Documentation: add incomplete intro to OpenID Connect
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Eric Chiang
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# An overview of OpenID Connect
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This document attempts to provide an overview of the [OpenID Connect protocl](
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https://openid.net/connect/), a flavor of OAuth2 that dex implements. While
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this document isn't complete we hope it provides a enough to get users up and
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running.
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## OAuth2
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OAuth2 should be familiar to anyone who's used something similar to a "Login
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with Facebook" button. In these cases an application has chosen to let an
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outside provider, in this case Facebook, attest to your identity instead of
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having you set a username and password with the app itself.
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The general flow for server side apps is:
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1. A new user visits an application.
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1. The application redirects the user to Facebook.
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1. The user logs into Facebook, then is asked if it's okay to let the
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application view the user's profile, post on their behalf, etc.
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1. If the user clicks okay, Facebook redirects the user back to the application
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with a code.
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1. The application redeems that code with provider for a token that can be used
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to access the authorized actions, such as viewing a users profile or posting on
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their wall.
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In these cases, dex is acting as Facebook (called the "provider" in OpenID
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Connect) while clients apps redirect to it for the end user's identity.
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## ID Tokens
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Unfortunately the access token applications get from OAuth2 providers is
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completely opaque to the client and unique to the provider. The token you
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receive from Facebook will be completely different from the one you'd get from
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Twitter or GitHub.
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OpenID Connect's primary extension of OAuth2 is an additional token returned in
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the token response called the ID Token. This token is a [JSON Web Token](
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https://tools.ietf.org/html/rfc7519) signed by the OpenID Connect server, with
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well known fields for user ID, name, email, etc. A typical token response from
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an OpenID Connect looks like (with less whitespace):
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```
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HTTP/1.1 200 OK
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Content-Type: application/json
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Cache-Control: no-store
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Pragma: no-cache
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{
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"access_token": "SlAV32hkKG",
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"token_type": "Bearer",
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"refresh_token": "8xLOxBtZp8",
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"expires_in": 3600,
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"id_token": "eyJhbGciOiJSUzI1NiIsImtpZCI6IjFlOWdkazcifQ.ewogImlzc
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yI6ICJodHRwOi8vc2VydmVyLmV4YW1wbGUuY29tIiwKICJzdWIiOiAiMjQ4Mjg5
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NzYxMDAxIiwKICJhdWQiOiAiczZCaGRSa3F0MyIsCiAibm9uY2UiOiAibi0wUzZ
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fV3pBMk1qIiwKICJleHAiOiAxMzExMjgxOTcwLAogImlhdCI6IDEzMTEyODA5Nz
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AKfQ.ggW8hZ1EuVLuxNuuIJKX_V8a_OMXzR0EHR9R6jgdqrOOF4daGU96Sr_P6q
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Jp6IcmD3HP99Obi1PRs-cwh3LO-p146waJ8IhehcwL7F09JdijmBqkvPeB2T9CJ
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NqeGpe-gccMg4vfKjkM8FcGvnzZUN4_KSP0aAp1tOJ1zZwgjxqGByKHiOtX7Tpd
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QyHE5lcMiKPXfEIQILVq0pc_E2DzL7emopWoaoZTF_m0_N0YzFC6g6EJbOEoRoS
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K5hoDalrcvRYLSrQAZZKflyuVCyixEoV9GfNQC3_osjzw2PAithfubEEBLuVVk4
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XUVrWOLrLl0nx7RkKU8NXNHq-rvKMzqg"
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}
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```
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That ID Token is a JWT with three base64'd fields separated by dots. The first
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is a header, the second is a payload, and the third is a signature of the first
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two fields. When parsed we can see the payload of this value is.
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```
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{
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"iss": "http://server.example.com",
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"sub": "248289761001",
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"aud": "s6BhdRkqt3",
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"nonce": "n-0S6_WzA2Mj",
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"exp": 1311281970,
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"iat": 1311280970
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}
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```
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This has a few interesting fields such as
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* The server that issued this token (`iss`).
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* The token's subject (`sub`). In this case a unique ID of the end user.
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* The token's audience (`aud`). The ID of the OAuth2 client this was issued for.
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TODO: Add examples of payloads with "email" fields.
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## Discovery
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OpenID Connect servers have a discovery mechanism for OAuth2 endpoints, scopes
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supported, and indications of various other OpenID Connect features.
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```
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$ curl http://127.0.0.1:5556/.well-known/openid-configuration
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{
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"issuer": "http://127.0.0.1:5556",
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"authorization_endpoint": "http://127.0.0.1:5556/auth",
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"token_endpoint": "http://127.0.0.1:5556/token",
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"jwks_uri": "http://127.0.0.1:5556/keys",
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"response_types_supported": [
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"code"
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],
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"subject_types_supported": [
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"public"
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],
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"id_token_signing_alg_values_supported": [
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"RS256"
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],
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"scopes_supported": [
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"openid",
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"email",
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"profile"
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]
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}
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```
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Importantly, we've discovered the authorization endpoint, token endpoint, and
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the location of the server's public keys. OAuth2 clients should be able to use
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the token and auth endpoints immediately, while a JOSE library can be used to
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parse the keys. The keys endpoint returns a [JSON Web Key](
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https://tools.ietf.org/html/rfc7517) Set of public keys that will look
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something like this:
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```
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$ curl http://127.0.0.1:5556/keys
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{
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"keys": [
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{
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"use": "sig",
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"kty": "RSA",
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"kid": "5d19a0fde5547960f4edaa1e1e8293e5534169ba",
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"alg": "RS256",
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"n": "5TAXCxkAQqHEqO0InP81z5F59PUzCe5ZNaDsD1SXzFe54BtXKn_V2a3K-BUNVliqMKhC2LByWLuI-A5ZlA5kXkbRFT05G0rusiM0rbkN2uvRmRCia4QlywE02xJKzeZV3KH6PldYqV_Jd06q1NV3WNqtcHN6MhnwRBfvkEIm7qWdPZ_mVK7vayfEnOCFRa7EZqr-U_X84T0-50wWkHTa0AfnyVvSMK1eKL-4yc26OWkmjh5ALfQFtnsz30Y2TOJdXtEfn35Y_882dNBDYBxtJV4PaSjXCxhiaIuBHp5uRS1INyMXCx2ve22ASNx_ERorv6BlXQoMDqaML2bSiN9N8Q",
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"e": "AQAB"
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}
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]
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}
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```
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