# Wax

WebAuthn library for elixir

<img src="https://raw.githubusercontent.com/tanguilp/wax/master/wax.png" width="128"/>

Goal: implement a *comprehensive* FIDO2 library on the server side
(*Relying party* or RP in the WebAuthn terminology) to authenticate users with WebAuthn.

For semantics (FIDO2, WebAuthn, FIDO...), read
[this article](https://medium.com/@herrjemand/introduction-to-webauthn-api-5fd1fb46c285)

## Demo app

You can try out and study WebAuthn authentication with Wax thanks to the
[wax_demo](https://github.com/tanguilp/wax_demo) test application.

See also a video demonstration of an authentication flow which allows replacing the password
authentication scheme by a WebAuthn password-less authentication:

[![Demo screenshot](https://raw.githubusercontent.com/tanguilp/wax_demo/master/assets/static/images/demo_screenshot.png)](https://vimeo.com/358361625)

## Project status

- Support the FIDO2 standard (especially all types of attestation statement formats and
all mandatory algorithms). See the "Support of FIDO2" section for further information
- **Passes all the 170 tests** of the official test suite (tested using
[WaxFidoTestSuiteServer](https://github.com/tanguilp/wax_fido_test_suite_server))
- This library has **not** be reviewed by independent security / FIDO2 specialists - use
it at your own risks or blindly trust its author!
- This library does not come with a javascript library to handle WebAuthn calls

## Compatibility

OTP25+

## Installation

Add the following line to your list of dependencies in `mix.exs`:

```elixir
def deps do
  [
    {:wax_, "~> 0.6.0"}
  ]
end
```

Note that due to a [name collision](https://github.com/resuelve/wax/issues/16), the application
name is `:wax_`, and not `:wax`. It doesn't cause issues using both library because the other's
package doesn't use the atom `Wax` as the base module name.

[Documentation](https://hexdocs.pm/wax_)

## Usage

To use FIDO2 for authentication, you must first *register* a new FIDO2 key for a user. The
process is therefore the following:
1. Register a FIDO2 key for a user
2. Authenticate as much as you want the user using the FIDO2 key registered in step 1

Optionaly, you might want to store more than one key, for instance if the user have
several authenticators.

The Wax library doesn't provide with a user data store to store the key generated in step
1 and to retrieve it in step 2. Instead, it lets you use any data store. The data to
be stored is described in the `Wax` module's documentation.

### Registration

Wax provides with 2 functions for registration:
1. `Wax.new_registration_challenge/1`: generates a challenge that must subsequently be sent
to the client for use by the javascript WebAuthn API
2. `Wax.register/3`: takes into parameter the response of the WebAuthn javascript API and
the challenge generated in step 1, and verifies it

Since the challenge generated in step 1 must be passed as a parameter in step 2, it is
required to persist it on the server side, for instance in the session:

```elixir
# generating a challenge

challenge = Wax.new_registration_challenge()

conn
|> put_session(:challenge, challenge)
|> render(register_key_page, challenge: challenge.bytes)
# the challenge is to be sent on the client one way or another
# this can be direct within the HTML, or using an API call
```

to be then retrieved when verifying the assertion:

```elixir
challenge = get_session(conn, :challenge)

case Wax.register(attestation_object, client_data_json, challenge) do
	{:ok, {authenticator_data, attestation_result}} ->
		# success case

	{:error, e} ->
		# verification failure
end
```

In the success case, a server will save the credential id (generated by the WebAuthn
javascript call) and the cose key in its user database for use for authentication.

Authenticator data contains the COSE key generated by the authenticator, which can be
found in `authenticator_data.attested_credential_data.credential_public_key`:

```elixir
%{
  -3 => <<182, 81, 183, 218, 92, 107, 106, 120, 60, 51, 75, 104, 141, 130,
    119, 232, 34, 245, 84, 203, 246, 165, 148, 179, 169, 31, 205, 126, 241,
    188, 241, 176>>,
  -2 => <<89, 29, 193, 225, 4, 234, 101, 162, 32, 6, 15, 14, 130, 179, 223,
    207, 53, 2, 134, 184, 178, 127, 51, 145, 57, 180, 104, 242, 138, 96, 27,
    221>>,
  -1 => 1,
  1 => 2,
  3 => -7
}
```

It probably doesn't need to be searchable or indexed, which is why one can store as a binary.
To convert back and forth Elixir data structures to binary and store the keys in a database
(SQL, for instance), take a look at the Erlang functions `:erlang.term_to_binary/1` and
`:erlang.binary_to_term/1`.

For further information, refer to the `Wax` module documentation.

