1 /*
2 * License : The MIT License
3 * Copyright(c) 2022 Olyutorskii
4 */
5
6 package io.github.olyutorskii.aletojio.bijection;
7
8 /**
9 * Diffusion of parity information.
10 *
11 * <p>Take the XOR of all even-numbered input bytes to obtain the odd-parity byte.
12 * The output is the XOR of the parity byte and the original each byte value.
13 *
14 * <p>This conversion is bijection.
15 * Perform this operation an even number of times to return to the original value.
16 *
17 * <p>Used as a pre-processor for 8bit S-box conversion to improve confusion.
18 *
19 * <p>byte-length must be even.
20 *
21 * <p>If 1 bit of N bytes input array changes,
22 * N-1 bits of output array changes.
23 * Hamming distance is N-1 bits.
24 *
25 * <p>ex) Input I has 4 bytes (I0-I3)
26 * Output O has 4 bytes (O0-O3)
27 *
28 * <p>{@code X = I0 ^ I1 ^ I2 ^ I3}
29 *
30 * <p>{@code O0 = I0 ^ X, O1 = I1 ^ X, O2 = I3 ^ X, O3 = I3 ^ X}
31 */
32 public final class ParitySpreader {
33
34 private static final String ERRMSG_ODDLEN = "length must be even";
35 private static final String ERRMSG_TOOSMALL = "too small length";
36 private static final String ERRMSG_TOOLARGE = "too large length";
37
38 private static final int MASK_BYTE = (1 << Byte.SIZE) - 1;
39
40
41 /**
42 * Hidden constructor.
43 */
44 private ParitySpreader() {
45 assert false;
46 }
47
48
49 /**
50 * Spread parity information.
51 *
52 * @param iVal int value
53 * @return result
54 */
55 public static int spread(int iVal) {
56 int i0 = iVal & MASK_BYTE;
57 int i1 = (iVal >>> (Byte.SIZE * 1)) & MASK_BYTE;
58 int i2 = (iVal >>> (Byte.SIZE * 2)) & MASK_BYTE;
59 int i3 = (iVal >>> (Byte.SIZE * 3)) & MASK_BYTE;
60
61 int xorVal = (i0 ^ i1) ^ (i2 ^ i3);
62
63 int o0 = i0 ^ xorVal;
64 int o1 = (i1 ^ xorVal) << (Byte.SIZE * 1);
65 int o2 = (i2 ^ xorVal) << (Byte.SIZE * 2);
66 int o3 = (i3 ^ xorVal) << (Byte.SIZE * 3);
67
68 int result = (o0 ^ o1) ^ (o2 ^ o3);
69
70 return result;
71 }
72
73 /**
74 * Spread parity information.
75 *
76 * @param bytes input and output bytes array
77 * @param length length of input must be even and smaller than input array.
78 * @throws IndexOutOfBoundsException length is negative or too large
79 * @throws IllegalArgumentException length is odd number
80 */
81 public static void spread(byte[] bytes, int length)
82 throws IndexOutOfBoundsException, IllegalArgumentException {
83 if (length < 0) throw new IndexOutOfBoundsException(ERRMSG_TOOSMALL);
84 if ((length & 1) == 1) throw new IllegalArgumentException(ERRMSG_ODDLEN);
85 if (bytes.length < length) throw new IndexOutOfBoundsException(ERRMSG_TOOLARGE);
86
87 byte xorVal = 0;
88 for (int idx = 0; idx < length; idx++) {
89 xorVal ^= bytes[idx];
90 }
91
92 for (int idx = 0; idx < length; idx++) {
93 byte bVal = bytes[idx];
94 bVal ^= xorVal;
95 bytes[idx] = bVal;
96 }
97
98 return;
99 }
100
101 }