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)) }