3 * $Id: rand.c,v 1.1 1997/08/07 09:46:05 mdw Exp $
5 * Random number generation
10 /*----- Licencing notice --------------------------------------------------*
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22 * GNU General Public License for more details.
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26 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
29 /*----- Revision history --------------------------------------------------*
32 * Revision 1.1 1997/08/07 09:46:05 mdw
33 * New source file added to maintain a randomness pool.
37 /*----- Header files ------------------------------------------------------*/
39 /* --- ANSI headers --- */
48 /* --- Local headers --- */
58 /*----- Magic numbers -----------------------------------------------------*/
60 #define rand__seedBits 512 /* Number of random seed bits */
62 /*----- Persistant state --------------------------------------------------*/
64 static unsigned char rand__pool[rand__seedBits / 8]; /* Entropy pool */
65 static size_t rand__i = 0; /* Index into entropy pool */
66 static size_t rand__r = 0; /* Rotation to apply to next byte */
68 /*----- Main code ---------------------------------------------------------*/
70 /* --- @rand_read@ --- *
72 * Arguments: @FILE *fp@ = pointer to file to read from
76 * Use: Reads a random number seed from the stream.
79 void rand_read(FILE *fp)
81 tx_getBits(rand__pool, rand__seedBits, fp);
84 T( traceblk(TRACE_RAND, "rand: loaded randomness pool",
85 rand__pool, sizeof(rand__pool)); )
88 /* --- @rand_write@ --- *
90 * Arguments: @FILE *fp@ = pointer to file to write on
94 * Use: Writes a random number seed back to the stream.
97 void rand_write(FILE *fp)
100 tx_putBits(rand__pool, rand__seedBits, fp);
103 /* --- @rand_clear@ --- *
109 * Use: Clears the random number pool.
112 void rand_clear(void)
114 memset(rand__pool, 0, sizeof(rand__pool));
115 T( trace(TRACE_RAND, "rand: cleared randomness pool"); )
120 /* --- @rand_encrypt@ --- *
122 * Arguments: @icrypt_job *j@ = pointer to an encryption job to apply
126 * Use: Encrypts the randomness pool with a given key. This should
127 * be done before use, in case the pool gets compromised.
130 void rand_encrypt(icrypt_job *j)
132 icrypt_encrypt(j, rand__pool, rand__pool, sizeof(rand__pool));
135 T( traceblk(TRACE_RAND, "encrypted randomness pool",
136 rand__pool, sizeof(rand__pool)); )
139 /* --- @rand_add@ --- *
141 * Arguments: @const void *p@ = pointer to some data
142 * @size_t sz@ = size of the data in bytes
146 * Use: Adds entropy to the pool.
149 void rand_add(const void *p, size_t sz)
153 * Entropy is inserted byte-by-byte. The method used is derived from
154 * Linux's `drivers/char/random.c', which is in turn appears to be inspired
157 * Imagine the randomness pool as 8 parallel shift registers. To insert
158 * a byte into the pool, XOR it with bytes picked according to a primitive
159 * polynomial mod 2, and rotate one place to the left. (The rotation is
160 * from SHA-1; it introduces some interaction between the registers.)
164 const unsigned char *cp = p;
165 size_t mask = (rand__seedBits / 8) - 1;
169 /* --- Ensure the polynomial is valid --- *
171 * The current polynomal is %$x^{64} + x^4 + x^3 + x + 1$% (picked from
172 * Schneier's excellent book). If the pool size changes, though, I'll
173 * need another polynomial.
176 #if rand__seedBits / 8 != 64
177 # error Change the polynomal in rand_add
180 T( traceblk(TRACE_RAND, "rand: incoming entropy", p, sz); )
182 /* --- Add values to the entropy pool --- */
187 x = ((x << r) | (x >> (8 - r)));
188 x ^= (rand__pool[i] ^
189 rand__pool[(i + 1) & mask] ^
190 rand__pool[(i + 3) & mask] ^
191 rand__pool[(i + 4) & mask]);
192 rand__pool[i] = ((x << 1) | (x >> 7)) & 0xffu;
198 /* --- Update the global counters --- */
203 T( traceblk(TRACE_RAND, "rand: added some entropy",
204 rand__pool, sizeof(rand__pool)); )
207 /* --- @rand_extract@ --- *
209 * Arguments: @unsigned char *b@ = pointer to output buffer
210 * @size_t sz@ = number of bytes wanted
214 * Use: Produces a number of random bytes.
217 void rand_extract(unsigned char *b, size_t sz)
221 * Hash the buffer with a secure hash (or, in this case, with MD5 and hope
222 * for the best...). If this gives us enough bytes, fill the output
223 * buffer; otherwise copy the whole hash out. The contribute the hash back
224 * into the randomness pool with @rand_add@ to provide some feedback.
226 * The secure hash gives us good mixing over the whole of the randomness
227 * pool, and attempts to ensure that an attacker receiving our random
228 * numbers can't predict any numbers we don't actually give him.
232 unsigned char mdbuf[MD5_HASHSIZE];
235 T( trace(TRACE_RAND, "rand: extracting entropy"); )
238 c = (sz >= sizeof(mdbuf)) ? sizeof(mdbuf) : sz;
240 md5_buffer(&md, rand__pool, sizeof(rand__pool));
241 md5_final(&md, mdbuf);
246 burn(mdbuf); burn(md);
248 T( trace(TRACE_RAND, "rand: finished extracting entropy"); )
251 /* --- @rand_churn@ --- *
257 * Use: Churns the randomness pool completely. The pool is replaced
258 * by repeated MD5-ings of itself.
261 void rand_churn(void)
264 unsigned char mdbuf[MD5_HASHSIZE];
265 size_t sz = sizeof(rand__pool);
268 T( trace(TRACE_RAND, "rand: churning pool"); )
274 c = (sz >= sizeof(mdbuf)) ? sizeof(mdbuf) : sz;
276 md5_buffer(&md, rand__pool, sizeof(rand__pool));
277 md5_final(&md, mdbuf);
285 burn(mdbuf); burn(md);
287 T( trace(TRACE_RAND, "rand: finished churning pool"); )
290 /*----- That's all, folks -------------------------------------------------*/