fips/docs/reference/security.md

Security Reference

Consolidated security reference covering the nftables baseline, peer ACL file format, cryptographic primitives, rekey defaults, replay window, filesystem permissions, threat-resistance matrix, and default network exposures per transport. For the threat-model design and rationale, see ../design/fips-security.md. For the operator activation steps and drop-in recipes, see ../how-to/enable-mesh-firewall.md.

nftables Baseline

The shipped baseline is /etc/fips/fips.nft. It defines a single nftables table inet fips with one chain hooked at input, structured as follows:

Step	Rule	Effect
1	iifname != "fips0" return	Match only traffic arriving on fips0; everything else short-circuits.
2	ct state established,related accept	Allow conntrack replies and related ICMPv6 errors.
3	icmpv6 type echo-request accept	Allow IPv6 echo (ping6 reachability).
4	include "/etc/fips/fips.d/*.nft"	Splice in operator drop-ins (empty matches nothing).
5	counter drop	Default-deny everything else; counter increments on every drop.

Outbound from fips0 is unrestricted. The baseline is a documented dpkg conffile — operator edits to /etc/fips/fips.nft are preserved across upgrades.

The systemd unit is fips-firewall.service (oneshot). It is not enabled by default; activation is an explicit operator gesture documented in ../how-to/enable-mesh-firewall.md.

Drop-In File Format

Operator extensions live under /etc/fips/fips.d/ with the .nft suffix. Each file is included inline into the inbound chain at the marked point and may contain any nftables rule lines valid in that context.

Naming convention: <purpose>-from-<source>.nft keeps drop-ins easy to scan. Examples shipped in the design discussion:

• ssh-from-bastion.nft — accept TCP/22 from a single mesh-node address
• http-from-cluster.nft — accept TCP/80 from a /64 mesh-address prefix
• dns-public.nft — accept UDP/53 and TCP/53 from any mesh node
• git-from-trusted.nft — accept TCP/9418 from a set of mesh-node addresses

After editing, reload via sudo systemctl reload-or-restart fips-firewall.service (or equivalently sudo nft -f /etc/fips/fips.nft since the file is idempotent).

Cryptographic Primitives

Component	Choice	Where Used
Curve	secp256k1	FMP IK, FSP XK, Schnorr signatures
Diffie-Hellman	ECDH on secp256k1 (x-only normalized)	Noise IK, Noise XK
AEAD	ChaCha20-Poly1305	FMP link encryption, FSP session encryption
Hash	SHA-256	NodeAddr derivation, Noise key schedule
Key derivation	HKDF-SHA256	Noise key schedule
Signatures	secp256k1 Schnorr	TreeAnnounce, LookupResponse proof, Nostr adverts
Noise pattern (link)	Noise_IK_secp256k1_ChaChaPoly_SHA256, with the deviation below	FMP link layer (IK with epoch payload)
Noise pattern (session)	Noise_XK_secp256k1_ChaChaPoly_SHA256, with the deviation below	FSP session layer (XK with epoch payload)

These choices align with the Nostr cryptographic stack (secp256k1 + ChaCha20-Poly1305 + SHA-256) and the NIP-44 encrypted messaging standard.

Deviation: Empty Associated Data in the Handshake AEAD

Both Noise patterns above deviate from the standard construction in one respect. The handshake AEAD uses an empty associated-data field where standard Noise EncryptAndHash uses the handshake hash h.

The choice was deliberate. Using secp256k1 rather than 25519 already put the construction outside standard Noise, so no standard-Noise peer could be confused with it, and the transcript hash bought no distinguishing value.

That argument is about domain separation, and on those grounds it holds. It does not cover transcript binding, which is the property actually absent. Domain separation and DH binding survive through the chaining key ck, which mix_key chains from ck = h, seeded from the protocol name in SymmetricState::initialize (src/noise/handshake.rs). The handshake hash h is maintained at every step and is never fed to the AEAD, so it binds nothing.

Rekey Defaults

Both link-layer and session-layer Noise sessions rekey under one of two triggers, configurable under node.rekey.*:

Parameter	Default	Description
enabled	true	Master switch.
after_secs	120	Time-based rekey threshold.
after_messages	65536	Message-count rekey threshold.

In addition to the configurable triggers, the daemon retains the old session keys for a fixed 10-second drain window after each cutover (compile-time constant DRAIN_WINDOW_SECS in src/node/handlers/rekey.rs). Rekey rotates the Noise key schedule and the session indices; old session keys are kept in previous_session for the drain window so in-flight packets encrypted under the old keys still decrypt.

Replay Window

Both layers use explicit per-packet counters with a sliding bitmap window for replay protection. The bitmap is 2048 entries at both layers — large enough to accommodate UDP reordering and packet loss without false-positive replay rejection. Counters older than the window are rejected. The same ReplayWindow and decrypt_with_replay_check() implementation is used at both the FMP and FSP layers.

Peer ACL

Mesh-level ACL files at /etc/fips/peers.allow and /etc/fips/peers.deny give the operator allowlist/blocklist control over which npubs may complete the FMP Noise IK link handshake.

File format:

• One entry per line. An entry is either a bech32 npub1..., an alias defined in /etc/fips/hosts, or the literal ALL wildcard (case-insensitive).
• Lines beginning with # are comments.
• Blank lines are ignored.

