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token-generator.go
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token-generator.go
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package main
import (
"crypto/rsa"
"crypto/x509"
"encoding/json"
"encoding/pem"
"flag"
"fmt"
"golang.org/x/oauth2/google"
"golang.org/x/oauth2/jws"
"io/ioutil"
"log"
"net/http"
"net/url"
"os"
"time"
)
var cs *jws.ClaimSet
var hdr *jws.Header
var privateKey *rsa.PrivateKey
const authUrl = "https://www.googleapis.com/oauth2/v4/token"
func main() {
port := flag.String("port", "8080", "port of the server")
jsonFile := flag.String("file", "", "full path of json file")
flag.Parse()
file := os.Getenv("GOOGLE_APPLICATION_CREDENTIALS")
if *jsonFile != "" {
file = *jsonFile
}
if file == "" {
log.Fatal("Provide a json file either in the GOOGLE_APPLICATION_CREDENTIALS env variable or with the -file param ")
}
b, _ := ioutil.ReadFile(file)
fromJSON, err := google.JWTConfigFromJSON(b)
if err != nil {
fmt.Println(err)
log.Fatalf("Impossible to read the json file %s as a service account secret file", file)
}
cs = &jws.ClaimSet{
Iss: fromJSON.Email,
Sub: fromJSON.Email,
Aud: authUrl,
}
hdr = &jws.Header{
Algorithm: "RS256",
Typ: "JWT",
KeyID: fromJSON.PrivateKeyID,
}
privateKey = ParseKey(fromJSON.PrivateKey)
http.HandleFunc("/", tokenGenerator)
http.ListenAndServe(":"+*port, nil)
}
func ParseKey(key []byte) *rsa.PrivateKey {
block, _ := pem.Decode(key)
if block != nil {
key = block.Bytes
}
parsedKey, err := x509.ParsePKCS8PrivateKey(key)
if err != nil {
parsedKey, err = x509.ParsePKCS1PrivateKey(key)
if err != nil {
return nil
}
}
parsed, ok := parsedKey.(*rsa.PrivateKey)
if !ok {
return nil
}
return parsed
}
func tokenGenerator(w http.ResponseWriter, r *http.Request) {
iat := time.Now()
exp := iat.Add(time.Hour)
aud := r.URL.Query().Get("aud")
if aud == "" {
w.WriteHeader(http.StatusBadRequest)
fmt.Fprintf(w, "audience (host name of the service to reach) must be specified. add &aud=<HOST> to your request")
}
cs.PrivateClaims = map[string]interface{}{"target_audience": aud}
cs.Iat = iat.Unix()
cs.Exp = exp.Unix()
msg, err := jws.Encode(hdr, cs, privateKey)
if err != nil {
fmt.Println(fmt.Errorf("google: could not encode JWT: %v", err))
w.WriteHeader(http.StatusInternalServerError)
fmt.Fprintf(w, err.Error())
}
f := url.Values{
"grant_type": {"urn:ietf:params:oauth:grant-type:jwt-bearer"},
"assertion": {msg},
}
res, err := http.PostForm(authUrl, f)
if err != nil {
fmt.Println(err)
w.WriteHeader(http.StatusInternalServerError)
fmt.Fprintf(w, err.Error())
}
c, err := ioutil.ReadAll(res.Body)
defer res.Body.Close()
if err != nil {
fmt.Println(err)
w.WriteHeader(http.StatusInternalServerError)
fmt.Fprintf(w, err.Error())
}
type resIdToken struct {
IdToken string `json:"id_token"`
}
id := &resIdToken{}
json.Unmarshal(c, id)
fmt.Fprintf(w, id.IdToken)
}