Botan: src/block/idea/idea.cpp Source File

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idea.cpp
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1 /*
2 * IDEA
3 * (C) 1999-2010 Jack Lloyd
4 *
5 * Distributed under the terms of the Botan license
6 */
7
8 #include <botan/idea.h>
9 #include <botan/loadstor.h>
10
11 namespace Botan {
12
13 namespace {
14
15 /*
16 * Multiplication modulo 65537
17 */
18 inline u16bit mul(u16bit x, u16bit y)
19 {
20 const u32bit P = static_cast<u32bit>(x) * y;
21
22 // P ? 0xFFFF : 0
23 const u16bit P_mask = !P - 1;
24
25 const u32bit P_hi = P >> 16;
26 const u32bit P_lo = P & 0xFFFF;
27
28 const u16bit r_1 = (P_lo - P_hi) + (P_lo < P_hi);
29 const u16bit r_2 = 1 - x - y;
30
31 return (r_1 & P_mask) | (r_2 & ~P_mask);
32 }
33
34 /*
35 * Find multiplicative inverses modulo 65537
36 *
37 * 65537 is prime; thus Fermat's little theorem tells us that
38 * x^65537 == x modulo 65537, which means
39 * x^(65537-2) == x^-1 modulo 65537 since
40 * x^(65537-2) * x == 1 mod 65537
41 *
42 * Do the exponentiation with a basic square and multiply: all bits are
43 * of exponent are 1 so we always multiply
44 */
45 u16bit mul_inv(u16bit x)
46 {
47 u16bit y = x;
48
49 for(size_t i = 0; i != 15; ++i)
50 {
51 y = mul(y, y); // square
52 y = mul(y, x);
53 }
54
55 return y;
56 }
57
58 /**
59 * IDEA is involutional, depending only on the key schedule
60 */
61 void idea_op(const byte in[], byte out[], size_t blocks, const u16bit K[52])
62 {
63 const size_t BLOCK_SIZE = 8;
64
65 for(size_t i = 0; i != blocks; ++i)
66 {
67 u16bit X1 = load_be<u16bit>(in, 0);
68 u16bit X2 = load_be<u16bit>(in, 1);
69 u16bit X3 = load_be<u16bit>(in, 2);
70 u16bit X4 = load_be<u16bit>(in, 3);
71
72 for(size_t j = 0; j != 8; ++j)
73 {
74 X1 = mul(X1, K[6*j+0]);
75 X2 += K[6*j+1];
76 X3 += K[6*j+2];
77 X4 = mul(X4, K[6*j+3]);
78
79 u16bit T0 = X3;
80 X3 = mul(X3 ^ X1, K[6*j+4]);
81
82 u16bit T1 = X2;
83 X2 = mul((X2 ^ X4) + X3, K[6*j+5]);
84 X3 += X2;
85
86 X1 ^= X2;
87 X4 ^= X3;
88 X2 ^= T0;
89 X3 ^= T1;
90 }
91
92 X1 = mul(X1, K[48]);
93 X2 += K[50];
94 X3 += K[49];
95 X4 = mul(X4, K[51]);
96
97 store_be(out, X1, X3, X2, X4);
98
99 in += BLOCK_SIZE;
100 out += BLOCK_SIZE;
101 }
102 }
103
104 }
105
106 /*
107 * IDEA Encryption
108 */
109 void IDEA::encrypt_n(const byte in[], byte out[], size_t blocks) const
110 {
111 idea_op(in, out, blocks, &EK[0]);
112 }
113
114 /*
115 * IDEA Decryption
116 */
117 void IDEA::decrypt_n(const byte in[], byte out[], size_t blocks) const
118 {
119 idea_op(in, out, blocks, &DK[0]);
120 }
121
122 /*
123 * IDEA Key Schedule
124 */
125 void IDEA::key_schedule(const byte key[], size_t)
126 {
127 for(size_t i = 0; i != 8; ++i)
128 EK[i] = load_be<u16bit>(key, i);
129
130 for(size_t i = 1, j = 8, offset = 0; j != 52; i %= 8, ++i, ++j)
131 {
132 EK[i+7+offset] = static_cast<u16bit>((EK[(i % 8) + offset] << 9) |
133 (EK[((i+1) % 8) + offset] >> 7));
134 offset += (i == 8) ? 8 : 0;
135 }
136
137 DK[51] = mul_inv(EK[3]);
138 DK[50] = -EK[2];
139 DK[49] = -EK[1];
140 DK[48] = mul_inv(EK[0]);
141
142 for(size_t i = 1, j = 4, counter = 47; i != 8; ++i, j += 6)
143 {
144 DK[counter--] = EK[j+1];
145 DK[counter--] = EK[j];
146 DK[counter--] = mul_inv(EK[j+5]);
147 DK[counter--] = -EK[j+3];
148 DK[counter--] = -EK[j+4];
149 DK[counter--] = mul_inv(EK[j+2]);
150 }
151
152 DK[5] = EK[47];
153 DK[4] = EK[46];
154 DK[3] = mul_inv(EK[51]);
155 DK[2] = -EK[50];
156 DK[1] = -EK[49];
157 DK[0] = mul_inv(EK[48]);
158 }
159
160 }
Botan::byte
unsigned char byte
Definition: types.h:22
Botan::load_be< u16bit >
u16bit load_be< u16bit >(const byte in[], size_t off)
Definition: loadstor.h:132
Botan::u16bit
unsigned short u16bit
Definition: types.h:27
Botan::IDEA::decrypt_n
void decrypt_n(const byte in[], byte out[], size_t blocks) const
Definition: idea.cpp:117
Botan
Definition: algo_base.h:14
Botan::store_be
void store_be(u16bit in, byte out[2])
Definition: loadstor.h:412
Botan::IDEA::encrypt_n
void encrypt_n(const byte in[], byte out[], size_t blocks) const
Definition: idea.cpp:109
Botan::u32bit
unsigned int u32bit
Definition: types.h:32