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// Copyright 2016 The go-github AUTHORS. All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file provides functions for validating payloads from GitHub Webhooks.
// GitHub docs: https://developer.github.com/webhooks/securing/#validating-payloads-from-github
package github
import (
"crypto/hmac"
"crypto/sha1"
"crypto/sha256"
"crypto/sha512"
"encoding/hex"
"errors"
"fmt"
"hash"
"io/ioutil"
"net/http"
"strings"
)
const (
// sha1Prefix is the prefix used by GitHub before the HMAC hexdigest.
sha1Prefix = "sha1"
// sha256Prefix and sha512Prefix are provided for future compatibility.
sha256Prefix = "sha256"
sha512Prefix = "sha512"
// signatureHeader is the GitHub header key used to pass the HMAC hexdigest.
signatureHeader = "X-Hub-Signature"
)
// genMAC generates the HMAC signature for a message provided the secret key
// and hashFunc.
func genMAC(message, key []byte, hashFunc func() hash.Hash) []byte {
mac := hmac.New(hashFunc, key)
mac.Write(message)
return mac.Sum(nil)
}
// checkMAC reports whether messageMAC is a valid HMAC tag for message.
func checkMAC(message, messageMAC, key []byte, hashFunc func() hash.Hash) bool {
expectedMAC := genMAC(message, key, hashFunc)
return hmac.Equal(messageMAC, expectedMAC)
}
// messageMAC returns the hex-decoded HMAC tag from the signature and its
// corresponding hash function.
func messageMAC(signature string) ([]byte, func() hash.Hash, error) {
if signature == "" {
return nil, nil, errors.New("missing signature")
}
sigParts := strings.SplitN(signature, "=", 2)
if len(sigParts) != 2 {
return nil, nil, fmt.Errorf("error parsing signature %q", signature)
}
var hashFunc func() hash.Hash
switch sigParts[0] {
case sha1Prefix:
hashFunc = sha1.New
case sha256Prefix:
hashFunc = sha256.New
case sha512Prefix:
hashFunc = sha512.New
default:
return nil, nil, fmt.Errorf("unknown hash type prefix: %q", sigParts[0])
}
buf, err := hex.DecodeString(sigParts[1])
if err != nil {
return nil, nil, fmt.Errorf("error decoding signature %q: %v", signature, err)
}
return buf, hashFunc, nil
}
// ValidatePayload validates an incoming GitHub Webhook event request
// and returns the (JSON) payload.
// secretKey is the GitHub Webhook secret message.
//
// Example usage:
//
// func (s *GitHubEventMonitor) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// payload, err := github.ValidatePayload(r, s.webhookSecretKey)
// if err != nil { ... }
// // Process payload...
// }
//
func ValidatePayload(r *http.Request, secretKey []byte) (payload []byte, err error) {
payload, err = ioutil.ReadAll(r.Body)
if err != nil {
return nil, err
}
sig := r.Header.Get(signatureHeader)
if err := validateSignature(sig, payload, secretKey); err != nil {
return nil, err
}
return payload, nil
}
// validateSignature validates the signature for the given payload.
// signature is the GitHub hash signature delivered in the X-Hub-Signature header.
// payload is the JSON payload sent by GitHub Webhooks.
// secretKey is the GitHub Webhook secret message.
//
// GitHub docs: https://developer.github.com/webhooks/securing/#validating-payloads-from-github
func validateSignature(signature string, payload, secretKey []byte) error {
messageMAC, hashFunc, err := messageMAC(signature)
if err != nil {
return err
}
if !checkMAC(payload, messageMAC, secretKey, hashFunc) {
return errors.New("payload signature check failed")
}
return nil
}
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