refactor: use io.Reader instead of custom method

algo/kwm/kwm_cipher.go

@@ -0,0 +1,31 @@
+package kwm
+
+import (
+	"encoding/binary"
+	"strconv"
+)
+
+type kwmCipher struct {
+	mask []byte
+}
+
+func newKwmCipher(key []byte) *kwmCipher {
+	return &kwmCipher{mask: generateMask(key)}
+}
+
+func generateMask(key []byte) []byte {
+	keyInt := binary.LittleEndian.Uint64(key)
+	keyStr := strconv.FormatUint(keyInt, 10)
+	keyStrTrim := padOrTruncate(keyStr, 32)
+	mask := make([]byte, 32)
+	for i := 0; i < 32; i++ {
+		mask[i] = keyPreDefined[i] ^ keyStrTrim[i]
+	}
+	return mask
+}
+
+func (c kwmCipher) Decrypt(buf []byte, offset int) {
+	for i := range buf {
+		buf[i] ^= c.mask[(offset+i)&0x1F] // equivalent: [i % 32]
+	}
+}