Add generic OpenID Connect provider (OAuth2)

This adds the oauth2 provider `oidc`. It needs an additional argument, the OIDC discovery endpoint to figure out where the auth and token URLs are.

Configuration is similar to setting up the Google Authentication with these changes:

 * `OAUTH2_PROVIDER = oidc`
 * `OAUTH2_OIDC_DISCOVERY_ENDPOINT = https://auth.exampe.org/discovery`

vendor/github.com/coreos/go-oidc/oidc.go

@@ -0,0 +1,409 @@
+// Package oidc implements OpenID Connect client logic for the golang.org/x/oauth2 package.
+package oidc
+
+import (
+	"context"
+	"crypto/sha256"
+	"crypto/sha512"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"hash"
+	"io/ioutil"
+	"mime"
+	"net/http"
+	"strings"
+	"time"
+
+	"golang.org/x/oauth2"
+	jose "gopkg.in/square/go-jose.v2"
+)
+
+const (
+	// ScopeOpenID is the mandatory scope for all OpenID Connect OAuth2 requests.
+	ScopeOpenID = "openid"
+
+	// ScopeOfflineAccess is an optional scope defined by OpenID Connect for requesting
+	// OAuth2 refresh tokens.
+	//
+	// Support for this scope differs between OpenID Connect providers. For instance
+	// Google rejects it, favoring appending "access_type=offline" as part of the
+	// authorization request instead.
+	//
+	// See: https://openid.net/specs/openid-connect-core-1_0.html#OfflineAccess
+	ScopeOfflineAccess = "offline_access"
+)
+
+var (
+	errNoAtHash      = errors.New("id token did not have an access token hash")
+	errInvalidAtHash = errors.New("access token hash does not match value in ID token")
+)
+
+// ClientContext returns a new Context that carries the provided HTTP client.
+//
+// This method sets the same context key used by the golang.org/x/oauth2 package,
+// so the returned context works for that package too.
+//
+//    myClient := &http.Client{}
+//    ctx := oidc.ClientContext(parentContext, myClient)
+//
+//    // This will use the custom client
+//    provider, err := oidc.NewProvider(ctx, "https://accounts.example.com")
+//
+func ClientContext(ctx context.Context, client *http.Client) context.Context {
+	return context.WithValue(ctx, oauth2.HTTPClient, client)
+}
+
+func doRequest(ctx context.Context, req *http.Request) (*http.Response, error) {
+	client := http.DefaultClient
+	if c, ok := ctx.Value(oauth2.HTTPClient).(*http.Client); ok {
+		client = c
+	}
+	return client.Do(req.WithContext(ctx))
+}
+
+// Provider represents an OpenID Connect server's configuration.
+type Provider struct {
+	issuer      string
+	authURL     string
+	tokenURL    string
+	userInfoURL string
+	algorithms  []string
+
+	// Raw claims returned by the server.
+	rawClaims []byte
+
+	remoteKeySet KeySet
+}
+
+type cachedKeys struct {
+	keys   []jose.JSONWebKey
+	expiry time.Time
+}
+
+type providerJSON struct {
+	Issuer      string   `json:"issuer"`
+	AuthURL     string   `json:"authorization_endpoint"`
+	TokenURL    string   `json:"token_endpoint"`
+	JWKSURL     string   `json:"jwks_uri"`
+	UserInfoURL string   `json:"userinfo_endpoint"`
+	Algorithms  []string `json:"id_token_signing_alg_values_supported"`
+}
+
+// supportedAlgorithms is a list of algorithms explicitly supported by this
+// package. If a provider supports other algorithms, such as HS256 or none,
+// those values won't be passed to the IDTokenVerifier.
+var supportedAlgorithms = map[string]bool{
+	RS256: true,
+	RS384: true,
+	RS512: true,
+	ES256: true,
+	ES384: true,
+	ES512: true,
+	PS256: true,
+	PS384: true,
+	PS512: true,
+}
+
+// NewProvider uses the OpenID Connect discovery mechanism to construct a Provider.
+//
+// The issuer is the URL identifier for the service. For example: "https://accounts.google.com"
+// or "https://login.salesforce.com".
+func NewProvider(ctx context.Context, issuer string) (*Provider, error) {
+	wellKnown := strings.TrimSuffix(issuer, "/") + "/.well-known/openid-configuration"
+	req, err := http.NewRequest("GET", wellKnown, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := doRequest(ctx, req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	body, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("unable to read response body: %v", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%s: %s", resp.Status, body)
+	}
+
+	var p providerJSON
