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1 | /* |
2 | * misc.c: Miscellaneous helpful functions. |
3 | */ |
4 | |
5 | #include <assert.h> |
6 | #include <stdlib.h> |
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7 | #include <string.h> |
8 | #include <stdio.h> |
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9 | |
10 | #include "puzzles.h" |
11 | |
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12 | void free_cfg(config_item *cfg) |
13 | { |
14 | config_item *i; |
15 | |
16 | for (i = cfg; i->type != C_END; i++) |
17 | if (i->type == C_STRING) |
18 | sfree(i->sval); |
19 | sfree(cfg); |
20 | } |
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21 | |
22 | /* |
23 | * The Mines (among others) game descriptions contain the location of every |
24 | * mine, and can therefore be used to cheat. |
25 | * |
26 | * It would be pointless to attempt to _prevent_ this form of |
27 | * cheating by encrypting the description, since Mines is |
28 | * open-source so anyone can find out the encryption key. However, |
29 | * I think it is worth doing a bit of gentle obfuscation to prevent |
30 | * _accidental_ spoilers: if you happened to note that the game ID |
31 | * starts with an F, for example, you might be unable to put the |
32 | * knowledge of those mines out of your mind while playing. So, |
33 | * just as discussions of film endings are rot13ed to avoid |
34 | * spoiling it for people who don't want to be told, we apply a |
35 | * keyless, reversible, but visually completely obfuscatory masking |
36 | * function to the mine bitmap. |
37 | */ |
38 | void obfuscate_bitmap(unsigned char *bmp, int bits, int decode) |
39 | { |
40 | int bytes, firsthalf, secondhalf; |
41 | struct step { |
42 | unsigned char *seedstart; |
43 | int seedlen; |
44 | unsigned char *targetstart; |
45 | int targetlen; |
46 | } steps[2]; |
47 | int i, j; |
48 | |
49 | /* |
50 | * My obfuscation algorithm is similar in concept to the OAEP |
51 | * encoding used in some forms of RSA. Here's a specification |
52 | * of it: |
53 | * |
54 | * + We have a `masking function' which constructs a stream of |
55 | * pseudorandom bytes from a seed of some number of input |
56 | * bytes. |
57 | * |
58 | * + We pad out our input bit stream to a whole number of |
59 | * bytes by adding up to 7 zero bits on the end. (In fact |
60 | * the bitmap passed as input to this function will already |
61 | * have had this done in practice.) |
62 | * |
63 | * + We divide the _byte_ stream exactly in half, rounding the |
64 | * half-way position _down_. So an 81-bit input string, for |
65 | * example, rounds up to 88 bits or 11 bytes, and then |
66 | * dividing by two gives 5 bytes in the first half and 6 in |
67 | * the second half. |
68 | * |
69 | * + We generate a mask from the second half of the bytes, and |
70 | * XOR it over the first half. |
71 | * |
72 | * + We generate a mask from the (encoded) first half of the |
73 | * bytes, and XOR it over the second half. Any null bits at |
74 | * the end which were added as padding are cleared back to |
75 | * zero even if this operation would have made them nonzero. |
76 | * |
77 | * To de-obfuscate, the steps are precisely the same except |
78 | * that the final two are reversed. |
79 | * |
80 | * Finally, our masking function. Given an input seed string of |
81 | * bytes, the output mask consists of concatenating the SHA-1 |
82 | * hashes of the seed string and successive decimal integers, |
83 | * starting from 0. |
84 | */ |
85 | |
86 | bytes = (bits + 7) / 8; |
87 | firsthalf = bytes / 2; |
88 | secondhalf = bytes - firsthalf; |
89 | |
90 | steps[decode ? 1 : 0].seedstart = bmp + firsthalf; |
91 | steps[decode ? 1 : 0].seedlen = secondhalf; |
92 | steps[decode ? 1 : 0].targetstart = bmp; |
93 | steps[decode ? 1 : 0].targetlen = firsthalf; |
94 | |
95 | steps[decode ? 0 : 1].seedstart = bmp; |
96 | steps[decode ? 0 : 1].seedlen = firsthalf; |
97 | steps[decode ? 0 : 1].targetstart = bmp + firsthalf; |
98 | steps[decode ? 0 : 1].targetlen = secondhalf; |
99 | |
100 | for (i = 0; i < 2; i++) { |
101 | SHA_State base, final; |
102 | unsigned char digest[20]; |
103 | char numberbuf[80]; |
104 | int digestpos = 20, counter = 0; |
105 | |
106 | SHA_Init(&base); |
107 | SHA_Bytes(&base, steps[i].seedstart, steps[i].seedlen); |
108 | |
109 | for (j = 0; j < steps[i].targetlen; j++) { |
110 | if (digestpos >= 20) { |
111 | sprintf(numberbuf, "%d", counter++); |
112 | final = base; |
113 | SHA_Bytes(&final, numberbuf, strlen(numberbuf)); |
114 | SHA_Final(&final, digest); |
115 | digestpos = 0; |
116 | } |
117 | steps[i].targetstart[j] ^= digest[digestpos++]; |
118 | } |
119 | |
120 | /* |
121 | * Mask off the pad bits in the final byte after both steps. |
122 | */ |
123 | if (bits % 8) |
124 | bmp[bits / 8] &= 0xFF & (0xFF00 >> (bits % 8)); |
125 | } |
126 | } |
127 | |
128 | /* err, yeah, these two pretty much rely on unsigned char being 8 bits. |
129 | * Platforms where this is not the case probably have bigger problems |
130 | * than just making these two work, though... */ |
131 | char *bin2hex(const unsigned char *in, int inlen) |
132 | { |
133 | char *ret = snewn(inlen*2 + 1, char), *p = ret; |
134 | int i; |
135 | |
136 | for (i = 0; i < inlen*2; i++) { |
137 | int v = in[i/2]; |
138 | if (i % 2 == 0) v >>= 4; |
139 | *p++ = "0123456789abcdef"[v & 0xF]; |
140 | } |
141 | *p = '\0'; |
142 | return ret; |
143 | } |
144 | |
145 | unsigned char *hex2bin(const char *in, int outlen) |
146 | { |
147 | unsigned char *ret = snewn(outlen, unsigned char); |
148 | int i; |
149 | |
150 | debug(("hex2bin: in '%s'", in)); |
151 | |
152 | memset(ret, 0, outlen*sizeof(unsigned char)); |
153 | for (i = 0; i < outlen*2; i++) { |
154 | int c = in[i]; |
155 | int v; |
156 | |
157 | assert(c != 0); |
158 | if (c >= '0' && c <= '9') |
159 | v = c - '0'; |
160 | else if (c >= 'a' && c <= 'f') |
161 | v = c - 'a' + 10; |
162 | else if (c >= 'A' && c <= 'F') |
163 | v = c - 'A' + 10; |
164 | else |
165 | v = 0; |
166 | |
167 | ret[i / 2] |= v << (4 * (1 - (i % 2))); |
168 | } |
169 | return ret; |
170 | } |
171 | |
172 | /* vim: set shiftwidth=4 tabstop=8: */ |