Unify hashing for avatar (#22289)

- Unify the hashing code for repository and user avatars into a
function.
- Use a sane hash function instead of MD5.
- Only require hashing once instead of twice(w.r.t. hashing for user
avatar).
- Improve the comment for the hashing code of why it works.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Yarden Shoham <hrsi88@gmail.com>

modules/avatar/hash.go

@@ -0,0 +1,28 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package avatar
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"strconv"
+)
+
+// HashAvatar will generate a unique string, which ensures that when there's a
+// different unique ID while the data is the same, it will generate a different
+// output. It will generate the output according to:
+// HEX(HASH(uniqueID || - || data))
+// The hash being used is SHA256.
+// The sole purpose of the unique ID is to generate a distinct hash Such that
+// two unique IDs with the same data will have a different hash output.
+// The "-" byte is important to ensure that data cannot be modified such that
+// the first byte is a number, which could lead to a "collision" with the hash
+// of another unique ID.
+func HashAvatar(uniqueID int64, data []byte) string {
+	h := sha256.New()
+	h.Write([]byte(strconv.FormatInt(uniqueID, 10)))
+	h.Write([]byte{'-'})
+	h.Write(data)
+	return hex.EncodeToString(h.Sum(nil))
+}

services/repository/avatar.go

@@ -5,7 +5,6 @@ package repository
 
 import (
 	"context"
-	"crypto/md5"
 	"fmt"
 	"image/png"
 	"io"
@@ -27,7 +26,7 @@ func UploadAvatar(repo *repo_model.Repository, data []byte) error {
 		return err
 	}
 
-	newAvatar := fmt.Sprintf("%d-%x", repo.ID, md5.Sum(data))
+	newAvatar := avatar.HashAvatar(repo.ID, data)
 	if repo.Avatar == newAvatar { // upload the same picture
 		return nil
 	}

services/repository/avatar_test.go

@@ -5,14 +5,13 @@ package repository
 
 import (
 	"bytes"
-	"crypto/md5"
-	"fmt"
 	"image"
 	"image/png"
 	"testing"
 
 	repo_model "code.gitea.io/gitea/models/repo"
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/avatar"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -28,7 +27,7 @@ func TestUploadAvatar(t *testing.T) {
 
 	err := UploadAvatar(repo, buff.Bytes())
 	assert.NoError(t, err)
-	assert.Equal(t, fmt.Sprintf("%d-%x", 10, md5.Sum(buff.Bytes())), repo.Avatar)
+	assert.Equal(t, avatar.HashAvatar(10, buff.Bytes()), repo.Avatar)
 }
 
 func TestUploadBigAvatar(t *testing.T) {

services/user/user.go

@@ -5,7 +5,6 @@ package user
 
 import (
 	"context"
-	"crypto/md5"
 	"fmt"
 	"image/png"
 	"io"
@@ -241,11 +240,7 @@ func UploadAvatar(u *user_model.User, data []byte) error {
 	defer committer.Close()
 
 	u.UseCustomAvatar = true
-	// Different users can upload same image as avatar
+	u.Avatar = avatar.HashAvatar(u.ID, data)
-	// If we prefix it with u.ID, it will be separated
-	// Otherwise, if any of the users delete his avatar
-	// Other users will lose their avatars too.
-	u.Avatar = fmt.Sprintf("%x", md5.Sum([]byte(fmt.Sprintf("%d-%x", u.ID, md5.Sum(data)))))
 	if err = user_model.UpdateUserCols(ctx, u, "use_custom_avatar", "avatar"); err != nil {
 		return fmt.Errorf("updateUser: %w", err)
 	}