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unlock-music-cli/algo/qmc/cipher_rc4.go

125 lines
2.6 KiB
Go

package qmc
import (
"errors"
)
// A rc4Cipher is an instance of RC4 using a particular key.
type rc4Cipher struct {
box []byte
key []byte
hash uint32
n int
}
// newRC4Cipher creates and returns a new rc4Cipher. The key argument should be the
// RC4 key, at least 1 byte and at most 256 bytes.
func newRC4Cipher(key []byte) (*rc4Cipher, error) {
n := len(key)
if n == 0 {
return nil, errors.New("qmc/cipher_rc4: invalid key size")
}
var c = rc4Cipher{key: key, n: n}
c.box = make([]byte, n)
for i := 0; i < n; i++ {
c.box[i] = byte(i)
}
var j = 0
for i := 0; i < n; i++ {
j = (j + int(c.box[i]) + int(key[i%n])) % n
c.box[i], c.box[j] = c.box[j], c.box[i]
}
c.getHashBase()
return &c, nil
}
func (c *rc4Cipher) getHashBase() {
c.hash = 1
for i := 0; i < c.n; i++ {
v := uint32(c.key[i])
if v == 0 {
continue
}
nextHash := c.hash * v
if nextHash == 0 || nextHash <= c.hash {
break
}
c.hash = nextHash
}
}
const (
rc4SegmentSize = 5120
rc4FirstSegmentSize = 128
)
func (c *rc4Cipher) Decrypt(src []byte, offset int) {
toProcess := len(src)
processed := 0
markProcess := func(p int) (finished bool) {
offset += p
toProcess -= p
processed += p
return toProcess == 0
}
if offset < rc4FirstSegmentSize {
blockSize := toProcess
if blockSize > rc4FirstSegmentSize-offset {
blockSize = rc4FirstSegmentSize - offset
}
c.encFirstSegment(src[:blockSize], offset)
if markProcess(blockSize) {
return
}
}
if offset%rc4SegmentSize != 0 {
blockSize := toProcess
if blockSize > rc4SegmentSize-offset%rc4SegmentSize {
blockSize = rc4SegmentSize - offset%rc4SegmentSize
}
c.encASegment(src[processed:processed+blockSize], offset)
if markProcess(blockSize) {
return
}
}
for toProcess > rc4SegmentSize {
c.encASegment(src[processed:processed+rc4SegmentSize], offset)
markProcess(rc4SegmentSize)
}
if toProcess > 0 {
c.encASegment(src[processed:], offset)
}
}
func (c *rc4Cipher) encFirstSegment(buf []byte, offset int) {
for i := 0; i < len(buf); i++ {
buf[i] ^= c.key[c.getSegmentSkip(offset+i)]
}
}
func (c *rc4Cipher) encASegment(buf []byte, offset int) {
box := make([]byte, c.n)
copy(box, c.box)
j, k := 0, 0
skipLen := (offset % rc4SegmentSize) + c.getSegmentSkip(offset/rc4SegmentSize)
for i := -skipLen; i < len(buf); i++ {
j = (j + 1) % c.n
k = (int(box[j]) + k) % c.n
box[j], box[k] = box[k], box[j]
if i >= 0 {
buf[i] ^= box[(int(box[j])+int(box[k]))%c.n]
}
}
}
func (c *rc4Cipher) getSegmentSkip(id int) int {
seed := int(c.key[id%c.n])
idx := int64(float64(c.hash) / float64((id+1)*seed) * 100.0)
return int(idx % int64(c.n))
}