Get domain wide delegation token using credentials API.

#jwt_token_source

.

jwt_token_source.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package jwt

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"io/ioutil"
	"net/url"
	"strings"
	"time"
 
	credentialspb "cloud.google.com/go/iam/credentials/apiv1/credentialspb"
	credentials "cloud.google.com/go/iam/credentials/apiv1"
	"golang.org/x/oauth2"
	"golang.org/x/oauth2/jws"
)
const GOOGLE_OAUTH2_TOKEN_ENDPOINT = "https://oauth2.googleapis.com/token"
const DEFAULT_GRANT_TYPE = "urn:ietf:params:oauth:grant-type:jwt-bearer"
 
// provide an email of a service account that your default credentials has permission to call signJWT:
// https://cloud.google.com/iam/docs/reference/credentials/rest/v1/projects.serviceAccounts/signJwt
func TokenSource(ctx context.Context, email string, scopes []string, subject string) oauth2.TokenSource {
	return oauth2.ReuseTokenSource(nil, jwtSource{ctx, email, scopes, subject})
}

type jwtSource struct {
	ctx  context.Context
	email string
	scopes          []string
	subject		string
}
 
func (js jwtSource) Token() (*oauth2.Token, error) {
	hc := oauth2.NewClient(js.ctx, nil)
	iamc, err := credentials.NewIamCredentialsClient(js.ctx)
 
	if err != nil {
		return  nil, err
	}
	defer iamc.Close()
	iat := time.Now()
	exp := iat.Add(time.Hour)
	claimSet := map[string]interface{}{
		"iss":	       js.email,
		"scope":       strings.Join(js.scopes, " "),
		"aud":         GOOGLE_OAUTH2_TOKEN_ENDPOINT,
		"iat":         iat.Unix(),
		"exp":         exp.Unix(),
	}
	if subject := js.subject; subject != "" {
		claimSet["sub"] = subject
		claimSet["prn"] = subject
	}
	payload, _ := json.Marshal(claimSet)
	req := &credentialspb.SignJwtRequest{
		Name: "projects/-/serviceAccounts/" + js.email,
		Payload: string(payload),
	}
	signResp, err := iamc.SignJwt(js.ctx, req)
	if err != nil {
		return nil, err
	}
	v := url.Values{}
	v.Set("grant_type", DEFAULT_GRANT_TYPE)
	v.Set("assertion", signResp.SignedJwt)
	resp, err := hc.PostForm(GOOGLE_OAUTH2_TOKEN_ENDPOINT, v)
	if err != nil {
		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
	}
	defer resp.Body.Close()
	body, err := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
	}
	if c := resp.StatusCode; c < 200 || c > 299 {
		return nil, &oauth2.RetrieveError{
			Response: resp,
			Body:     body,
		}
	}
	var tokenRes struct {
		AccessToken string `json:"access_token"`
		TokenType   string `json:"token_type"`
		IDToken     string `json:"id_token"`
		ExpiresIn   int64  `json:"expires_in"` // relative seconds from now
	}
	if err := json.Unmarshal(body, &tokenRes); err != nil {
		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
	}
	token := &oauth2.Token{
		AccessToken: tokenRes.AccessToken,
		TokenType:   tokenRes.TokenType,
	}
	raw := make(map[string]interface{})
	json.Unmarshal(body, &raw) // no error checks for optional fields
	token = token.WithExtra(raw)
 
	if secs := tokenRes.ExpiresIn; secs > 0 {
		token.Expiry = time.Now().Add(time.Duration(secs) * time.Second)
	}
	if v := tokenRes.IDToken; v != "" {
		claimSet, err := jws.Decode(v)
		if err != nil {
			return nil, fmt.Errorf("oauth2: error decoding JWT token: %v", err)
		}
		token.Expiry = time.Unix(claimSet.Exp, 0)
	}
	return token, nil
}

LICENSE

Copyright (c) 2009 The Go Authors. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

   * Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following disclaimer
in the documentation and/or other materials provided with the
distribution.
   * Neither the name of Google Inc. nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.