refactor: use io.Reader instead of custom method

algo/ncm/ncm_cipher.go

@@ -0,0 +1,42 @@
+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
+}