### Authentication

The process is quite similar, with 2 functions for authentication:
1. `Wax.new_authentication_challenge/1`: generates a challenge from a list of
`{credential id, key}` saved during the registration processes. It also has to be sent to
the client for use by the javascript WebAuthn API
2. `Wax.authenticate/5`: to be called to verify the WebAuthn javascript API response
with the returned data (composed of signature, authenticator data, etc.) with the
challenge generated in step 1

This also requires storing the challenge:
```elixir
cred_ids_and_keys = UserStore.get_keys(username)

challenge = Wax.new_authentication_challenge(allow_credentials: cred_ids_and_keys)

conn
|> put_session(:authentication_challenge, challenge)
|> render(auth_verify_page, challenge: challenge.bytes, creds: cred_ids_and_keys)
# the challenge is to be sent on the client one way or another
# this can be direct within the HTML, or using an API call
```

to be passed as a paramter to the `Wax.authenticate/5` function:

```elixir
challenge = get_session(conn, :authentication_challenge)

case Wax.authenticate(raw_id, authenticator_data, sig, client_data_json, challenge) do
	{:ok, _} ->
		# ok, user authenticated

	{:error, _} ->
		# invalid WebAuthn response
end
```

For further information, refer to the `Wax` module documentation.

## Options
The options are set when generating the challenge (for both registration and
authentication). Options can be configured either globally in the configuration
file or when generating the challenge. Some also have default values.

Option values set during challenge generation take precedence over globally configured
options, which takes precedence over default values.

These options are:

|  Option       |  Type         |  Applies to       |  Default value                | Notes |
|:-------------:|:-------------:|-------------------|:-----------------------------:|-------|
|`attestation`|`"none"` or `"direct"`|registration|`"none"`| |
|`origin`|`String.t()`|registration & authentication| | **Mandatory**. Example: `https://www.example.com` |
|`rp_id`|`String.t()` or `:auto`|registration & authentication|If set to `:auto`, automatically determined from the `origin` (set to the host) | With `:auto`, it defaults to the full host (e.g.: `www.example.com`). This option allow you to set the `rp_id` to another valid value (e.g.: `example.com`) |
|`user_verification`|`"discouraged"`, `"preferred"` or `"required"`|registration & authentication|`"preferred"`| |
|`trusted_attestation_types`|`[t:Wax.Attestation.type/0]`|registration|`[:none, :basic, :uncertain, :attca, :anonca, :self]`| |
|`verify_trust_root`|`boolean()`|registration|`true`|Only for `u2f` and `packed` attestation. `tpm` attestation format is always checked against metadata|
|`acceptable_authenticator_statuses`|`[String.t()]`|registration|`["FIDO_CERTIFIED", "FIDO_CERTIFIED_L1",  "FIDO_CERTIFIED_L1plus", "FIDO_CERTIFIED_L2", "FIDO_CERTIFIED_L2plus", "FIDO_CERTIFIED_L3", "FIDO_CERTIFIED_L3plus"]`| The `"UPDATE_AVAILABLE"` status is not whitelisted by default |
|`timeout`|`non_neg_integer()`|registration & authentication|`20 * 60`| The validity duration of a challenge, in seconds |
|`android_key_allow_software_enforcement`|`boolean()`|registration|`false`| When registration is a Android key, determines whether software enforcement is acceptable (`true`) or only hardware enforcement is (`false`) |
|`silent_authentication_enabled`|`boolean()`|authentication|`false`| See [https://github.com/fido-alliance/conformance-tools-issues/issues/434](https://github.com/fido-alliance/conformance-tools-issues/issues/434) |

## FIDO2 Metadata

If you use attestation, you need to enabled metadata.

### Configuring MDSv3 metadata

This is the official metadata service of the FIDO foundation.

Set the `:update_metadata` environment variable to `true` and metadata will load
automatically through HTTP from
[https://mds3.fidoalliance.org/](https://fidoalliance.org/metadata/).

### Loading FIDO2 metadata from a directory

In addition to the FIDO2 metadata service, it is possible to load metadata from a directory.
To do so, the `:metadata_dir` application environment variable must be set to one of:
- a `String.t()`: the path to the directory containing the metadata files
- an `atom()`: in this case, the files are loaded from the `"fido2_metadata"` directory of the
private (`"priv/"`) directory of the application (whose name is the atom)

In both case, Wax tries to load all files (even directories and other special files).

#### Example configuration

```elixir
config :wax_,
  origin: "http://localhost:4000",
  rp_id: :auto,
  metadata_dir: :my_application
```

will try to load all files of the `"priv/fido2_metadata/"` of the `:my_application` as FIDO2
metadata statements. On failure, a warning is emitted.

