Sign generated MXC URIs

directmedia.go

@@ -18,6 +18,7 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
 	"errors"
 	"fmt"
 	"io"
@@ -48,6 +49,8 @@ type DirectMediaAPI struct {
 	log    zerolog.Logger
 	proxy  http.Client
 
+	signatureKey [32]byte
+
 	attachmentCache     map[AttachmentCacheKey]AttachmentCacheValue
 	attachmentCacheLock sync.Mutex
 }
@@ -88,6 +91,7 @@ func newDirectMediaAPI(br *DiscordBridge) *DirectMediaAPI {
 		os.Exit(11)
 		return nil
 	}
+	dma.signatureKey = sha256.Sum256(parsed.Priv.Seed())
 	dma.ks = &federation.KeyServer{
 		KeyProvider: &federation.StaticServerKey{
 			ServerName: dma.cfg.ServerName,
@@ -139,7 +143,7 @@ func newDirectMediaAPI(br *DiscordBridge) *DirectMediaAPI {
 func (dma *DirectMediaAPI) makeMXC(data MediaIDData) id.ContentURI {
 	return id.ContentURI{
 		Homeserver: dma.cfg.ServerName,
-		FileID:     data.Wrap().String(),
+		FileID:     data.Wrap().SignedString(dma.signatureKey),
 	}
 }
 
@@ -319,7 +323,7 @@ func (dma *DirectMediaAPI) FetchNewAttachmentURL(ctx context.Context, meta *Atta
 
 func (dma *DirectMediaAPI) GetMediaURL(ctx context.Context, encodedMediaID string) (url string, expiry time.Time, err error) {
 	var mediaID *MediaID
-	mediaID, err = ParseMediaID(encodedMediaID)
+	mediaID, err = ParseMediaID(encodedMediaID, dma.signatureKey)
 	if err != nil {
 		err = &RespError{
 			Code:    mautrix.MNotFound.ErrCode,

directmedia_id.go

@@ -18,6 +18,8 @@ package main
 
 import (
 	"bytes"
+	"crypto/hmac"
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/binary"
 	"errors"
@@ -51,11 +53,18 @@ type MediaID struct {
 	Data      MediaIDData
 }
 
-func ParseMediaID(id string) (*MediaID, error) {
+func ParseMediaID(id string, key [32]byte) (*MediaID, error) {
 	data, err := base64.RawURLEncoding.DecodeString(id)
 	if err != nil {
 		return nil, fmt.Errorf("failed to decode base64: %w", err)
 	}
+	hasher := hmac.New(sha256.New, key[:])
+	checksum := data[len(data)-TruncatedHashLength:]
+	data = data[:len(data)-TruncatedHashLength]
+	hasher.Write(data)
+	if !hmac.Equal(checksum, hasher.Sum(nil)[:TruncatedHashLength]) {
+		return nil, ErrMediaIDChecksumMismatch
+	}
 	mid := &MediaID{}
 	err = mid.Read(bytes.NewReader(data))
 	if err != nil {
@@ -64,9 +73,14 @@ func ParseMediaID(id string) (*MediaID, error) {
 	return mid, nil
 }
 
-func (mid *MediaID) String() string {
-	buf := bytes.NewBuffer(make([]byte, 0, mid.Data.Size()))
+const TruncatedHashLength = 16
+
+func (mid *MediaID) SignedString(key [32]byte) string {
+	buf := bytes.NewBuffer(make([]byte, 0, mid.Size()))
 	mid.Write(buf)
+	hasher := hmac.New(sha256.New, key[:])
+	hasher.Write(buf.Bytes())
+	buf.Write(hasher.Sum(nil)[:TruncatedHashLength])
 	return base64.RawURLEncoding.EncodeToString(buf.Bytes())
 }
 
@@ -77,9 +91,14 @@ func (mid *MediaID) Write(to io.Writer) {
 	mid.Data.Write(to)
 }
 
+func (mid *MediaID) Size() int {
+	return len(MediaIDPrefix) + 2 + mid.Data.Size() + TruncatedHashLength
+}
+
 var (
-	ErrInvalidMediaID     = errors.New("invalid media ID")
-	ErrUnsupportedMediaID = errors.New("unsupported media ID")
+	ErrInvalidMediaID          = errors.New("invalid media ID")
+	ErrMediaIDChecksumMismatch = errors.New("invalid checksum in media ID")
+	ErrUnsupportedMediaID      = errors.New("unsupported media ID")
 )
 
 func (mid *MediaID) Read(from io.Reader) error {

example-config.yaml

@@ -183,6 +183,7 @@ bridge:
         # Optionally, you can force redirects and not allow proxying at all by setting this to false.
         allow_proxy: true
         # Matrix server signing key to make the federation tester pass, same format as synapse's .signing.key file.
+        # This key is also used to sign the mxc:// URIs to ensure only the bridge can generate them.
         server_key: generate
     # Settings for converting animated stickers.
     animated_sticker: