67 lines
1.3 KiB
Go
67 lines
1.3 KiB
Go
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package kgm
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import (
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"crypto/md5"
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"fmt"
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"io"
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)
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// kgmCryptoV3 is kgm file crypto v3
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type kgmCryptoV3 struct {
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slotBox []byte
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fileBox []byte
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rd io.Reader
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offset int
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}
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var kgmV3Slot2Key = map[uint32][]byte{
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1: {0x6C, 0x2C, 0x2F, 0x27},
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}
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func newKgmCryptoV3(header *Header, body io.Reader) (io.Reader, error) {
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c := &kgmCryptoV3{rd: body}
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slotKey, ok := kgmV3Slot2Key[header.CryptoSlot]
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if !ok {
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return nil, fmt.Errorf("kgm3: unknown crypto slot %d", header.CryptoSlot)
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}
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c.slotBox = kugouMD5(slotKey)
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c.fileBox = append(kugouMD5(header.CryptoKey), 0x6b)
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return c, nil
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}
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func (d *kgmCryptoV3) Read(buf []byte) (int, error) {
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n, err := d.rd.Read(buf)
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if n > 0 {
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d.decrypt(buf[:n], d.offset)
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d.offset += n
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}
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return n, err
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}
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func (d *kgmCryptoV3) decrypt(b []byte, offset int) {
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for i := 0; i < len(b); i++ {
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b[i] ^= d.fileBox[(offset+i)%len(d.fileBox)]
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b[i] ^= b[i] << 4
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b[i] ^= d.slotBox[(offset+i)%len(d.slotBox)]
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b[i] ^= xorCollapseUint32(uint32(offset + i))
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}
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}
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func xorCollapseUint32(i uint32) byte {
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return byte(i) ^ byte(i>>8) ^ byte(i>>16) ^ byte(i>>24)
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}
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func kugouMD5(b []byte) []byte {
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digest := md5.Sum(b)
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ret := make([]byte, 16)
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for i := 0; i < md5.Size; i += 2 {
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ret[i] = digest[14-i]
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ret[i+1] = digest[14-i+1]
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}
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return ret
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}
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