application, or a collection of unrelated public microservices.

The baseline implementation shape is:

- one source repository and release process;
- one separately built executable for each online security role;
- a networkless Composer executable and image profile;
- small protocol libraries with strict dependency direction;
- a public client SDK with a deliberately narrow surface;
- a simulator and conformance corpus in the same repository;
- local Podman deployment only for the functional PoC;
- independent hosts and operators where a deployment makes anonymity claims.

Separate processes are justified by independent deployment, key ownership,
network policy, compromise containment, and operator ownership. Application
features that do not need a server-side trust boundary MUST remain modules,
not daemons.

## 3. Core Architectural Invariants

The following invariants apply to every profile.

### ARC-001: No universal trust domain

No process, host, operator account, database, management system, or private key
MAY be required to observe or control the complete user-to-storage path.

### ARC-002: The Composer is networkless

`fog-compose` MUST have no network interface. All external input and output
cross explicit bounded transfer interfaces. A network-capable convenience
mode is not a conforming Composer profile.

### ARC-003: No data-plane bypass

Native user operations MUST traverse an entry, all configured mix layers, and
a terminal courier or service endpoint. Failure handling MUST NOT introduce a
direct client-to-courier, client-to-storage, entry-to-storage, or
mix-to-clearnet path.

### ARC-004: Three distinct cryptographic layers

Message end-to-end protection, KEMSphinx routing protection, and adjacent-node
Noise transport MUST use purpose-separated protocols and keys. Success or
failure of one layer MUST NOT be interpreted as validation of another.

### ARC-005: Complete authenticated network view

Route construction, entry eligibility, node roles, packet profiles, security
parameters, and key validity MUST derive from a complete threshold-authenticated
consensus accepted by the Composer. A relay-supplied partial view MUST NOT
control route construction.

### ARC-006: Uniform external behavior

Native applications MUST share consensus-authorized packet classes, routing
rules, retry classes, and cover processing. Core nodes MUST NOT vary packet
geometry or parser selection according to `fog-drop`, `fog-mailbox`, or
`fog-im`.

### ARC-007: One private-key owner

Each private key MUST have one owning role and one purpose. Separate roles MUST
NOT share private key files, secret volumes, writable state directories, or
backup credentials.

### ARC-008: Bounded state and parsing

Every queue, cache, request, response, parser, retry sequence, and storage
operation MUST have explicit size, time, and resource bounds. Unknown-critical,
non-canonical, expired, or unauthenticated input MUST fail closed.

### ARC-009: Privacy-aware failure

Availability recovery MUST preserve the declared layer count, packet class,
authenticated profile, entry constraints, cover policy, and consensus
freshness. If it cannot, the affected operation MUST stop and report a local
coarse failure.

### ARC-010: No inherited external guarantee

Tor, Nym, Katzenpost, YAMN, SMTP, NNTP, and foreign Sphinx networks are not
runtime dependencies of the FOG core. An edge bridge terminates the FOG trust
claim and MUST publish a separate threat model.

## 4. System Planes

FOG separates four planes:

1. **Offline user plane**: plaintext, identities, contacts, message state,
   storage capabilities, route construction, and validation inside the
   Composer.
2. **Anonymous data plane**: entry, stratified mixes, courier, native service
   endpoints, and storage replicas carrying fixed-class opaque traffic.
3. **Consensus control plane**: node descriptors, authority voting,
   threshold-signed consensus, revocation, profile authorization, and
   equivocation evidence.
4. **Operations and release plane**: local administration, signed builds,
   configuration generation, coarse metrics, backups, incident response, and
   updates.

These planes MAY share public code and formats. They MUST NOT share universal
credentials or writable runtime state.

```text
                         consensus control plane
             node descriptors -> authorities -> signed consensus
                                      |
                                      v
offline user plane               anonymous data plane

Composer -> transfer -> blind relay -> entry -> L1 -> L2 -> L3 -> courier
   ^                    |                                           |
   |                    |                                           v
   +---- opaque input <-+                                  storage replicas

                         operations and release plane
          signed artifacts, local administration, coarse aggregates
```

## 5. Data Classification

Architecture and protocol documents MUST label fields and state using these
classes or a stricter derived scheme.

| Class | Meaning | Examples |
| --- | --- | --- |
| `PUBLIC-AUTH` | Public but authenticity or freshness matters | consensus, descriptors, release metadata, protocol profiles |
| `OPAQUE-META` | Encrypted content with sensitive timing or relationship metadata | KEMSphinx packets, transfer bundles, reply envelopes, relay queues |
| `CAPABILITY` | Possession grants a read, write, reply, or recovery operation | mailbox capabilities, reply blocks, rendezvous handles |
| `SECRET-ROLE` | Private material owned by one online role | Noise, KEMSphinx, authority, replica, queue-sealing keys |
| `SECRET-USER` | User secret or conversation state | identity keys, ratchet state, contact state, backup keys |
| `PLAINTEXT-USER` | Decrypted user content | drafts, messages, rendered attachment content |
| `AGGREGATE` | Delayed and privacy-reviewed operational data | thresholded loss or availability measurements |

`CAPABILITY`, `SECRET-ROLE`, `SECRET-USER`, and `PLAINTEXT-USER` MUST NOT enter
logs, metrics, command lines, crash reports, descriptors, consensus, or support
bundles.

## 6. Trust Domains and Deployment Units

Each row is a distinct security domain even when the PoC places containers on
one host.

| Domain | Baseline deployable | Primary state owner | Network position |
| --- | --- | --- | --- |
| Offline user | `fog-compose` | user identity, contacts, message and capability state | no network |
| Transfer receiver | restricted adapter owned by `fog-client-relay` | bounded incomplete transfer state | local physical input only |
| Online user relay | `fog-client-relay` | opaque queues, schedules, temporary entry and return state | client edge |
| Directory authority | `fog-authority` | descriptors, votes, consensus and revocation state | control plane only |
| Entry | `fog-entry` | bounded ingress and temporary return state | edge of data plane |
| Mix layer | `fog-mix` with fixed layer assignment | delay queues, replay state, epoch keys | one stratified layer |
| Courier | `fog-courier` | bounded request deduplication and replica dispatch state | terminal mix service |
| Storage | `fog-store` | opaque boxes, tombstones, replica metadata | behind courier |
| Optional native service | `fog-service-*` | minimum service-specific opaque state | terminal mix service |
| Observer | `fog-observer` | delayed coarse aggregates | operations plane |
| Release | offline release tooling | release signing and provenance state | outside runtime network |
| External bridge | `fog-bridge-*` | bridge-specific state | outside core guarantee |

A conforming executable MUST activate exactly one online role. Multi-role
configuration flags are forbidden outside explicitly labeled local simulation
fixtures.