+	err = unmarshalResp(resp, body, &p)
+	if err != nil {
+		return nil, fmt.Errorf("oidc: failed to decode provider discovery object: %v", err)
+	}
+
+	if p.Issuer != issuer {
+		return nil, fmt.Errorf("oidc: issuer did not match the issuer returned by provider, expected %q got %q", issuer, p.Issuer)
+	}
+	var algs []string
+	for _, a := range p.Algorithms {
+		if supportedAlgorithms[a] {
+			algs = append(algs, a)
+		}
+	}
+	return &Provider{
+		issuer:       p.Issuer,
+		authURL:      p.AuthURL,
+		tokenURL:     p.TokenURL,
+		userInfoURL:  p.UserInfoURL,
+		algorithms:   algs,
+		rawClaims:    body,
+		remoteKeySet: NewRemoteKeySet(ctx, p.JWKSURL),
+	}, nil
+}
+
+// Claims unmarshals raw fields returned by the server during discovery.
+//
+//    var claims struct {
+//        ScopesSupported []string `json:"scopes_supported"`
+//        ClaimsSupported []string `json:"claims_supported"`
+//    }
+//
+//    if err := provider.Claims(&claims); err != nil {
+//        // handle unmarshaling error
+//    }
+//
+// For a list of fields defined by the OpenID Connect spec see:
+// https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata
+func (p *Provider) Claims(v interface{}) error {
+	if p.rawClaims == nil {
+		return errors.New("oidc: claims not set")
+	}
+	return json.Unmarshal(p.rawClaims, v)
+}
+
+// Endpoint returns the OAuth2 auth and token endpoints for the given provider.
+func (p *Provider) Endpoint() oauth2.Endpoint {
+	return oauth2.Endpoint{AuthURL: p.authURL, TokenURL: p.tokenURL}
+}
+
+// UserInfo represents the OpenID Connect userinfo claims.
+type UserInfo struct {
+	Subject       string `json:"sub"`
+	Profile       string `json:"profile"`
+	Email         string `json:"email"`
+	EmailVerified bool   `json:"email_verified"`
+
+	claims []byte
+}
+
+// Claims unmarshals the raw JSON object claims into the provided object.
+func (u *UserInfo) Claims(v interface{}) error {
+	if u.claims == nil {
+		return errors.New("oidc: claims not set")
+	}
+	return json.Unmarshal(u.claims, v)
+}
+
+// UserInfo uses the token source to query the provider's user info endpoint.
+func (p *Provider) UserInfo(ctx context.Context, tokenSource oauth2.TokenSource) (*UserInfo, error) {
+	if p.userInfoURL == "" {
+		return nil, errors.New("oidc: user info endpoint is not supported by this provider")
+	}
+
+	req, err := http.NewRequest("GET", p.userInfoURL, nil)
+	if err != nil {
+		return nil, fmt.Errorf("oidc: create GET request: %v", err)
+	}
+
+	token, err := tokenSource.Token()
+	if err != nil {
+		return nil, fmt.Errorf("oidc: get access token: %v", err)
+	}
+	token.SetAuthHeader(req)
+
+	resp, err := doRequest(ctx, req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	body, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%s: %s", resp.Status, body)
+	}
+
+	var userInfo UserInfo
+	if err := json.Unmarshal(body, &userInfo); err != nil {
+		return nil, fmt.Errorf("oidc: failed to decode userinfo: %v", err)
+	}
+	userInfo.claims = body
+	return &userInfo, nil
+}
+
+// IDToken is an OpenID Connect extension that provides a predictable representation
+// of an authorization event.
+//
+// The ID Token only holds fields OpenID Connect requires. To access additional
+// claims returned by the server, use the Claims method.
+type IDToken struct {
+	// The URL of the server which issued this token. OpenID Connect
+	// requires this value always be identical to the URL used for
+	// initial discovery.
+	//
+	// Note: Because of a known issue with Google Accounts' implementation
+	// this value may differ when using Google.
+	//
+	// See: https://developers.google.com/identity/protocols/OpenIDConnect#obtainuserinfo
+	Issuer string
+
+	// The client ID, or set of client IDs, that this token is issued for. For
+	// common uses, this is the client that initialized the auth flow.
+	//
+	// This package ensures the audience contains an expected value.
+	Audience []string
+
+	// A unique string which identifies the end user.
+	Subject string
+
+	// Expiry of the token. Ths package will not process tokens that have
+	// expired unless that validation is explicitly turned off.
+	Expiry time.Time
+	// When the token was issued by the provider.
