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