## 7. Role Contracts

### 7.1 `fog-compose`

The Composer is the only core component authorized to handle user plaintext,
user identity private keys, contact secrets, message state, and mailbox
capabilities.

It MUST:

- verify release, consensus, contact, recovery, and message authenticity;
- persist the highest accepted consensus epoch and rollback state;
- build message-level ciphertext before export;
- select internal routes from the authenticated consensus;
- construct KEMSphinx packets, reply material, and storage operations;
- create fixed-class opaque work items for the blind relay;
- consume inbound bundles only after complete bounded parsing and
  cryptographic verification;
- store mutable secrets in an authenticated encrypted state store;
- minimize plaintext lifetime and exclude plaintext from generic desktop
  services, swap, previews, indexing, and crash dumps.

It MUST NOT:

- open a network socket or expose an HTTP, RPC, shell, or plugin server;
- delegate message encryption, route selection, capability derivation, or
  contact authentication to the blind relay;
- import generic archives, office documents, HTML, scripts, executable files,
  or unbounded media;
- accept a lower consensus epoch without an explicit authenticated recovery
  procedure.

`fog-drop`, `fog-mailbox`, and `fog-im` are initially Composer-side application
modules over shared messaging and storage protocol libraries. They are not
independent network daemons in the initial architecture.

Threats addressed: `TM-ENDPOINT-01`, `TM-ENDPOINT-02`, `TM-ENDPOINT-03`,
`TM-APP-01`, `TM-CRYPTO-01`, `TM-PKI-02`.

### 7.2 Transfer receiver

The transfer receiver is a minimal adapter at the blind-relay boundary. It MAY
decode QR or one explicitly configured FOG-SX physical backend. It MUST output
only a bounded opaque FOG bundle to the relay queue.

It MUST run without access to relay network credentials, queue decryption
keys, user identity keys, or general filesystems. FEC and transport checksums
MUST be completed before the inner bundle enters the relay, but they MUST NOT
be treated as authenticity.

The high-assurance offline-to-online path MUST have no automatic reverse data
channel. Online-to-offline input uses a separately controlled visual or
receive-only import path and is not the reverse channel of FOG-SX.

Threats addressed: `TM-ENDPOINT-02`, `TM-AVAIL-01`.

### 7.3 `fog-client-relay`

The blind relay is the online scheduling and transport agent for one local
Composer profile. It is allowed to know that its local user is using FOG and
to observe local submission and import times. It MUST remain blind to
plaintext, contacts, capabilities, application type, and the internal mix
route.

It MUST:

- maintain bounded encrypted-at-rest queues of opaque work items;
- fetch complete consensus documents and pass them unmodified to the Composer;
- verify enough public consensus metadata to reject obvious network misuse,
  while treating the Composer as the final consensus authority for user work;
- choose an entry only from the Composer-authorized temporary candidate set;
- follow consensus-authorized transmission, retrieval, cover, retry, and
  failure classes;
- generate client cover and loop traffic without using user identity or
  contact secrets;
- maintain only short-lived, opaque return rendezvous state;
- transfer received opaque bundles to the controlled Composer import path;
- erase expired work, return state, and incomplete transfers according to
  explicit limits.

It MUST NOT:

- parse message or service payloads;
- construct or rewrite the Composer's internal route;
- hold message identity keys, mailbox capabilities, contact vouchers, ratchet
  state, or release signing keys;
- bypass an entry after failure;
- expose a general-purpose proxy, SOCKS, VPN, TUN, SMTP, NNTP, or web API;
- use a stable remote account identity as the default entry authentication
  mechanism.

The relay MAY select among opaque entry-specific submission variants prepared
by the Composer. The exact entry-capsule and fallback construction is defined
by `FOG-WIRE` and `FOG-SPHINX-PROFILES`; the relay MUST NOT learn the first
internal mix hop from that construction.

Threats addressed: `TM-NET-01`, `TM-NET-02`, `TM-NET-03`, `TM-NET-06`,
`TM-ROLE-01`, `TM-ENDPOINT-01`, `TM-OPS-01`.

### 7.4 `fog-authority`

Authorities collectively define the authenticated network view. They do not
carry user data-plane packets.

Each authority MUST:

- authenticate and validate descriptors for admitted role identities;
- record declared operator families and deployment attributes;
- validate role, layer, key, profile, address, epoch, and revocation rules;
- exchange signed votes with peer authorities;
- produce canonical consensus only at the configured threshold;
- archive sufficient signed material to diagnose rollback and equivocation;
- publish current and next public keys with bounded overlap;
- separate long-term authority identity and recovery material from online
  voting state;
- fail deterministically when a quorum or time condition is not met.

An authority MUST NOT:

- inject data-plane traffic through privileged paths;
- assign one process to multiple data-plane roles;
- unilaterally create a valid consensus;
- automatically trust a replacement authority or node identity;
- receive user capabilities, packet identifiers, or fine-grained flow data.

Consensus distribution MAY use multiple mirrors, but mirrors are untrusted
transport. Signatures, epoch monotonicity, validity, and profile authorization
are verified at every consumer.

Threats addressed: `TM-PKI-01`, `TM-PKI-02`, `TM-PKI-03`, `TM-NET-06`,
`TM-SUPPLY-01`.

### 7.5 `fog-entry`

The entry is an access gateway distinct from the three privacy-relevant mix
layers. It accepts relay sessions, normalizes ingress handling, and dispatches
opaque KEMSphinx packets to layer 1.

