125 lines
2.6 KiB
Go
125 lines
2.6 KiB
Go
package qmc
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import (
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"errors"
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)
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// A rc4Cipher is an instance of RC4 using a particular key.
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type rc4Cipher struct {
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box []byte
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key []byte
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hash uint32
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n int
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}
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// NewRC4Cipher creates and returns a new rc4Cipher. The key argument should be the
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// RC4 key, at least 1 byte and at most 256 bytes.
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func NewRC4Cipher(key []byte) (*rc4Cipher, error) {
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n := len(key)
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if n == 0 {
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return nil, errors.New("qmc/cipher_rc4: invalid key size")
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}
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var c = rc4Cipher{key: key, n: n}
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c.box = make([]byte, n)
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for i := 0; i < n; i++ {
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c.box[i] = byte(i)
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}
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var j = 0
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for i := 0; i < n; i++ {
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j = (j + int(c.box[i]) + int(key[i%n])) % n
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c.box[i], c.box[j] = c.box[j], c.box[i]
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}
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c.getHashBase()
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return &c, nil
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}
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func (c *rc4Cipher) getHashBase() {
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c.hash = 1
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for i := 0; i < c.n; i++ {
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v := uint32(c.key[i])
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if v == 0 {
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continue
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}
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nextHash := c.hash * v
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if nextHash == 0 || nextHash <= c.hash {
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break
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}
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c.hash = nextHash
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}
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}
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const (
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rc4SegmentSize = 5120
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rc4FirstSegmentSize = 128
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)
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func (c *rc4Cipher) Decrypt(src []byte, offset int) {
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toProcess := len(src)
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processed := 0
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markProcess := func(p int) (finished bool) {
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offset += p
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toProcess -= p
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processed += p
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return toProcess == 0
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}
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if offset < rc4FirstSegmentSize {
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blockSize := toProcess
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if blockSize > rc4FirstSegmentSize-offset {
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blockSize = rc4FirstSegmentSize - offset
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}
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c.encFirstSegment(src[:blockSize], offset)
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if markProcess(blockSize) {
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return
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}
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}
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if offset%rc4SegmentSize != 0 {
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blockSize := toProcess
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if blockSize > rc4SegmentSize-offset%rc4SegmentSize {
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blockSize = rc4SegmentSize - offset%rc4SegmentSize
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}
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c.encASegment(src[processed:processed+blockSize], offset)
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if markProcess(blockSize) {
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return
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}
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}
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for toProcess > rc4SegmentSize {
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c.encASegment(src[processed:processed+rc4SegmentSize], offset)
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markProcess(rc4SegmentSize)
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}
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if toProcess > 0 {
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c.encASegment(src[processed:], offset)
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}
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}
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func (c *rc4Cipher) encFirstSegment(buf []byte, offset int) {
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for i := 0; i < len(buf); i++ {
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buf[i] ^= c.key[c.getSegmentSkip(offset+i)]
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}
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}
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func (c *rc4Cipher) encASegment(buf []byte, offset int) {
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box := make([]byte, c.n)
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copy(box, c.box)
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j, k := 0, 0
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skipLen := (offset % rc4SegmentSize) + c.getSegmentSkip(offset/rc4SegmentSize)
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for i := -skipLen; i < len(buf); i++ {
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j = (j + 1) % c.n
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k = (int(box[j]) + k) % c.n
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box[j], box[k] = box[k], box[j]
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if i >= 0 {
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buf[i] ^= box[(int(box[j])+int(box[k]))%c.n]
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}
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}
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}
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func (c *rc4Cipher) getSegmentSkip(id int) int {
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seed := int(c.key[id%c.n])
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idx := int64(float64(c.hash) / float64((id+1)*seed) * 100.0)
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return int(idx % int64(c.n))
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}
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