dec-music/decrypt/ncm/ncm.go

package ncm

import (
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"io"

	"jsuse.com/dev/dec-music/decrypt"
	"jsuse.com/dev/dec-music/internal/crypto"
)

const magicHeader = "CTENFDAM"

var (
	keyCore = []byte{
		0x68, 0x7a, 0x48, 0x52, 0x41, 0x6d, 0x73, 0x6f,
		0x35, 0x6b, 0x49, 0x6e, 0x62, 0x61, 0x78, 0x57,
	}
	keyMeta = []byte{
		0x23, 0x31, 0x34, 0x6C, 0x6A, 0x6B, 0x5F, 0x21,
		0x5C, 0x5D, 0x26, 0x30, 0x55, 0x3C, 0x27, 0x28,
	}
)

func newDecoder(p *decrypt.DecoderParams) decrypt.Decoder {
	return &decoder{rd: p.Reader}
}

type decoder struct {
	rd     io.ReadSeeker
	offset int
	cipher decrypt.StreamDecoder
}

func (d *decoder) Validate() error {
	if err := d.validateMagicHeader(); err != nil {
		return err
	}

	if _, err := d.rd.Seek(2, io.SeekCurrent); err != nil {
		return fmt.Errorf("ncm seek file: %w", err)
	}

	keyData, err := d.readKeyData()
	if err != nil {
		return err
	}

	if err := d.skipMetaData(); err != nil {
		return fmt.Errorf("ncm skip meta: %w", err)
	}

	if _, err := d.rd.Seek(5, io.SeekCurrent); err != nil {
		return fmt.Errorf("ncm seek gap: %w", err)
	}

	if err := d.skipCoverData(); err != nil {
		return fmt.Errorf("ncm skip cover: %w", err)
	}

	d.cipher = newNcmCipher(keyData)
	return nil
}

func (d *decoder) validateMagicHeader() error {
	header := make([]byte, len(magicHeader))
	if _, err := d.rd.Read(header); err != nil {
		return fmt.Errorf("ncm read magic header: %w", err)
	}
	if !bytes.Equal([]byte(magicHeader), header) {
		return errors.New("ncm magic header not match")
	}
	return nil
}

func (d *decoder) readKeyData() ([]byte, error) {
	bKeyLen := make([]byte, 4)
	if _, err := io.ReadFull(d.rd, bKeyLen); err != nil {
		return nil, fmt.Errorf("ncm read key length: %w", err)
	}
	iKeyLen := binary.LittleEndian.Uint32(bKeyLen)

	bKeyRaw := make([]byte, iKeyLen)
	if _, err := io.ReadFull(d.rd, bKeyRaw); err != nil {
		return nil, fmt.Errorf("ncm read key data: %w", err)
	}
	for i := uint32(0); i < iKeyLen; i++ {
		bKeyRaw[i] ^= 0x64
	}

	return crypto.PKCS7UnPadding(crypto.DecryptAES128ECB(bKeyRaw, keyCore))[17:], nil
}

func (d *decoder) skipMetaData() error {
	bMetaLen := make([]byte, 4)
	if _, err := io.ReadFull(d.rd, bMetaLen); err != nil {
		return fmt.Errorf("ncm read meta length: %w", err)
	}
	iMetaLen := binary.LittleEndian.Uint32(bMetaLen)
	if iMetaLen == 0 {
		return nil
	}
	_, err := d.rd.Seek(int64(iMetaLen), io.SeekCurrent)
	return err
}

func (d *decoder) skipCoverData() error {
	bCoverFrameLen := make([]byte, 4)
	if _, err := io.ReadFull(d.rd, bCoverFrameLen); err != nil {
		return fmt.Errorf("ncm read cover frame length: %w", err)
	}

	coverFrameStartOffset, err := d.rd.Seek(0, io.SeekCurrent)
	if err != nil {
		return err
	}
	coverFrameLen := binary.LittleEndian.Uint32(bCoverFrameLen)

	bCoverLen := make([]byte, 4)
	if _, err := io.ReadFull(d.rd, bCoverLen); err != nil {
		return fmt.Errorf("ncm read cover length: %w", err)
	}
	iCoverLen := binary.LittleEndian.Uint32(bCoverLen)

	if _, err := io.ReadFull(d.rd, make([]byte, iCoverLen)); err != nil {
		return fmt.Errorf("ncm skip cover data: %w", err)
	}

	offsetAudioData := coverFrameStartOffset + int64(coverFrameLen) + 4
	_, err = d.rd.Seek(offsetAudioData, io.SeekStart)
	return err
}

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() {
	decrypt.RegisterDecoder("ncm", false , newDecoder)
}