It MUST:

- authenticate itself through the fixed entry link profile;
- authorize submission using short-lived, consensus-bound material rather
  than a stable user account by default;
- accept only fixed-class opaque submissions for the active epoch;
- decrypt or validate only the entry capsule needed to learn the authorized
  first internal hop and packet binding;
- forward only to consensus-authorized layer-1 nodes;
- maintain bounded queues and non-amplifying failure behavior;
- process real and cover submissions through the same path;
- support bounded, opaque, short-lived return rendezvous where required by the
  reply protocol;
- perform exactly one terminal KEMSphinx unwrap on an authorized reply route
  when the return profile requires it, using a dedicated entry-return key and
  never a mix-layer or entry-capsule key.

It MUST NOT:

- process an internal KEMSphinx mix layer or any forward KEMSphinx hop merely
  because it is an entry;
- perform a reply-terminal unwrap outside an authenticated return-rendezvous
  context or forward the result as if it were another mix hop;
- learn the complete route, final service, mailbox capability, contact, or
  plaintext;
- select or rewrite the internal route;
- forward directly to layer 2, layer 3, courier, storage, bridge, or Internet;
- retain a durable per-user mailbox or account in the core profile;
- produce application-specific errors or timing classes.

The exact entry capsule and return rendezvous are protocol decisions. They
MUST preserve Composer route authority and relay blindness. An implementation
convenience MUST NOT silently make the entry the first of only three total
hops.

Threats addressed: `TM-NET-01`, `TM-NET-03`, `TM-NET-06`, `TM-ROLE-01`,
`TM-AVAIL-01`.

### 7.6 `fog-mix`

Every mix executable has one fixed layer assignment from the current
consensus. It is a cryptographic router, not an application server.

It MUST:

- authenticate adjacent eligible nodes through the fixed node link profile;
- accept packets only from roles permitted for its layer;
- perform exactly one KEMSphinx hop transformation;
- validate the packet profile, epoch, replay tag, routing command, and delay
  bounds before enqueueing;
- persist replay state for the required maximum packet lifetime;
- delay and schedule packets according to the authenticated profile;
- process real, loop, decoy, forwarded, and reply traffic uniformly within
  their authorized packet class;
- forward only to the next authorized layer or terminal role;
- enforce bounded queues, connection counts, cryptographic work, and retries;
- emit only coarse local health aggregates.

It MUST NOT:

- expose application plugins, storage APIs, user accounts, or general proxy
  functions;
- select an arbitrary next layer or skip a layer;
- parse message, capability, courier, storage, or bridge payloads;
- share KEMSphinx private keys or replay databases with another role;
- log packets, replay tags, routes, per-packet delay, or fine-grained timing.

Loss of valid replay state places the node outside the active privacy profile.
It MUST stop packet processing until safe state is restored or a new epoch
with fresh keys begins according to the replay specification.

Threats addressed: `TM-NET-01`, `TM-NET-03`, `TM-NET-04`, `TM-NET-05`,
`TM-NET-06`, `TM-ROLE-02`, `TM-OPS-01`, `TM-AVAIL-01`.

### 7.7 `fog-courier`

The courier is the terminal data-plane mediator between anonymous requests and
storage or a separately declared native service. It terminates only the final
KEMSphinx service envelope and MUST not learn the client network origin.

It MUST:

- accept terminal packets only after all configured mix layers;
- parse one bounded, versioned courier command set;
- dispatch opaque encrypted operations to eligible replicas;
- keep replica selection knowledge separate from final capability-derived
  record location where the storage construction requires it;
- keep only bounded, expiring request-deduplication and reply state;
- use single-use reply material or an equivalently reviewed construction;
- return replies through a consensus-authorized anonymous reply route;
- apply identical outer behavior to reads, writes, misses, retries, expected
  outcomes, and cover operations as defined by `FOG-STORAGE`;
- enforce non-amplifying limits before expensive or fan-out work.

A courier acceptance reply is not replica durability or message delivery. The
courier MUST preserve opaque final-replica receipt material without creating
or interpreting it.

It MUST NOT:

- receive user identity keys, message plaintext, contact state, or long-term
  mailbox capabilities;
- connect directly to a blind relay or Composer;
- expose replicas directly to clients or mixes;
- become a durable mailbox database;
- load arbitrary third-party plugins in the core process;
- send data to the Internet or an external bridge under the core profile.

Threats addressed: `TM-NET-01`, `TM-NET-02`, `TM-NET-05`, `TM-ROLE-03`,
`TM-APP-01`, `TM-AVAIL-01`.

### 7.8 `fog-store`

Storage replicas hold authenticated encrypted records addressed through
capability-derived, rotating, pseudorandom locations. They are not public
mailbox servers.

Each replica MUST:

- authenticate couriers and eligible replica peers through a fixed node link
  profile;
- accept only bounded courier or replica protocol operations;
- validate record authentication before committing state;
- issue purpose-separated authenticated receipts only after durable local
  commit under the exact active storage manifest;
- implement idempotent writes, explicit expiry, tombstones, quotas, bounded
  retention, and garbage collection;
- keep replica and storage-envelope keys separate from node link keys;
- return fixed-class authenticated encrypted results;
- support the replica-independence and intermediate/final separation required
  by the selected storage profile;
- encrypt storage media and backups as defense in depth without treating disk
  encryption as end-to-end protection.

It MUST NOT:

- accept connections from Composers, blind relays, entries, or ordinary mixes;
- receive user plaintext or message identity private keys;
- expose record existence through a public unauthenticated lookup interface;
- share a writable database, database credentials, or backup key with another
  replica operator;
- publish per-record or fine-grained access metrics.

The intended privacy profile requires at least four independently operated
replicas. A smaller local fixture is functional testing only and MUST disable
the corresponding unlinkability claim.

Threats addressed: `TM-NET-02`, `TM-ROLE-03`, `TM-ENDPOINT-03`,
`TM-CRYPTO-01`, `TM-AVAIL-01`.

### 7.9 Native application modules and optional services

The initial `fog-drop`, `fog-mailbox`, and `fog-im` state machines run inside
the Composer and produce a common bounded message/storage envelope. Their
network traffic MUST be indistinguishable within the declared packet class.

