Support using ID token as the ouath access token.

jwt/jwt.go

@@ -69,6 +69,9 @@ type Config struct {
 
 	// PrivateClaims optionally specifies private claims in the JWT.
 	PrivateClaims map[string]interface{}
+
+	// UseIDToken optionally uses ID token instead of access token.
+	UseIDToken bool
 }
 
 // TokenSource returns a JWT TokenSource using the configuration
@@ -168,6 +171,9 @@ func (js jwtSource) Token() (*oauth2.Token, error) {
 		if err != nil {
 			return nil, fmt.Errorf("oauth2: error decoding JWT token: %v", err)
 		}
+		if js.conf.UseIDToken {
+			token.AccessToken = tokenRes.IDToken
+		}
 		token.Expiry = time.Unix(claimSet.Exp, 0)
 	}
 	return token, nil