Fix infohash decoding (#813)

Calculate the info hash of the uploaded torrent file
instead of the re-encoded torrent file.

The re-encoded torrent files only contain a subset
of the original info values and thus have a different hash.

router/upload.go

@@ -1,7 +1,6 @@
 package router
 
 import (
-	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -172,12 +171,17 @@ func (f *uploadForm) ExtractInfo(r *http.Request) error {
 		if len(f.Magnet) != 0 {
 			return errTorrentPlusMagnet
 		}
-		binInfohash, err := torrent.Infohash()
-		if err != nil {
-			return err
+
+		_, seekErr = tfile.Seek(0, io.SeekStart)
+		if seekErr != nil {
+			return seekErr
 		}
-		f.Infohash = strings.ToUpper(hex.EncodeToString(binInfohash[:]))
-		f.Magnet = util.InfoHashToMagnet(f.Infohash, f.Name, trackers...)
+		infohash, err := metainfo.DecodeInfohash(tfile)
+		if err != nil {
+			return metainfo.ErrInvalidTorrentFile
+		}
+		f.Infohash = infohash
+		f.Magnet = util.InfoHashToMagnet(infohash, f.Name, trackers...)
 
 		// extract filesize
 		f.Filesize = int64(torrent.TotalSize())

service/api/api.go

@@ -198,11 +198,15 @@ func (r *TorrentRequest) ValidateMultipartUpload(req *http.Request) (int64, erro
 			r.Name = torrent.TorrentName()
 		}
 
-		binInfohash, err := torrent.Infohash()
+		_, err = tfile.Seek(0, io.SeekStart)
 		if err != nil {
 			return 0, err, http.StatusInternalServerError
 		}
-		r.Hash = strings.ToUpper(hex.EncodeToString(binInfohash[:]))
+		infohash, err := metainfo.DecodeInfohash(tfile)
+		if err != nil {
+			return 0, err, http.StatusInternalServerError
+		}
+		r.Hash = infohash
 
 		// extract filesize
 		filesize := int64(torrent.TotalSize())

util/metainfo/metainfo.go

@@ -4,9 +4,11 @@ package metainfo
 
 import (
 	"crypto/sha1"
+	"encoding/hex"
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/zeebo/bencode"
 )
@@ -120,19 +122,6 @@ func (tf *TorrentFile) IsPrivate() bool {
 	return tf.Info.Private != nil && *tf.Info.Private == 1
 }
 
-// Infohash : calculate infohash
-func (tf *TorrentFile) Infohash() (ih [20]byte, err error) {
-	s := sha1.New()
-	enc := bencode.NewEncoder(s)
-	err = enc.Encode(&tf.Info)
-	if err != nil {
-		return
-	}
-	d := s.Sum(nil)
-	copy(ih[:], d[:])
-	return
-}
-
 // IsSingleFile : return true if this torrent is for a single file
 func (tf *TorrentFile) IsSingleFile() bool {
 	return tf.Info.Length > 0
@@ -151,3 +140,27 @@ func (tf *TorrentFile) Decode(r io.Reader) (err error) {
 	err = dec.Decode(tf)
 	return
 }
+
+type torrentRaw struct {
+	InfoRaw bencode.RawMessage `bencode:"info"`
+}
+
+// DecodeInfohash : Decode and calculate the info hash
+func DecodeInfohash(r io.Reader) (hash string, err error) {
+	var t torrentRaw
+	d := bencode.NewDecoder(r)
+	err = d.Decode(&t)
+	if err != nil {
+		return
+	}
+
+	s := sha1.New()
+	_, err = s.Write(t.InfoRaw)
+	if err != nil {
+		return
+	}
+	rawHash := s.Sum(nil)
+
+	hash = strings.ToUpper(hex.EncodeToString(rawHash))
+	return
+}