A future feature MAY require a server-side `fog-service-*` executable. Such a
service MUST:

- be a separately keyed terminal role behind all mix layers;
- declare its exact request fields and information exposure;
- accept one fixed bounded protocol, not arbitrary code or generic HTTP;
- use anonymous reply routes;
- receive no privilege to call unrelated services or public Internet targets;
- add a threat-model extension and independent conformance tests.

No service is permitted to weaken the common packet profile or make the
courier parse application plaintext.

Threats addressed: `TM-NET-06`, `TM-APP-01`, `TM-ROLE-03`.

### 7.10 `fog-observer`

The observer receives delayed, coarse, thresholded aggregates. It is not a
packet-flow collector.

It MUST:

- accept only a versioned aggregate schema;
- enforce release delay, minimum population, and suppression rules;
- separate operator health views from public views;
- expire raw signed aggregate submissions after bounded processing;
- publish the aggregation and differencing-risk policy.

It MUST NOT:

- receive packet IDs, replay tags, capabilities, user IDs, full routes,
  connection logs, queue contents, or event-level timestamps;
- require universal read access to node logs or databases;
- instruct nodes to enable debug logging;
- be required for packet forwarding or consensus validity.

Threats addressed: `TM-NET-03`, `TM-OPS-01`.

### 7.11 Release system

Release signing is outside every runtime role. Release keys MUST NOT exist on
mix, authority, entry, courier, storage, observer, relay, or general CI
workers.

The release process MUST produce canonical signed metadata, artifact hashes,
version and compatibility information, rollback constraints, and provenance.
Composer and node update verification MUST fail closed. Emergency revocation
MUST use an authenticated path distinct from ordinary online administration.

Threats addressed: `TM-SUPPLY-01`, `TM-ENDPOINT-01`, `TM-CRYPTO-01`,
`TM-CRYPTO-02`.

### 7.12 `fog-bridge-*`

An external bridge is never part of the core trust claim. It MUST be a
separate executable, identity, service descriptor, process, host policy, data
store, log policy, and threat-model appendix.

It MUST NOT share a process with courier, storage, entry, or mix roles. Its
output is governed by the external protocol, and FOG cannot conceal metadata
that the external endpoint reveals.

## 8. Trust-Boundary Interfaces

Every interface MUST have one owning specification, exact framing, maximum
size, authentication rule, replay rule, timeout, failure class, and test
vectors.

| ID | Interface | Producer -> consumer | Data class | Owning specification |
| --- | --- | --- | --- | --- |
| `IF-01` | Composer export | Composer -> transfer receiver -> relay | `OPAQUE-META` | `FOG-COMPOSER`, `FOG-SX` |
| `IF-02` | Composer import | relay or controlled medium -> Composer | `PUBLIC-AUTH`, `OPAQUE-META`, `CAPABILITY` | `FOG-COMPOSER` |
| `IF-03` | Descriptor upload | node -> authorities | `PUBLIC-AUTH` | `FOG-PKI`, `FOG-WIRE` |
| `IF-04` | Authority vote | authority -> authority | `PUBLIC-AUTH` | `FOG-PKI`, `FOG-WIRE` |
| `IF-05` | Consensus distribution | authorities or mirrors -> all roles | `PUBLIC-AUTH` | `FOG-PKI` |
| `IF-06` | Relay submission | relay -> entry | `OPAQUE-META` | `FOG-WIRE`, `FOG-SPHINX-PROFILES` |
| `IF-07` | Layer forwarding | entry/L1/L2/L3 -> next role | `OPAQUE-META` | `FOG-WIRE`, `FOG-SPHINX-PROFILES` |
| `IF-08` | Terminal request | L3 -> courier or service | `OPAQUE-META` | `FOG-WIRE`, `FOG-SPHINX-PROFILES`, service contract |
| `IF-09` | Replica operation | courier <-> replicas; replica <-> replica | `OPAQUE-META` | `FOG-STORAGE`, `FOG-WIRE` |
| `IF-10` | Anonymous reply | courier/service -> mix route -> entry/relay | `OPAQUE-META`, `CAPABILITY` | `FOG-SPHINX-PROFILES`, `FOG-WIRE` |
| `IF-11` | Aggregate submission | role -> observer | `AGGREGATE` | `FOG-OBSERVABILITY`, `FOG-WIRE` |
| `IF-12` | Signed update | release distribution -> role | `PUBLIC-AUTH` | `FOG-UPDATE` |
| `IF-13` | Local administration | operator -> one role | role-local | deployment profile |

No generic RPC bus, shared event bus, shared SQL database, shared Redis, or
service mesh identity MAY span these trust boundaries in the core profile.

## 9. Control-Plane Flows

### 9.1 Node admission and descriptor publication

1. An operator creates separate role and node identities using an offline or
   controlled enrollment procedure.
2. The operator submits a signed admission request with role, family, layer,
   address, public keys, supported profiles, and declared infrastructure.
3. Authorities validate policy and operator-family conflicts.
4. An admitted node creates an epoch descriptor and signs canonical bytes.
5. The node submits the descriptor independently to the authority set.
6. Authorities validate and include eligible descriptors in their votes.

Admission does not imply health, honesty, independence, or permanent
eligibility. These are separately governed and measured.

### 9.2 Consensus production

1. Authorities exchange authenticated votes and commitments.
2. Each authority derives canonical topology and network parameters.
3. A consensus becomes valid only with the configured threshold of signatures
   over identical canonical bytes.
4. Current and next key material overlap only for the specified window.
5. Authorities and mirrors publish signed votes, consensus, and revocations.

FOG-PKI defines deterministic responses to split votes, missing quorum, clock
skew, stale epochs, rollback, freeze, and equivocation. Consumers MUST NOT
merge partial views locally.

### 9.3 Consensus consumption by an offline Composer

1. The relay obtains complete consensus from more than one retrieval path
   where practical.
2. The relay passes the bytes and available consistency evidence through the
   controlled import boundary.
3. The Composer performs canonical parsing, threshold signature validation,
   epoch and rollback checks, profile checks, and trust-anchor checks.
4. Only the accepted consensus may drive routes, entry candidates, packet
   profiles, and traffic parameters.
5. The Composer persists the highest accepted epoch before exporting work
   based on it.

An untrusted relay can withhold or replay data and cause denial of service, but
it MUST NOT be able to make a forged or rolled-back view valid.

## 10. Outbound User Flow

The logical outbound flow is:

1. A Composer application module creates a bounded semantic message.
2. The Composer stages one authenticated message transition, encrypts the
   message for the recipient, and atomically persists the advanced state with
   the exact immutable envelope before that envelope becomes exportable.
3. The Composer atomically derives the required storage box and capability
   transition, persists its immutable storage request generation, and only
   then makes the operation exportable.
4. The Composer selects a consensus-valid terminal role and one node from each
   mix layer, obeying operator-family constraints.
5. The Composer creates the KEMSphinx packet, reply material, and entry-bound
   opaque submission variants.
6. The Composer exports a fixed-class work bundle over `IF-01`.
7. The relay validates only the outer work-bundle contract, queues it, and
   selects one Composer-authorized entry variant under the current schedule.
8. The relay sends the opaque variant to the selected entry over `IF-06`.
9. The entry validates the entry capsule and forwards the bound packet to the
   authorized layer-1 node.
10. Each mix performs exactly one transformation, delay, replay check, and
    forwarding decision.
11. Layer 3 delivers the terminal packet to the selected courier or native
    service.
12. The courier performs the bounded opaque operation and, where applicable,
    contacts storage replicas through `IF-09`.
13. A result returns only through the supplied anonymous reply mechanism.

At no point may the relay reconstruct the internal route, the entry change it,
or a mix interpret the user message.

The exact choice between a complete packet per entry variant and a smaller
entry capsule bound to a shared packet remains an explicit protocol decision.
It MUST be resolved with packet-size, replay, fallback, and correlation
analysis before implementation.

## 11. Retrieval and Reply Flow

FOG does not require direct user-to-user sessions. Retrieval is a Composer-
constructed anonymous service operation.

1. The relay obtains short-lived return-rendezvous material from eligible
   entries or maintains an eligible live session under the active profile.
2. The relay transfers the opaque public or capability-bound rendezvous
   material to the Composer through `IF-02`.
3. The Composer validates it against accepted consensus and binds a single-use
   reply route to a retrieval or write operation.
4. The outbound operation traverses entry, all mix layers, and the courier.
5. The courier or service returns one fixed-class response using the supplied
   single-use reply material.
6. The reply traverses the consensus-authorized mix route and terminates at a
   bounded entry rendezvous or eligible live relay session.
7. The relay stores only the opaque response until expiry or controlled
   Composer import.
8. The Composer stages authentication, decryption, deduplication, and message
   state advancement, then atomically commits all effects before rendering
   content or scheduling an acknowledgment.

An entry return rendezvous MUST be random, short-lived, bounded, and unrelated
to a stable global username. It MUST NOT become a durable provider mailbox.
Exact queueing, polling, retransmission, acknowledgment, and SURB behavior is
defined by the packet, wire, messaging, and storage specifications.

Empty reads, hits, misses, replies, retries, and acknowledgments MUST fit the
same declared external traffic classes. A relay MUST continue its configured
cover and retrieval schedule independently of whether the Composer has a real
operation pending.

## 12. Cover and Loop Traffic Architecture

Cover generation is a protocol subsystem, not an optional application feature.

- The Composer prepares user-protocol decoys when they require message or
  storage semantics unavailable to the relay.
- The blind relay generates network cover and loop traffic using only public
  consensus and ephemeral local state.
- Entries process cover submissions identically to real submissions.
- Mixes generate and process consensus-authorized loop or decoy packets.
- Couriers, services, and replicas implement indistinguishable bounded cover
  outcomes defined by their protocol.
- Nodes export only delayed aggregate loss and health measurements.

The simulator determines cover rate, destination selection, delay, loop,
retry, polling, and shutdown parameters. Operators MUST NOT tune privacy-
critical distributions independently outside an authenticated profile.

`FOG-SIMULATION.md` records the first deterministic comparison matrix. It
confirms that sparse use and the functional PoC do not support anonymity
claims, and that cover volume, local pool overlap, latency, and long-term
observation proxies trade different resources. It deliberately selects no
numeric profile. Loop, polling, retry, queue, degraded-mode, formal observer,
and trace-driven extensions remain required before parameters are frozen.

If the minimum traffic or cover conditions attached to a claim disappear,
nodes follow the specified degraded-mode or shutdown policy. They MUST NOT
silently continue under the stronger claim.

## 13. State Ownership

| Role | Permitted durable state | Forbidden durable state |
| --- | --- | --- |
| Composer | encrypted identity, contacts, message state, capabilities, drafts, trust anchors, highest epoch | online session credentials for other roles, node private keys |
| Relay | encrypted opaque queues, schedule state, temporary entries, return handles, highest relay-checked epoch | plaintext, contact state, mailbox capabilities, internal routes |
| Authority | descriptors, votes, consensus history, admission and revocation records | user packets, capabilities, data-plane queues |
| Entry | node configuration, bounded replay-independent ingress state, short-lived opaque return state | durable user accounts, plaintext, internal routes, mailboxes |
| Mix | epoch keys, replay state, bounded delay queues, local aggregate counters | message state, application data, full routes, user identities |
| Courier | bounded deduplication, retry and reply state | durable mailboxes, user identity state, plaintext |
| Store | opaque records, tombstones, expiry and replica state | user identities, plaintext, courier dedup state |
| Service | explicitly specified minimum opaque service state | unrelated application state, client network identity |
| Observer | delayed aggregate submissions and published aggregates | event streams, packet identifiers, role secrets |

State schemas MUST include version, ownership, integrity, maximum size,
retention, migration, backup, restore, corruption, and deletion behavior.
Copying a database between roles or replicas is not a recovery mechanism.

