go.chromium.org/luci@v0.0.0-20240309015107-7cdc2e660f33/server/encryptedcookies/internal/pkce.go (about)

     1  // Copyright 2021 The LUCI Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //      http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package internal
    16  
    17  import (
    18  	"crypto/rand"
    19  	"crypto/sha256"
    20  	"encoding/base64"
    21  	"fmt"
    22  )
    23  
    24  // GenerateCodeVerifier generates a random string used as a code_verifier in
    25  // the PKCE protocol.
    26  //
    27  // See https://tools.ietf.org/html/rfc7636.
    28  func GenerateCodeVerifier() string {
    29  	blob := make([]byte, 50)
    30  	if _, err := rand.Read(blob); err != nil {
    31  		panic(fmt.Sprintf("failed to generate code verifier: %s", err))
    32  	}
    33  	// Note: there are exactly 64 symbols here. We exclude technically allowed '_'
    34  	// and '~' to simplify random number processing below needed to get fair
    35  	// distribution of probabilities.
    36  	const alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-."
    37  	for i := range blob {
    38  		blob[i] = alphabet[blob[i]&63]
    39  	}
    40  	return string(blob)
    41  }
    42  
    43  // DeriveCodeChallenge derives code_challenge from the code_verifier.
    44  func DeriveCodeChallenge(codeVerifier string) string {
    45  	codeVerifierS256 := sha256.Sum256([]byte(codeVerifier))
    46  	return base64.RawURLEncoding.EncodeToString(codeVerifierS256[:])
    47  }