split senc decoding into two phases to be able to use data from other…

… boxes

README.md

@@ -200,6 +200,7 @@ The examples are:
     This tool has been extended to support generation of segments with multiple tracks as well
 	as reading and writing `mdat` in lazy mode
 4. `multitrack` parses a fragmented file with multiple tracks
+5. `decrypt-cenc` decrypts a segmented mp4 file encrypted in `cenc` mode
 
 ## Stability
 The APIs should be fairly stable, but minor non-backwards-compatible changes may happen until version 1.

examples/decrypt-cenc/decrypt_cenc_test.go

@@ -1,13 +1,34 @@
 package main
 
-import "testing"
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/go-test/deep"
+)
 
 func TestDecodeCenc(t *testing.T) {
 	inFile := "testdata/prog_8s_enc_dashinit.mp4"
-	outFile := "testdata/dec.mp4"
+	expectedOutFile := "testdata/prog_8s_dec_dashinit.mp4"
 	hexString := "63cb5f7184dd4b689a5c5ff11ee6a328"
-	err := start(inFile, outFile, hexString)
+	ifh, err := os.Open(inFile)
+	if err != nil {
+		t.Error(err)
+	}
+	buf := bytes.Buffer{}
+	err = start(ifh, &buf, hexString)
 	if err != nil {
 		t.Error(err)
 	}
+	expectedOut, err := ioutil.ReadFile(expectedOutFile)
+	if err != nil {
+		t.Error(err)
+	}
+	gotOut := buf.Bytes()
+	diff := deep.Equal(expectedOut, gotOut)
+	if diff != nil {
+		t.Errorf("Mismatch: %s", diff)
+	}
 }

examples/decrypt-cenc/main.go

@@ -1,3 +1,8 @@
+// decrypt-cenc  - decrypt a segmented mp4 file encrypted in cenc mode
+//
+// The output is in the same format as the input but with samples decrypted
+// and encryption information boxes such as pssh and schm removed.
+// An example file is given in testdata/prog_8s_enc_dashinit.mp4
 package main
 
 import (
@@ -13,25 +18,26 @@ import (
 )
 
 func main() {
-
 	inFilePath := flag.String("i", "", "Required: Path to input file")
 	outFilePath := flag.String("o", "", "Required: Output file")
 	hexKey := flag.String("k", "", "Required: key (hex)")
 
-	err := start(*inFilePath, *outFilePath, *hexKey)
+	ifh, err := os.Open(*inFilePath)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer ifh.Close()
+	ofh, err := os.Create(*outFilePath)
+	if err != nil {
+		log.Fatal(err)
+	}
+	err = start(ifh, ofh, *hexKey)
 	if err != nil {
 		log.Fatalln(err)
 	}
-
 }
 
-func start(inPath, outPath, hexKey string) error {
-
-	ifh, err := os.Open(inPath)
-	if err != nil {
-		return err
-	}
-	defer ifh.Close()
+func start(r io.Reader, w io.Writer, hexKey string) error {
 
 	if len(hexKey) != 32 {
 		return fmt.Errorf("Hex key must have length 32 chars")
@@ -41,7 +47,7 @@ func start(inPath, outPath, hexKey string) error {
 		return err
 	}
 
-	err = decryptMP4withCenc(ifh, key, outPath)
+	err = decryptMP4withCenc(r, key, w)
 	if err != nil {
 		return err
 	}
@@ -64,7 +70,7 @@ func findTrackInfo(tracks []trackInfo, trackID uint32) trackInfo {
 }
 
 // decryptMP4withCenc - decrypt segmented mp4 file with CENC encryption
-func decryptMP4withCenc(r io.Reader, key []byte, outPath string) error {
+func decryptMP4withCenc(r io.Reader, key []byte, w io.Writer) error {
 	inMp4, err := mp4.DecodeFile(r)
 	if err != nil {
 		return err
@@ -73,11 +79,6 @@ func decryptMP4withCenc(r io.Reader, key []byte, outPath string) error {
 		return fmt.Errorf("file not fragmented. Not supported")
 	}
 
-	ofh, err := os.Create(outPath)
-	if err != nil {
-		return err
-	}
-
 	tracks := make([]trackInfo, 0, len(inMp4.Init.Moov.Traks))
 
 	moov := inMp4.Init.Moov
@@ -135,16 +136,15 @@ func decryptMP4withCenc(r io.Reader, key []byte, outPath string) error {
 	}
 
 	// Write the modified init segment
-	err = inMp4.Init.Encode(ofh)
+	err = inMp4.Init.Encode(w)
 	if err != nil {
 		return err
 	}
 
-	err = decryptAndWriteSegments(inMp4.Segments, tracks, key, ofh)
+	err = decryptAndWriteSegments(inMp4.Segments, tracks, key, w)
 	if err != nil {
 		return err
 	}
-	ofh.Close()
 	return nil
 }
 