## 14. Key Ownership

Each narrower protocol specification MUST refine this table into exact key
lifecycle entries.

| Key class | Sole owning role | Authorized use | Baseline persistence |
| --- | --- | --- | --- |
| Pairwise contact roots and handshake identities | Composer | private contact authentication and session establishment | long-term encrypted user state, with profile-specific recovery rules |
| Ratchet, outbox, deduplication, and conversation state | Composer | atomic message send, retry, receive, and acknowledgment transitions | encrypted mutable user state, non-resumable after stale restore |
| Directional stream capability and storage outbox state | Composer | rotating box derivation, immutable request generations, reads, writes, tombstones, and receipt validation | encrypted mutable user state, non-resumable after stale restore |
| Composer state-encryption key | Composer | local authenticated encryption | profile-specific protected storage |
| Backup or recovery key | user offline recovery domain | restore Composer state | separate from backup ciphertext |
| Relay queue-sealing key | blind relay | local opaque queue protection | local service-protected storage |
| Relay ephemeral cover state | blind relay | cover and loop construction | bounded or ephemeral |
| Node identity key | one online node | descriptor and role authentication | role-local protected storage |
| Noise link key | one online node | fixed adjacent-link profile | role-local, rotated by profile |
| Entry capsule key | one entry | open entry-bound submission capsule | epoch-bounded role-local state |
| Entry return KEMSphinx key | one entry | exactly one terminal reply unwrap for a short-lived rendezvous | epoch-bounded role-local state |
| Mix KEMSphinx key | one mix | exactly one layer transformation | epoch-bounded role-local state |
| Terminal KEMSphinx key | one courier or service | open terminal service envelope | epoch-bounded role-local state |
| Replica envelope key | one storage replica | replica request and response protection | epoch or storage-profile bounded |
| Replica receipt key | one storage replica | authenticate local durable storage results to the Composer | storage-manifest bounded, separate from envelope, identity, and Noise keys |
| Storage-at-rest key | one replica operator | defense-in-depth disk or database encryption | deployment-specific |
| Authority identity key | one authority | authority identity and authenticated recovery | preferably offline or hardware-protected |
| Authority online vote key | one authority | epoch vote and consensus participation | short-lived or tightly controlled online state |
| Authority wire key | one authority wire service | authenticated descriptor and authority-peer transport | root-certified bounded online state |
| Aggregate signing key | one reporting role | authenticate coarse metrics | role-local |
| Release signing key | offline release domain | sign canonical releases and metadata | offline, never on runtime hosts |

Public consensus contains only public keys and authenticated parameters.
Private keys MUST NOT be copied through consensus, container images, shared
volumes, environment templates, logs, or support artifacts.

No role may use a long-term user identity key as a network account, storage
capability, node identity, transport key, release key, or backup key.

## 15. Network Reachability Policy

Default-deny reachability is part of the architecture.

| Source | Permitted destinations | Explicitly forbidden destinations |
| --- | --- | --- |
| Composer | none | every network target |
| Relay | consensus mirrors, eligible entries, explicitly configured local transfer interface | mixes, courier, storage, external bridges, Internet proxy targets |
| Authority | peer authorities, descriptor submitters, consensus publication endpoints, local administration | user data plane, storage records |
| Entry | eligible relays, layer-1 nodes, bounded reply-route peers required by profile | layer 2, layer 3, storage, public Internet |
| Layer 1 mix | entries and layer-2 nodes | relay, layer 3, storage, public Internet |
| Layer 2 mix | layer-1 and layer-3 nodes | relay, entry, storage, public Internet |
| Layer 3 mix | layer-2 and terminal courier/service nodes | relay, entry, storage, public Internet |
| Courier | layer-3 or authorized reply-route mix peers, eligible replicas | relay, Composer, public Internet |
| Store | eligible couriers and replica peers | Composer, relay, entry, ordinary mixes, public Internet |
| Observer | aggregate-reporting roles and publication endpoint | packet interfaces, role databases |
| Bridge | explicitly declared FOG terminal and external endpoint | all undeclared core interfaces |

The reply protocol MAY require a terminal role to connect to a
consensus-authorized first reply hop that differs from the forward table. That
exception MUST be explicit in `FOG-SPHINX-PROFILES`, constrained by role and
epoch, and tested as part of the firewall policy.

Management access is a separate operator-local boundary. No central
management credential may administer all authorities, all mix layers, the
courier, and the replica set in a claim-bearing deployment.

## 16. Co-Location and Operator Separation

### 16.1 Functional PoC

The local PoC MAY place separate containers on one host and MAY use simulated
authority or storage fixtures. It MUST still use separate executables, service
users, state paths, key files, ports, and default-deny container networks.

PoC co-location invalidates operator-independence, infrastructure-diversity,
and production anonymity claims.

`FOG-LOCAL-POC.md` and `../deploy/podman/topology.json` refine this into the
first deployment contract. The Composer and authority fixtures are
networkless. Relay, entry, one mix in each fixed layer, courier, and four
storage fixtures use only internal pairwise networks for `IF-06` through
`IF-09`. The contract forbids host networking, published ports, shared
writable volumes, shared secret scopes, and missing containment controls. No
Compose manifest is generated until role fixtures exist.

### 16.2 Alpha

An alpha profile MUST use:

- at least three authorities with a 2-of-3 quorum;
- three mix layers with at least two independently operated mixes per layer;
- at least four independently operated storage replicas;
- separate entry, mix, courier, and storage role identities;
- documented operator-family, provider, ASN, location, and administrative
  relationships;
- no route containing the same operator family twice.

One operator MAY run more than one role only when the relationship is declared,
route policy accounts for it, and the relevant security claim explicitly
allows it. One process or private key MUST NOT implement more than one role.

### 16.3 Claim-bearing production profile

The preferred initial topology is three mix nodes per layer. Authority,
entry, mix, courier, storage, observer, build, and release administration
SHOULD be organizationally separated where practical.

No provider account, orchestration control plane, backup service, monitoring
credential, or CI system SHOULD control enough nominal operators to defeat the
claim. Concealed common control remains a documented residual risk.

