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validate.go
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validate.go
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// Copyright 2020 Google LLC.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package idtoken
import (
"context"
"crypto"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/rsa"
"crypto/sha256"
"encoding/base64"
"encoding/json"
"fmt"
"math/big"
"net/http"
"strings"
"time"
"google.golang.org/api/option"
"google.golang.org/api/option/internaloption"
htransport "google.golang.org/api/transport/http"
)
const (
es256KeySize int = 32
googleIAPCertsURL string = "https://www.gstatic.com/iap/verify/public_key-jwk"
googleSACertsURL string = "https://www.googleapis.com/oauth2/v3/certs"
)
var (
defaultValidator = &Validator{client: newCachingClient(http.DefaultClient)}
// now aliases time.Now for testing.
now = time.Now
)
func defaultValidatorOpts() []ClientOption {
return []ClientOption{
internaloption.WithDefaultScopes("https://www.googleapis.com/auth/cloud-platform"),
option.WithoutAuthentication(),
}
}
// Payload represents a decoded payload of an ID Token.
type Payload struct {
Issuer string `json:"iss"`
Audience string `json:"aud"`
Expires int64 `json:"exp"`
IssuedAt int64 `json:"iat"`
Subject string `json:"sub,omitempty"`
Claims map[string]interface{} `json:"-"`
}
// jwt represents the segments of a jwt and exposes convenience methods for
// working with the different segments.
type jwt struct {
header string
payload string
signature string
}
// jwtHeader represents a parted jwt's header segment.
type jwtHeader struct {
Algorithm string `json:"alg"`
Type string `json:"typ"`
KeyID string `json:"kid"`
}
// certResponse represents a list jwks. It is the format returned from known
// Google cert endpoints.
type certResponse struct {
Keys []jwk `json:"keys"`
}
// jwk is a simplified representation of a standard jwk. It only includes the
// fields used by Google's cert endpoints.
type jwk struct {
Alg string `json:"alg"`
Crv string `json:"crv"`
Kid string `json:"kid"`
Kty string `json:"kty"`
Use string `json:"use"`
E string `json:"e"`
N string `json:"n"`
X string `json:"x"`
Y string `json:"y"`
}
// Validator provides a way to validate Google ID Tokens with a user provided
// http.Client.
type Validator struct {
client *cachingClient
}
// NewValidator creates a Validator that uses the options provided to configure
// a the internal http.Client that will be used to make requests to fetch JWKs.
func NewValidator(ctx context.Context, opts ...ClientOption) (*Validator, error) {
opts = append(defaultValidatorOpts(), opts...)
client, _, err := htransport.NewClient(ctx, opts...)
if err != nil {
return nil, err
}
return &Validator{client: newCachingClient(client)}, nil
}
// Validate is used to validate the provided idToken with a known Google cert
// URL. If audience is not empty the audience claim of the Token is validated.
// Upon successful validation a parsed token Payload is returned allowing the
// caller to validate any additional claims.
func (v *Validator) Validate(ctx context.Context, idToken string, audience string) (*Payload, error) {
return v.validate(ctx, idToken, audience)
}
// Validate is used to validate the provided idToken with a known Google cert
// URL. If audience is not empty the audience claim of the Token is validated.
// Upon successful validation a parsed token Payload is returned allowing the
// caller to validate any additional claims.
func Validate(ctx context.Context, idToken string, audience string) (*Payload, error) {
// TODO(codyoss): consider adding a check revoked version of the api. See: https://pkg.go.dev/firebase.google.com/go/auth?tab=doc#Client.VerifyIDTokenAndCheckRevoked
return defaultValidator.validate(ctx, idToken, audience)
}
// ParsePayload parses the given token and returns its payload.
//
// Warning: This function does not validate the token prior to parsing it.
//
// ParsePayload is primarily meant to be used to inspect a token's payload. This is
// useful when validation fails and the payload needs to be inspected.
//
// Note: A successful Validate() invocation with the same token will return an
// identical payload.
