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// Copyright 2023 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 oauth2
import (
const (
codeChallengeKey = "code_challenge"
codeChallengeMethodKey = "code_challenge_method"
codeVerifierKey = "code_verifier"
// GenerateVerifier generates a PKCE code verifier with 32 octets of randomness.
// This follows recommendations in RFC 7636.
// A fresh verifier should be generated for each authorization.
// S256ChallengeOption(verifier) should then be passed to Config.AuthCodeURL
// (or Config.DeviceAccess) and VerifierOption(verifier) to Config.Exchange
// (or Config.DeviceAccessToken).
func GenerateVerifier() string {
// "RECOMMENDED that the output of a suitable random number generator be
// used to create a 32-octet sequence. The octet sequence is then
// base64url-encoded to produce a 43-octet URL-safe string to use as the
// code verifier."
data := make([]byte, 32)
if _, err := rand.Read(data); err != nil {
return base64.RawURLEncoding.EncodeToString(data)
// VerifierOption returns a PKCE code verifier AuthCodeOption. It should be
// passed to Config.Exchange or Config.DeviceAccessToken only.
func VerifierOption(verifier string) AuthCodeOption {
return setParam{k: codeVerifierKey, v: verifier}
// S256ChallengeFromVerifier returns a PKCE code challenge derived from verifier with method S256.
// Prefer to use S256ChallengeOption where possible.
func S256ChallengeFromVerifier(verifier string) string {
sha := sha256.Sum256([]byte(verifier))
return base64.RawURLEncoding.EncodeToString(sha[:])
// S256ChallengeOption derives a PKCE code challenge derived from verifier with
// method S256. It should be passed to Config.AuthCodeURL or Config.DeviceAccess
// only.
func S256ChallengeOption(verifier string) AuthCodeOption {
return challengeOption{
challenge_method: "S256",
challenge: S256ChallengeFromVerifier(verifier),
type challengeOption struct{ challenge_method, challenge string }
func (p challengeOption) setValue(m url.Values) {
m.Set(codeChallengeMethodKey, p.challenge_method)
m.Set(codeChallengeKey, p.challenge)