## 17. Failure and Degraded Modes

| Condition | Required architectural response |
| --- | --- |
| Consensus signature or canonical parse failure | reject and retain last safely usable state only within its validity |
| Stale or rolled-back consensus | stop new route construction; require authenticated recovery |
| Authority quorum failure | no locally synthesized consensus; continue only under explicitly valid prior epoch rules |
| Entry failure | choose only another pre-authorized opaque variant under randomized bounded retry policy |
| Mix connection failure | bounded backoff; no layer skip or deterministic emergency route |
| Replay state unavailable | stop affected mix processing until safe recovery or specified fresh epoch |
| Cover process failure | leave the affected unobservability profile and follow explicit shutdown/degraded policy |
| Queue saturation | bounded shedding without detailed remote oracle or amplification |
| Courier or replica timeout | bounded randomized retry through the defined protocol; no direct client fallback |
| Replica loss | follow specified quorum or erasure behavior; never fabricate successful durability |
| Clock uncertainty outside bound | reject time-sensitive new state and surface coarse local fault |
| Update verification failure | retain last verified non-revoked version or stop if policy requires it |
| State corruption | quarantine state, avoid secret-bearing diagnostics, and use authenticated recovery |

Detailed error codes may exist inside a trusted local process boundary for
testing. Remote peers receive only bounded protocol outcomes that do not expose
parsing, capability, record-existence, or route oracles.

## 18. Configuration, Build, and Update Boundaries

Configuration MUST be role-specific and schema-validated. A deployment tool
MAY generate multiple role configurations, but the generated runtime artifacts
MUST contain only the public information and secrets required by that role.

Runtime services MUST use:

- a dedicated unprivileged account;
- a role-specific read-only executable and configuration;
- one role-specific writable state directory;
- no shared writable source or configuration checkout;
- explicit network allowlists;
- bounded resource controls;
- disabled core dumps and privacy-unsafe debug modes;
- local secret injection that does not place secrets in command arguments or
  container images.

Build, release signing, consensus signing, node operation, and user identity
management are separate authorities. A successful CI build does not authorize
a release, a release does not authorize a network epoch, and an epoch does not
authorize user messages.

### 18.1 Cost-minimal build and packaging profile

One release build MUST compile all required role executables once per supported
operating-system and architecture target. Nodes and operators MUST reuse those
verified artifacts; they MUST NOT require per-node or per-operator compilation.
The build SHOULD share dependency, object, and module caches across role
executables.

The functional fixture and local alpha laboratory MAY use one immutable
digest-pinned image containing multiple fixture executables, provided every
container activates exactly one role and preserves separate configuration,
service identity, state, secrets, and network policy. This packaging exception
does not permit a multi-role process.

A distributed alpha or claim-bearing deployment SHOULD assemble one minimal
image per runtime role from the already compiled artifact set. Role images
SHOULD share identical OCI base layers so registries and hosts deduplicate
storage and transfer. Image assembly MUST NOT trigger a separate compilation
for every image or node.

The supported build matrix MUST contain only platforms required by current
deployment or evidence gates. Independent reproducible rebuilds are release
verification evidence and MAY reuse the same source and pinned toolchain; they
are not required for routine startup of each node.

## 19. Repository and Module Shape

The future implementation repository SHOULD begin with this logical layout.
Exact language-specific names MAY vary without changing dependency direction.

```text
fog/
  cmd/
    fog-compose/
    fog-sx-send/
    fog-sx-receive/
    fog-client-relay/
    fog-authority/
    fog-entry/
    fog-mix/
    fog-courier/
    fog-store/
    fog-observer/
  protocol/
    encoding/
    pki/
    wire/
    sphinx/
    messaging/
    storage/
    composer/
    sx/
  roles/
    authority/
    entry/
    mix/
    courier/
    store/
    observer/
    clientrelay/
  sdk/
    client/
    composer/
  sim/
  specs/
  testvectors/
  tests/
    conformance/
    integration/
    fault/
    topology/
  deploy/
    podman/
  ops/
```

Dependency direction MUST be:

```text
cmd -> one role -> protocol packages
sdk ------------> protocol packages
sim ------------> protocol models and independent simulation models
tests ----------> public contracts and built executables
```

Protocol packages MUST NOT import role implementations, network listeners,
databases, container tooling, or operator configuration. One role package MUST
NOT import another role's implementation. Cross-role behavior occurs only
through versioned protocol interfaces.

The public SDK MUST expose message and packet construction contracts without
exposing node private APIs. Generic `utils`, shared mutable singletons, a
universal database package, and an in-process plugin bus are forbidden
substitutes for explicit boundaries.

The following MUST NOT be split yet:

- separate source repositories for every role;
- server daemons for `fog-drop`, `fog-mailbox`, and `fog-im`;
- a generic external bridge framework inside core nodes;
- operator-selectable cryptographic plugin systems;
- separate databases or queues where a role currently needs no durable state.

## 20. Architectural Verification

Before the local PoC, the repository MUST support or define tests for:

- one executable activating only one role;
- dependency rules preventing role-to-role implementation imports;
- configuration schema rejection of multi-role or unknown-critical settings;
- default-deny reachability for every row in the network policy table;
- inability of Composer images to create or receive network traffic;
- absence of shared writable volumes and private key files across roles;
- fixed packet-class behavior across all native applications;
- complete traversal of entry and all mix layers;
- replay-state loss and fail-closed restart behavior;
- voucher consume-once behavior and atomic ratchet, outbox, deduplication,
  inbox, and acknowledgment transactions;
- exact message-envelope and storage-request retry layering: immutable
  courier-envelope bytes per request generation with fresh KEMSphinx, route,
  entry, rendezvous, and reply material per transmission;
- deterministic storage shards, disjoint intermediates, receipt quorum,
  empty-read non-advancement, tombstone precedence, and non-resurrection;
- exact Composer bundle headers, hostile import limits, commit-before-effect,
  external-anchor reconciliation, non-resumable recovery, and update floors;
- exact FOG-SX object and frame headers, fixed profile tuple, size padding,
  CRC domains, FEC and conflict budgets, no-ACK behavior, and physical
  direction fixtures;
- consensus stale, rollback, split, and equivocation scenarios;
- queue, parser, retry, CPU, memory, connection, and storage bounds;
- log and metrics schemas that reject prohibited data classes;
- compromise fixtures showing the information available to each isolated role;
- update signature, version, rollback, and revocation behavior;
- deterministic conformance vectors for every trust-boundary interface.