Evaluation order (first match wins, default-allow on no match):

1. peers.allow — if the peer matches an entry here (or ALL is in peers.allow), the handshake is admitted, regardless of any peers.deny entry.
2. peers.deny — if the peer matches an entry here (or ALL is in peers.deny), the handshake is refused.
3. Otherwise the peer is admitted.

peers.allow is not an exclusive gate on its own: an unlisted peer falls through to step 3 and is admitted unless it appears in peers.deny. To turn peers.allow into a strict allowlist, place ALL in peers.deny so every unlisted peer is rejected at step 2.

The ALL wildcard makes the operator's posture explicit:

• ALL in peers.allow admits every peer (same effect as the default-allow behavior, but documented in the file).
• ALL in peers.deny blocks every peer except those listed in peers.allow — the "allowlist-strict" posture.

In practice this collapses to a few common postures:

• Default-allow with denylist: leave peers.allow empty; populate peers.deny. All npubs may peer except those listed.
• Allowlist-strict: populate peers.allow and put ALL in peers.deny. Only the listed npubs may peer; everyone else is rejected at step 2.

A populated peers.allow with an empty peers.deny is not a strict allowlist — it is equivalent to default-allow plus an explicit "always-admit" set. The strict variant requires ALL in peers.deny.

Aliases are resolved through /etc/fips/hosts at file-load time. If peers.allow lists core-vm and /etc/fips/hosts maps core-vm to a specific npub, that npub is admitted. If core-vm is later remapped to a different npub, the ACL re-resolves on the next mtime change. Operators should be aware that ACL semantics follow the hosts-file aliasing, not just the literal npubs visible in the file.

Both files are reloaded automatically when their mtime changes — no daemon restart or signal is needed. ACL evaluation runs after msg1 decryption but before any further peer-state mutation; rate-limited msg1s never reach the ACL.

Filesystem Permissions

Path	Owner	Mode	Purpose
/etc/fips/fips.key	root:root	0600	Persistent identity private key (sensitive).
/etc/fips/fips.pub	root:root	0644	Public key (npub).
/etc/fips/fips.yaml	root:root	0644	Daemon configuration (dpkg conffile).
/etc/fips/fips.nft	root:root	0644	nftables baseline (dpkg conffile).
/etc/fips/fips.d/	root:root	0755	Operator drop-in directory.
/etc/fips/hosts	root:root	0644	Optional hostname → npub map (dpkg conffile).
/etc/fips/peers.allow	root:root	0644	Optional peer allowlist.
/etc/fips/peers.deny	root:root	0644	Optional peer denylist.
/run/fips/control.sock	root:fips	0770	Control socket (members of fips group can use fipsctl).
/run/fips/	root:fips	0750	Control socket parent directory.

Adding a user to the fips group grants fipsctl access without requiring root. The daemon chowns the control socket and its parent directory at bind time.

Threat-Resistance Matrix

The link layer's threat-resistance matrix is consolidated here from the FMP design document:

Threat	Mitigation
Connection exhaustion	Token-bucket rate limit + connection count limit
CPU exhaustion (msg1 flood)	Rate limit before crypto operations
Replay attacks	Counter-based nonces with sliding window (2048 entries)
State confusion	Strict handshake state machine validation
Spoofed encrypted packets	Index lookup + AEAD verification
Spoofed msg2	Index lookup + Noise ephemeral key binding
Address spoofing	Cryptographic authority, not address-based
Session correlation	Index rotation on rekey
Inbound exposure on fips0	Default-deny nftables baseline (operator opt-in)
Sybil identities	Discretionary peering + handshake rate limiting + optional peer ACL
Eclipse attack	Diverse peering across independent operators and transports
Unauthorized peer admission	Optional peers.allow allowlist consulted before handshake

See ../design/fips-mesh-layer.md for the unauthenticated-attack-surface analysis (only handshake msg1 is reachable by unauthenticated parties), and ../design/fips-mesh-operation.md for the metadata-privacy model and the rejection of onion routing.

Default Network Exposures by Transport

Transport	Default Inbound	Default Bind	Opt-in
UDP	None until bind_addr set	0.0.0.0:2121 typical	Operator sets transports.udp.bind_addr
TCP	None until bind_addr set	None — outbound-only without bind	Operator sets transports.tcp.bind_addr
Ethernet	Listens on configured interface (raw AF_PACKET)	EtherType 0x2121 on selected interface	Per-flag discovery, announce, auto_connect, accept_connections
Tor	None until directory_service configured	127.0.0.1:8443 (loopback only)	Operator sets transports.tor.directory_service and configures HiddenServiceDir in torrc
BLE	Off by default	n/a	Operator enables transports.ble.*
Nostr discovery	Off by default	n/a (relay client, not a listener)	Operator sets node.discovery.nostr.enabled: true

The mesh-layer fips0 interface is reachable from any mesh node that can route to you, not only direct peers — your direct peers forward traffic from any reachable mesh node onto your fips0. The default-deny nftables baseline (operator opt-in) is the recommended way to restrict inbound traffic on fips0. See ../how-to/enable-mesh-firewall.md.

See also

• ../design/fips-security.md — threat model and design rationale for the fips0 baseline
• ../design/fips-mesh-layer.md — FMP link encryption, replay protection, rate limiting
• ../design/fips-session-layer.md — FSP end-to-end encryption, Noise XK, replay window
• ../how-to/enable-mesh-firewall.md — operator activation and drop-in recipes
• configuration.md — full node.rekey.*, node.rate_limit.* parameter tables