collin/anvil

9162f8306f5434eec386c071f5e11f804f6cd109 / docs / passkeys.md

1	# Passkeys
2
3	Sign in with Touch ID, Windows Hello, a phone, or a security key instead of an
4	account password. Passkeys are for login only — repository secrets
5	([secrets.md](secrets.md)) stay keyed to your ssh keys, because CI needs to
6	unlock them from a terminal where no authenticator is present.
7
8	## Using them
9
10	Register (account settings → Passkeys): name the device, press *Add
11	passkey*, approve the prompt. Registering a second passkey on the same
12	authenticator is refused by the browser rather than silently duplicated — anvil
13	sends the existing credential ids as `excludeCredentials`.
14
15	Sign in: the login page's Sign in with a passkey button. No username: a
16	passkey is a discoverable credential, so the authenticator tells anvil which
17	credential it used and that identifies the account.
18
19	Password sign-in keeps working, and remains the way in if you lose every
20	authenticator. Removing your last passkey is allowed for the same reason.
21
22	## What anvil stores, and what it means if the database leaks
23
24	Only public material: the credential id, the credential's public key, and the
25	counters WebAuthn asks a relying party to track. The private key stays in the
26	authenticator and is never transmitted, so — unlike a password hash — nothing in
27	the `passkeys` table can be turned into a login, offline or otherwise. A leak
28	costs users their registrations, not their accounts.
29
30	Two properties come from the protocol rather than from anvil's code:
31
32	- Phishing resistance. The authenticator binds every signature to anvil's
33	relying-party id. A look-alike site cannot get a usable signature, even with a
34	perfect replica of this UI.
35	- Replay resistance. Every ceremony is a fresh random challenge, held in
36	memory, valid for five minutes, and accepted exactly once.
37
38	## The relying-party id is your `base_url` host
39
40	WebAuthn scopes a credential to one host, taken here from `http.base_url`:
41
42	\| `base_url` \| RP id \|
43	\|-----------------------------------\|---------------------------\|
44	\| `https://anvil.richardscollin.com` \| `anvil.richardscollin.com` \|
45	\| `https://anvil.localhost` \| `anvil.localhost` \|
46	\| `http://localhost:3000` \| `localhost` \|
47
48	Consequences worth knowing before you move an instance:
49
50	- Change the host and existing passkeys stop working. They are not deleted,
51	they simply belong to a different site now; users re-register (password login
52	is the way back in).
53	- Passkeys do not travel between instances. One created against the local
54	Docker instance (`deploy/dev.sh`) is not usable on production, by design.
55	- WebAuthn requires a secure context: HTTPS, or plain `localhost`. A LAN IP
56	over HTTP will not offer passkeys at all. `deploy/dev.sh` + portless gives
57	local development real HTTPS, which is why passkeys can be tested there.
58
59	## Implementation
60
61	`crates/anvil-core/src/passkeys.rs` holds the credential storage and the
62	in-memory challenge registry; `crates/anvil-web/src/passkeys.rs` holds the two
63	ceremonies, the JSON, and the browser glue.
64
65	Verification is [`webauthn_rp`](https://crates.io/crates/webauthn_rp), chosen
66	over the better-known `webauthn-rs` for one hard reason: `webauthn-rs` depends
67	on OpenSSL, and anvil ships as a statically linked musl binary built by
68	`deploy/build.sh` with no C toolchain in the picture. `webauthn_rp` is pure Rust
69	and implements the spec's ceremony steps explicitly.
70
71	Only passkeys are supported — discoverable credentials with user verification
72	required. No attestation is requested (`none`), which is the norm for consumer
73	authenticators and avoids collecting hardware identifiers we have no use for.
74
75	The browser side hand-rolls the base64url ↔ ArrayBuffer conversions rather than
76	using `PublicKeyCredential.parseCreationOptionsFromJSON()` / `toJSON()`: those
77	are recent enough that relying on them would narrow support to new browsers for
78	no gain.