Tests MUST observe public contracts and externally visible behavior, not reach
through trust boundaries to share internal state.

## 21. Threat Traceability

| Threat | Primary architectural controls |
| --- | --- |
| `TM-NET-01` | uniform packet classes, relay scheduling, cover subsystem, stratified mixes |
| `TM-NET-02` | independent retrieval schedule, rotating capabilities, bounded rendezvous, storage separation |
| `TM-NET-03` | loop health, degraded-mode gate, bounded retries, no bypass |
| `TM-NET-04` | per-mix replay state, single-use reply material, idempotent terminal state |
| `TM-NET-05` | authenticated KEMSphinx processing, terminal validation, uniform failures |
| `TM-NET-06` | complete consensus, fixed profiles, application-independent core behavior |
| `TM-PKI-01` | permissioned admission, family declarations, route constraints |
| `TM-PKI-02` | threshold consensus, monotonic Composer state, consistency evidence |
| `TM-PKI-03` | distinct authorities, quorum rules, offline recovery, deterministic failure |
| `TM-ROLE-01` | blind relay, entry capsules, Composer route authority, no direct fallback |
| `TM-ROLE-02` | one fixed mix layer per process, family-separated routes, role-local keys |
| `TM-ROLE-03` | courier/replica separation, opaque commands, independent replicas |
| `TM-ENDPOINT-01` | networkless Composer, narrow devices, encrypted state, signed updates |
| `TM-ENDPOINT-02` | minimal transfer adapter, bounded formats, no reverse FOG-SX path |
| `TM-ENDPOINT-03` | role-owned state, separate backup keys, no database copying |
| `TM-APP-01` | Composer-side application state, bounded authenticated rendering, no active content |
| `TM-OPS-01` | separate observer, aggregate-only interface, prohibited data classes |
| `TM-SUPPLY-01` | offline release authority, provenance, signed updates, role-specific artifacts |
| `TM-CRYPTO-01` | sole key owners, purpose separation, lifecycle refinement requirement |
| `TM-CRYPTO-02` | consensus-authorized suites, no plugin negotiation or downgrade |
| `TM-AVAIL-01` | bounds at every interface, non-amplification, quotas, explicit degraded modes |

Every later specification MUST reference the applicable threat IDs and
architecture invariants. If it changes a trust boundary or information flow,
this document and the threat model MUST be updated before implementation.

## 22. Open Architectural Protocol Contracts

This architecture intentionally leaves the following to narrower reviewed
specifications:

- the exact entry submission capsule and packet binding;
- short-lived return rendezvous and offline Composer import mechanics;
- activation of an exact numeric message profile after the structural
  `FOG-MESSAGING` contract and its non-active PQXDH, Triple Ratchet, and
  ML-KEM Braid candidate pass byte-exact integration, vectors, implementation
  review, and independent review;
- the exact reviewed entry and mutual Noise or PQNoise suites and numeric
  wire-profile parameters within `FOG-WIRE-1`; X-Wing is the leading KEM to
  evaluate, not an active profile;
- activation of a final `FOG-SPHINX-1` primitive suite after the fixed
  structural profile and calculated candidate in `FOG-SPHINX-PROFILES.md` pass
  benchmarks, vectors, simulation, and independent review;
- activation of an exact numeric storage profile after the structural
  `FOG-STORAGE` contract and its non-active narrow BACAP/Pigeonhole candidate
  pass primitive review, receipt analysis, geometry, vectors, simulation, and
  independent review;
- activation of exact Composer MicroVM and Portable profiles after the
  structural `FOG-COMPOSER` contract and its Linux vault, Qubes, Portable, and
  TUF candidates pass platform, anchor, fault, recovery, and independent
  review;
- cover, loop, delay, retry, retrieval, and shutdown distributions;
- activation of one exact numeric FOG-SX joint profile after the structural
  `FOG-SX` contract and its non-active RaptorQ, Reed-Solomon, QR, Lightpipe,
  and MIDI candidates pass implementation, license and IPR, resource, vector,
  hardware-direction, and independent review;
- exact aggregate metrics and suppression thresholds;
- implementation languages, reviewed libraries, and activation evidence for
  the non-active cryptographic candidates in `FOG-CRYPTO-SUITES.md`.

These are explicit design dependencies. No daemon implementation may resolve
them through undocumented behavior.

## 23. References

- FOG threat model: `FOG-THREAT-MODEL.md`
- FOG public key infrastructure: `FOG-PKI.md`
- FOG wire protocol: `FOG-WIRE.md`
- FOG Sphinx profile framework: `FOG-SPHINX-PROFILES.md`
- FOG messaging protocol: `FOG-MESSAGING.md`
- FOG storage protocol: `FOG-STORAGE.md`
- FOG Composer protocol: `FOG-COMPOSER.md`
- FOG simplex transfer protocol: `FOG-SX.md`
- FOG observability protocol: `FOG-OBSERVABILITY.md`
- FOG cryptographic suite evaluation: `FOG-CRYPTO-SUITES.md`
- FOG traffic and topology simulation: `FOG-SIMULATION.md`
- FOG local Podman PoC: `FOG-LOCAL-POC.md`
- Katzenpost mix network specification:
  <https://katzenpost.network/docs/specs/mixnet/>
- Katzenpost public key infrastructure specification:
  <https://katzenpost.network/docs/specs/pki/>
- Katzenpost wire protocol specification:
  <https://katzenpost.network/docs/specs/wireprotocol/>
- Katzenpost Pigeonhole protocol specification:
  <https://katzenpost.network/docs/specs/pigeonhole/>
- Piotrowska et al., *The Loopix Anonymity System*:
  <https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/piotrowska>
- Infeld et al., *Echomix: a Strong Anonymity System with Messaging*:
  <https://arxiv.org/abs/2501.02933>
- Noise Protocol Framework:
  <https://noiseprotocol.org/noise.html>

These references inform role separation, stratified mixing, consensus,
transport, reply, and storage boundaries. FOG requires its own protocol
profiles, conformance evidence, simulation, deployment evidence, and review.