+	IssuedAt time.Time
+
+	// Initial nonce provided during the authentication redirect.
+	//
+	// This package does NOT provided verification on the value of this field
+	// and it's the user's responsibility to ensure it contains a valid value.
+	Nonce string
+
+	// at_hash claim, if set in the ID token. Callers can verify an access token
+	// that corresponds to the ID token using the VerifyAccessToken method.
+	AccessTokenHash string
+
+	// signature algorithm used for ID token, needed to compute a verification hash of an
+	// access token
+	sigAlgorithm string
+
+	// Raw payload of the id_token.
+	claims []byte
+
+	// Map of distributed claim names to claim sources
+	distributedClaims map[string]claimSource
+}
+
+// Claims unmarshals the raw JSON payload of the ID Token into a provided struct.
+//
+//		idToken, err := idTokenVerifier.Verify(rawIDToken)
+//		if err != nil {
+//			// handle error
+//		}
+//		var claims struct {
+//			Email         string `json:"email"`
+//			EmailVerified bool   `json:"email_verified"`
+//		}
+//		if err := idToken.Claims(&claims); err != nil {
+//			// handle error
+//		}
+//
+func (i *IDToken) Claims(v interface{}) error {
+	if i.claims == nil {
+		return errors.New("oidc: claims not set")
+	}
+	return json.Unmarshal(i.claims, v)
+}
+
+// VerifyAccessToken verifies that the hash of the access token that corresponds to the iD token
+// matches the hash in the id token. It returns an error if the hashes  don't match.
+// It is the caller's responsibility to ensure that the optional access token hash is present for the ID token
+// before calling this method. See https://openid.net/specs/openid-connect-core-1_0.html#CodeIDToken
+func (i *IDToken) VerifyAccessToken(accessToken string) error {
+	if i.AccessTokenHash == "" {
+		return errNoAtHash
+	}
+	var h hash.Hash
+	switch i.sigAlgorithm {
+	case RS256, ES256, PS256:
+		h = sha256.New()
+	case RS384, ES384, PS384:
+		h = sha512.New384()
+	case RS512, ES512, PS512:
+		h = sha512.New()
+	default:
+		return fmt.Errorf("oidc: unsupported signing algorithm %q", i.sigAlgorithm)
+	}
+	h.Write([]byte(accessToken)) // hash documents that Write will never return an error
+	sum := h.Sum(nil)[:h.Size()/2]
+	actual := base64.RawURLEncoding.EncodeToString(sum)
+	if actual != i.AccessTokenHash {
+		return errInvalidAtHash
+	}
+	return nil
+}
+
+type idToken struct {
+	Issuer       string                 `json:"iss"`
+	Subject      string                 `json:"sub"`
+	Audience     audience               `json:"aud"`
+	Expiry       jsonTime               `json:"exp"`
+	IssuedAt     jsonTime               `json:"iat"`
+	NotBefore    *jsonTime              `json:"nbf"`
+	Nonce        string                 `json:"nonce"`
+	AtHash       string                 `json:"at_hash"`
+	ClaimNames   map[string]string      `json:"_claim_names"`
+	ClaimSources map[string]claimSource `json:"_claim_sources"`
+}
+
+type claimSource struct {
+	Endpoint    string `json:"endpoint"`
+	AccessToken string `json:"access_token"`
+}
+
+type audience []string
+
+func (a *audience) UnmarshalJSON(b []byte) error {
+	var s string
+	if json.Unmarshal(b, &s) == nil {
+		*a = audience{s}
+		return nil
+	}
+	var auds []string
+	if err := json.Unmarshal(b, &auds); err != nil {
+		return err
+	}
+	*a = audience(auds)
+	return nil
+}
+
+type jsonTime time.Time
+
+func (j *jsonTime) UnmarshalJSON(b []byte) error {
+	var n json.Number
+	if err := json.Unmarshal(b, &n); err != nil {
+		return err
+	}
+	var unix int64
+
+	if t, err := n.Int64(); err == nil {
+		unix = t
+	} else {
+		f, err := n.Float64()
+		if err != nil {
+			return err
+		}
+		unix = int64(f)
+	}
+	*j = jsonTime(time.Unix(unix, 0))
+	return nil
+}
+
+func unmarshalResp(r *http.Response, body []byte, v interface{}) error {
+	err := json.Unmarshal(body, &v)
+	if err == nil {
+		return nil
+	}
+	ct := r.Header.Get("Content-Type")
+	mediaType, _, parseErr := mime.ParseMediaType(ct)
+	if parseErr == nil && mediaType == "application/json" {
+		return fmt.Errorf("got Content-Type = application/json, but could not unmarshal as JSON: %v", err)
+	}
+	return fmt.Errorf("expected Content-Type = application/json, got %q: %v", ct, err)
+}