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unlock-music-cli/algo/ncm/ncm_cipher.go

43 lines
686 B
Go

package ncm
type ncmCipher struct {
key []byte
box []byte
}
func newNcmCipher(key []byte) *ncmCipher {
return &ncmCipher{
key: key,
box: buildKeyBox(key),
}
}
func (c *ncmCipher) Decrypt(buf []byte, offset int) {
for i := 0; i < len(buf); i++ {
buf[i] ^= c.box[(i+offset)&0xff]
}
}
func buildKeyBox(key []byte) []byte {
box := make([]byte, 256)
for i := 0; i < 256; i++ {
box[i] = byte(i)
}
var j byte
for i := 0; i < 256; i++ {
j = box[i] + j + key[i%len(key)]
box[i], box[j] = box[j], box[i]
}
ret := make([]byte, 256)
var _i byte
for i := 0; i < 256; i++ {
_i = byte(i + 1)
si := box[_i]
sj := box[_i+si]
ret[i] = box[si+sj]
}
return ret
}