func ParsePayload(idToken string) (*Payload, error) {
jwt, err := parseJWT(idToken)
if err != nil {
return nil, err
}
return jwt.parsedPayload()
}
func (v *Validator) validate(ctx context.Context, idToken string, audience string) (*Payload, error) {
jwt, err := parseJWT(idToken)
if err != nil {
return nil, err
}
header, err := jwt.parsedHeader()
if err != nil {
return nil, err
}
payload, err := jwt.parsedPayload()
if err != nil {
return nil, err
}
sig, err := jwt.decodedSignature()
if err != nil {
return nil, err
}
if audience != "" && payload.Audience != audience {
return nil, fmt.Errorf("idtoken: audience provided does not match aud claim in the JWT")
}
if now().Unix() > payload.Expires {
return nil, fmt.Errorf("idtoken: token expired: now=%v, expires=%v", now().Unix(), payload.Expires)
}
switch header.Algorithm {
case "RS256":
if err := v.validateRS256(ctx, header.KeyID, jwt.hashedContent(), sig); err != nil {
return nil, err
}
case "ES256":
if err := v.validateES256(ctx, header.KeyID, jwt.hashedContent(), sig); err != nil {
return nil, err
}
default:
return nil, fmt.Errorf("idtoken: expected JWT signed with RS256 or ES256 but found %q", header.Algorithm)
}
return payload, nil
}
func (v *Validator) validateRS256(ctx context.Context, keyID string, hashedContent []byte, sig []byte) error {
certResp, err := v.client.getCert(ctx, googleSACertsURL)
if err != nil {
return err
}
j, err := findMatchingKey(certResp, keyID)
if err != nil {
return err
}
dn, err := decode(j.N)
if err != nil {
return err
}
de, err := decode(j.E)
if err != nil {
return err
}
pk := &rsa.PublicKey{
N: new(big.Int).SetBytes(dn),
E: int(new(big.Int).SetBytes(de).Int64()),
}
return rsa.VerifyPKCS1v15(pk, crypto.SHA256, hashedContent, sig)
}
func (v *Validator) validateES256(ctx context.Context, keyID string, hashedContent []byte, sig []byte) error {
certResp, err := v.client.getCert(ctx, googleIAPCertsURL)
if err != nil {
return err
}
j, err := findMatchingKey(certResp, keyID)
if err != nil {
return err
}
dx, err := decode(j.X)
if err != nil {
return err
}
dy, err := decode(j.Y)
if err != nil {
return err
}
pk := &ecdsa.PublicKey{
Curve: elliptic.P256(),
X: new(big.Int).SetBytes(dx),
Y: new(big.Int).SetBytes(dy),
}
r := big.NewInt(0).SetBytes(sig[:es256KeySize])
s := big.NewInt(0).SetBytes(sig[es256KeySize:])
if valid := ecdsa.Verify(pk, hashedContent, r, s); !valid {
return fmt.Errorf("idtoken: ES256 signature not valid")
}
return nil
}
func findMatchingKey(response *certResponse, keyID string) (*jwk, error) {
if response == nil {
return nil, fmt.Errorf("idtoken: cert response is nil")
}
for _, v := range response.Keys {
if v.Kid == keyID {
return &v, nil
}
}
return nil, fmt.Errorf("idtoken: could not find matching cert keyId for the token provided")
}
func parseJWT(idToken string) (*jwt, error) {
segments := strings.Split(idToken, ".")
if len(segments) != 3 {
return nil, fmt.Errorf("idtoken: invalid token, token must have three segments; found %d", len(segments))
}
return &jwt{
header: segments[0],
payload: segments[1],
signature: segments[2],
}, nil
}
// decodedHeader base64 decodes the header segment.
func (j *jwt) decodedHeader() ([]byte, error) {
dh, err := decode(j.header)
if err != nil {
return nil, fmt.Errorf("idtoken: unable to decode JWT header: %v", err)
}
return dh, nil
}
// decodedPayload base64 payload the header segment.
func (j *jwt) decodedPayload() ([]byte, error) {
p, err := decode(j.payload)
if err != nil {
return nil, fmt.Errorf("idtoken: unable to decode JWT payload: %v", err)
}
return p, nil
}
// decodedPayload base64 payload the header segment.
func (j *jwt) decodedSignature() ([]byte, error) {
p, err := decode(j.signature)
if err != nil {
return nil, fmt.Errorf("idtoken: unable to decode JWT signature: %v", err)
}
return p, nil
}
// parsedHeader returns a struct representing a JWT header.
func (j *jwt) parsedHeader() (jwtHeader, error) {
var h jwtHeader
dh, err := j.decodedHeader()
if err != nil {
return h, err
}
err = json.Unmarshal(dh, &h)
if err != nil {
return h, fmt.Errorf("idtoken: unable to unmarshal JWT header: %v", err)
}
return h, nil
}
// parsedPayload returns a struct representing a JWT payload.
func (j *jwt) parsedPayload() (*Payload, error) {
var p Payload
dp, err := j.decodedPayload()
if err != nil {
return nil, err
}
if err := json.Unmarshal(dp, &p); err != nil {
return nil, fmt.Errorf("idtoken: unable to unmarshal JWT payload: %v", err)
}
if err := json.Unmarshal(dp, &p.Claims); err != nil {
return nil, fmt.Errorf("idtoken: unable to unmarshal JWT payload claims: %v", err)
}
return &p, nil
}
// hashedContent gets the SHA256 checksum for verification of the JWT.
func (j *jwt) hashedContent() []byte {
signedContent := j.header + "." + j.payload
hashed := sha256.Sum256([]byte(signedContent))
return hashed[:]
}
func (j *jwt) String() string {
return fmt.Sprintf("%s.%s.%s", j.header, j.payload, j.signature)
}
func decode(s string) ([]byte, error) {
return base64.RawURLEncoding.DecodeString(s)
}