## Security considerations

- Make sure to understand the implications of not using attested credentials before
accepting `none` or `self` attestation types, or disabling it for `packed` and `u2f`
formats by disabling it with the `verify_trust_root` option
- This library has **not** be reviewed by independent security / FIDO2 specialists - use
it at your own risks or blindly trust its author! If you're knowledgeable about
FIDO2 and willing to help reviewing it, please contact the author

## Changes

See [CHANGELOG.md](CHANGELOG.md).

## Support of FIDO2

### Server Requirements and Transport Binding Profile

[2. Registration and Attestations](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#registration-and-attestation)
- [x] **Mandatory**: registration support
- [x] **Mandatory**: random challenge
- [2.1. Validating Attestation](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#validating-attestation)
  - [x] **Mandatory**: attestation validation
  - [x] **Mandatory**: attestation certificate chains (note: can be disabled through an option)
  - [x] **Mandatory**: validation of attestation through the FIDO Metadata Service 
- [2.2. Attestation Types](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#attestation-types)
  - [x] **Mandatory**: basic attestation
  - [x] **Mandatory**: self attestation
  - [x] **Mandatory**: private CA attestation
  - [ ] *Optional*: elliptic curve direct anonymous attestation
- [2.3. Attestation Formats](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#attestation-formats)
  - [x] **Mandatory**: packed attestation
  - [x] **Mandatory**: TPM attestation
  - [x] *Optional*: Android key attestation
  - [x] **Mandatory**: U2F attestation
  - [x] **Mandatory**: Android Safetynet attestation
  - [x] [**Mandatory**](https://medium.com/webauthnworks/webauthn-fido2-whats-new-in-mds3-migrating-from-mds2-to-mds3-a271d82cb774#c977): Apple Anonymous

[3. Authentication and Assertions](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#authn-and-assertion)
  - [x] **Mandatory**: authentication
  - [x] **Mandatory**: random challenge
  - [x] **Mandatory**: assertion signature validation
  - [x] **Mandatory**: TUP verification (note: and also user verified, through an option)

[4. Communication Channel Requirements](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#communication-channel-requirements)
  - [ ] *Optional*: TokenBinding support (won't be supported following Chrome drop of the now
  dead token binding standard)

[5. Extensions](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#extensions)
  - [x] **Mandatory**: registration and authentication support without extension
  - [ ] *Optional*: extension support
  - [ ] *Optional*: appid extension support

[6. Other](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#other)
  - [x] **Mandatory**: RS1 (RSASSA-PKCS1-v1_5 w/ SHA-1) algorithm support
  - [x] **Mandatory**: RS256 (RSASSA-PKCS1-v1_5 w/ SHA-256) algorithm support
  - [x] *Optional*: RS384 (RSASSA-PKCS1-v1_5 w/ SHA-384) algorithm support
  - [x] *Optional*: RS512 (RSASSA-PKCS1-v1_5 w/ SHA-512) algorithm support
  - [x] *Optional*: PS256 (RSASSA-PSS w/ SHA-256) algorithm support
  - [x] *Optional*: PS384 (RSASSA-PSS w/ SHA-384) algorithm support
  - [x] *Optional*: PS512 (RSASSA-PSS w/ SHA-512) algorithm support
  - [x] **Mandatory**: ES256 (ECDSA using P-256 and SHA-256) algorithm support
  - [x] *Optional*: ES384 (ECDSA using P-384 and SHA-384) algorithm support
  - [x] *Optional*: ES512 (ECDSA using P-521 and SHA-512) algorithm support
  - [x] *Optional*: EdDSA algorithm support
  - [x] *Optional*: ES256K (ECDSA using P-256K and SHA-256) algorithm support
  - [ ] **Mandatory**: compliance with the FIDO privacy principles (note: out-of-scope, to be implemented by the server using the Wax library)

[7. Transport Binding Profile](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html#transport-binding-profile)
  - [x] *optional*: API implementation ([`WaxAPIRest`](https://github.com/tanguilp/wax_api_rest))

### FIDO Metadata Service

[3.1.8 Metadata TOC object processing rules](https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-metadata-service-v2.0-rd-20180702.html#metadata-toc-object-processing-rules)
  - [ ] TOC verification against the `x5u` attribute (note: doesn't seem to be used)
  - [x] TOC verification against the `x5c` attribute
  - [x] TOC CRLs verification
  - [x] Loading and verification of metadata statements against the hased value of the TOC
  - [x] Handling of the status of the authenticator (through whitelisting, see the
  `:acceptable_authenticator_statuses` option)