init: dec-music 项目初始化

decrypt/kgm/kgm.go

@@ -0,0 +1,67 @@
+package kgm
+
+import (
+	"fmt"
+	"io"
+
+	"jsuse.com/dev/dec-music/decrypt"
+)
+
+type decoder struct {
+	rd io.ReadSeeker
+
+	cipher decrypt.StreamDecoder
+	offset int
+
+	header header
+}
+
+func newDecoder(p *decrypt.DecoderParams) decrypt.Decoder {
+	return &decoder{rd: p.Reader}
+}
+
+// Validate checks if the file is a valid Kugou (.kgm, .vpr, .kgma) file.
+// rd will be seeked to the beginning of the encrypted audio.
+func (d *decoder) Validate() (err error) {
+	if err := d.header.FromFile(d.rd); err != nil {
+		return err
+	}
+	// TODO; validate crypto version
+
+	switch d.header.CryptoVersion {
+	case 3:
+		d.cipher, err = newKgmCryptoV3(&d.header)
+		if err != nil {
+			return fmt.Errorf("kgm init crypto v3: %w", err)
+		}
+	default:
+		return fmt.Errorf("kgm: unsupported crypto version %d", d.header.CryptoVersion)
+	}
+
+	// prepare for read
+	if _, err := d.rd.Seek(int64(d.header.AudioOffset), io.SeekStart); err != nil {
+		return fmt.Errorf("kgm seek to audio: %w", err)
+	}
+
+	return nil
+}
+
+func (d *decoder) Read(buf []byte) (int, error) {
+	n, err := d.rd.Read(buf)
+	if n > 0 {
+		d.cipher.Decrypt(buf[:n], d.offset)
+		d.offset += n
+	}
+	return n, err
+}
+
+func init() {
+	// Kugou
+	decrypt.RegisterDecoder("kgm", false , newDecoder)
+	decrypt.RegisterDecoder("kgma", false , newDecoder)
+	// Viper
+	decrypt.RegisterDecoder("vpr", false , newDecoder)
+	// Kugou Android
+	decrypt.RegisterDecoder("kgm.flac", false , newDecoder)
+	decrypt.RegisterDecoder("vpr.flac", false , newDecoder)
+}

decrypt/kgm/kgm_header.go

@@ -0,0 +1,64 @@
+package kgm
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+)
+
+var (
+	vprHeader = []byte{
+		0x05, 0x28, 0xBC, 0x96, 0xE9, 0xE4, 0x5A, 0x43,
+		0x91, 0xAA, 0xBD, 0xD0, 0x7A, 0xF5, 0x36, 0x31,
+	}
+	kgmHeader = []byte{
+		0x7C, 0xD5, 0x32, 0xEB, 0x86, 0x02, 0x7F, 0x4B,
+		0xA8, 0xAF, 0xA6, 0x8E, 0x0F, 0xFF, 0x99, 0x14,
+	}
+
+	ErrKgmMagicHeader = errors.New("kgm magic header not matched")
+)
+
+// header is the header of a KGM file.
+type header struct {
+	MagicHeader    []byte // 0x00-0x0f: magic header
+	AudioOffset    uint32 // 0x10-0x13: offset of audio data
+	CryptoVersion  uint32 // 0x14-0x17: crypto version
+	CryptoSlot     uint32 // 0x18-0x1b: crypto key slot
+	CryptoTestData []byte // 0x1c-0x2b: crypto test data
+	CryptoKey      []byte // 0x2c-0x3b: crypto key
+}
+
+func (h *header) FromFile(rd io.ReadSeeker) error {
+	if _, err := rd.Seek(0, io.SeekStart); err != nil {
+		return fmt.Errorf("kgm seek start: %w", err)
+	}
+
+	buf := make([]byte, 0x3c)
+	if _, err := io.ReadFull(rd, buf); err != nil {
+		return fmt.Errorf("kgm read header: %w", err)
+	}
+
+	return h.FromBytes(buf)
+}
+
+func (h *header) FromBytes(buf []byte) error {
+	if len(buf) < 0x3c {
+		return errors.New("invalid kgm header length")
+	}
+
+	h.MagicHeader = buf[:0x10]
+	if !bytes.Equal(kgmHeader, h.MagicHeader) && !bytes.Equal(vprHeader, h.MagicHeader) {
+		return ErrKgmMagicHeader
+	}
+
+	h.AudioOffset = binary.LittleEndian.Uint32(buf[0x10:0x14])
+	h.CryptoVersion = binary.LittleEndian.Uint32(buf[0x14:0x18])
+	h.CryptoSlot = binary.LittleEndian.Uint32(buf[0x18:0x1c])
+	h.CryptoTestData = buf[0x1c:0x2c]
+	h.CryptoKey = buf[0x2c:0x3c]
+
+	return nil
+}

decrypt/kgm/kgm_v3.go

@@ -0,0 +1,55 @@
+package kgm
+
+import (
+	"crypto/md5"
+	"fmt"
+
+	"jsuse.com/dev/dec-music/decrypt"
+)
+
+// kgmCryptoV3 is kgm file crypto v3
+type kgmCryptoV3 struct {
+	slotBox []byte
+	fileBox []byte
+}
+
+var kgmV3Slot2Key = map[uint32][]byte{
+	1: {0x6C, 0x2C, 0x2F, 0x27},
+}
+
+func newKgmCryptoV3(header *header) (decrypt.StreamDecoder, error) {
+	c := &kgmCryptoV3{}
+
+	slotKey, ok := kgmV3Slot2Key[header.CryptoSlot]
+	if !ok {
+		return nil, fmt.Errorf("kgm3: unknown crypto slot %d", header.CryptoSlot)
+	}
+	c.slotBox = kugouMD5(slotKey)
+
+	c.fileBox = append(kugouMD5(header.CryptoKey), 0x6b)
+
+	return c, nil
+}
+
+func (d *kgmCryptoV3) Decrypt(b []byte, offset int) {
+	for i := 0; i < len(b); i++ {
+		b[i] ^= d.fileBox[(offset+i)%len(d.fileBox)]
+		b[i] ^= b[i] << 4
+		b[i] ^= d.slotBox[(offset+i)%len(d.slotBox)]
+		b[i] ^= xorCollapseUint32(uint32(offset + i))
+	}
+}
+
+func xorCollapseUint32(i uint32) byte {
+	return byte(i) ^ byte(i>>8) ^ byte(i>>16) ^ byte(i>>24)
+}
+
+func kugouMD5(b []byte) []byte {
+	digest := md5.Sum(b)
+	ret := make([]byte, 16)
+	for i := 0; i < md5.Size; i += 2 {
+		ret[i] = digest[14-i]
+		ret[i+1] = digest[14-i+1]
+	}
+	return ret
+}