@@ -176,13 +176,24 @@ func decryptFragment(frag *mp4.Fragment, tracks []trackInfo, key []byte) error {
 	moof := frag.Moof
 	var nrBytesRemoved uint64 = 0
 	for _, traf := range moof.Trafs {
-		senc := traf.Senc
+		hasSenc, isParsed := traf.ContainsSencBox()
+		if !hasSenc {
+			return fmt.Errorf("no senc box in traf")
+		}
 		ti := findTrackInfo(tracks, traf.Tfhd.TrackID)
+		if !isParsed {
+			defaultIVSize := ti.sinf.Schi.Tenc.DefaultPerSampleIVSize
+			err := traf.ParseReadSenc(defaultIVSize, moof.StartPos)
+			if err != nil {
+				return fmt.Errorf("parseReadSenc: %w", err)
+			}
+		}
 		samples, err := frag.GetFullSamples(ti.trex)
 		if err != nil {
 			return err
 		}
-		err = decryptSamplesInPlace(samples, key, senc)
+
+		err = decryptSamplesInPlace(samples, key, traf.Senc)
 		if err != nil {
 			return err
 		}

mp4/file.go

@@ -162,12 +162,31 @@ LoopBoxes:
 		if err != nil {
 			return nil, err
 		}
-		if boxType == "mdat" {
+		switch boxType {
+		case "mdat":
 			if f.isFragmented {
 				if lastBoxType != "moof" {
 					return nil, fmt.Errorf("Does not support %v between moof and mdat", lastBoxType)
 				}
 			}
+		case "moof":
+			moof := box.(*MoofBox)
+			for _, traf := range moof.Trafs {
+				if ok, parsed := traf.ContainsSencBox(); ok && !parsed {
+					defaultIVSize := byte(0) // Should get this from tenc in sinf
+					if f.Moov != nil {
+						trackID := traf.Tfhd.TrackID
+						sinf := f.Moov.GetSinf(trackID)
+						if sinf != nil && sinf.Schi != nil && sinf.Schi.Tenc != nil {
+							defaultIVSize = sinf.Schi.Tenc.DefaultPerSampleIVSize
+						}
+					}
+					err = traf.ParseReadSenc(defaultIVSize, moof.StartPos)
+					if err != nil {
+						return nil, err
+					}
+				}
+			}
 		}
 		f.AddChild(box, boxStartPos)
 		lastBoxType = boxType

mp4/moov.go

@@ -15,6 +15,7 @@ type MoovBox struct {
 	Pssh     *PsshBox
 	Psshs    []*PsshBox
 	Children []Box
+	StartPos uint64
 }
 
 // NewMoovBox - Generate a new empty moov box
@@ -24,7 +25,6 @@ func NewMoovBox() *MoovBox {
 
 // AddChild - Add a child box
 func (m *MoovBox) AddChild(box Box) {
-
 	switch box.Type() {
 	case "mvhd":
 		m.Mvhd = box.(*MvhdBox)
@@ -62,6 +62,7 @@ func DecodeMoov(hdr *boxHeader, startPos uint64, r io.Reader) (Box, error) {
 		return nil, err
 	}
 	m := NewMoovBox()
+	m.StartPos = startPos
 	for _, b := range l {
 		m.AddChild(b)
 	}
@@ -111,3 +112,19 @@ func (m *MoovBox) RemovePsshs() []*PsshBox {
 
 	return psshs
 }
+
+func (m *MoovBox) GetSinf(trackID uint32) *SinfBox {
+	for _, trak := range m.Traks {
+		if trak.Tkhd.TrackID == trackID {
+			stsd := trak.Mdia.Minf.Stbl.Stsd
+			sd := stsd.Children[0] // Get first (and only)
+			if visual, ok := sd.(*VisualSampleEntryBox); ok {
+				return visual.Sinf
+			}
+			if audio, ok := sd.(*AudioSampleEntryBox); ok {
+				return audio.Sinf
+			}
+		}
+	}
+	return nil
+}

mp4/saio.go

@@ -5,7 +5,7 @@ import (
 	"io/ioutil"
 )
 
-// SaioBox - Sample Auxiliary Information Offsets Box (saiz)
+// SaioBox - Sample Auxiliary Information Offsets Box (saiz) (in stbl or traf box)
 type SaioBox struct {
 	Version              byte
 	Flags                uint32
@@ -101,8 +101,9 @@ func (b *SaioBox) Info(w io.Writer, specificBoxLevels, indent, indentStep string
 	}
 	bd.write(" - sampleCount: %d", len(b.Offset))
 	level := getInfoLevel(b, specificBoxLevels)
+	bd.write(" - offset[%d]=%d", 1, b.Offset[0])
 	if level > 0 {
-		for i := 0; i < len(b.Offset); i++ {
+		for i := 1; i < len(b.Offset); i++ {
 			bd.write(" - offset[%d]=%d", i+1, b.Offset[i])
 		}
 	}

mp4/saiz.go

@@ -5,7 +5,7 @@ import (
 	"io/ioutil"
 )
 
-// SaizBox - Sample Auxiliary Information Sizes Box (saiz)
+// SaizBox - Sample Auxiliary Information Sizes Box (saiz)  (in stbl or traf box)
 type SaizBox struct {
 	Version               byte
 	Flags                 uint32

mp4/schm.go

@@ -76,7 +76,7 @@ func (b *SchmBox) Encode(w io.Writer) error {
 func (b *SchmBox) Info(w io.Writer, specificBoxLevels, indent, indentStep string) (err error) {
 	bd := newInfoDumper(w, indent, b, int(b.Version), b.Flags)
 	bd.write(" - schemeType: %s", b.SchemeType)
-	bd.write(" - schemeVersion: %d", b.SchemeVersion)
+	bd.write(" - schemeVersion: %d  (%d.%d)", b.SchemeVersion, b.SchemeVersion>>16, b.SchemeVersion&0xffff)
 	if b.Flags&0x01 != 0 {
 		bd.write(" - schemeURI: %q", b.SchemeURI)
 	}