3 * $Id: rho.c,v 1.1 2000/07/09 21:32:30 mdw Exp $
5 * Pollard's rho algorithm for discrete logs
7 * (c) 2000 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Catacomb.
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21 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 * GNU Library General Public License for more details.
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30 /*----- Revision history --------------------------------------------------*
33 * Revision 1.1 2000/07/09 21:32:30 mdw
34 * Pollard's rho algorithm for computing discrete logs.
38 /*----- Header files ------------------------------------------------------*/
46 /*----- Main code ---------------------------------------------------------*/
50 * Arguments: @rho_ctx *cc@ = pointer to the context structure
51 * @void *x, *y@ = two (equal) base values (try 1)
52 * @mp *a, *b@ = logs of %$x$% (see below)
54 * Returns: The discrete logarithm %$\log_g a$%, or null if the algorithm
55 * failed. (This is unlikely, though possible.)
57 * Use: Uses Pollard's rho algorithm to compute discrete logs in the
58 * group %$G$% generated by %$g$%.
60 * The algorithm works by finding a cycle in a pseudo-random
61 * walk. The function @ops->split@ should return an element
62 * from %$\{\,0, 1, 2\,\}$% according to its argument, in order
63 * to determine the walk. At each step in the walk, we know a
64 * group element %$x \in G$% together with its representation as
65 * a product of powers of %$g$% and $%a$% (i.e., we know that
66 * %$x = g^\alpha a^\beta$% for some %$\alpha$%, %$\beta$%).
68 * Locating a cycle gives us a collision
70 * %$g^{\alpha} a^{\beta} = g^{\alpha'} a^{\beta'}$%
72 * Taking logs of both sides (to base %$g$%) gives us that
74 * %$\log a\equiv\frac{\alpha-\alpha'}{\beta'-\beta}\bmod{n}$%
76 * Good initial values are %$x = y = 1$% (the multiplicative
77 * identity of %$G$%) and %$\alpha\equiv\beta\equiv0\bmod{n}$%.
78 * If that doesn't work then start choosing more `interesting'
81 * Note that the algorithm requires minimal space but
82 * %$O(\sqrt{n})$% time. Don't do this on large groups,
83 * particularly if you can find a decent factor base.
85 * Finally, note that this function will free the input values
86 * when it's finished with them. This probably isn't a great
90 static void step(rho_ctx *cc, void *x, mp **a, mp **b)
92 switch (cc->ops->split(x)) {
94 cc->ops->mul(x, cc->g, cc->c);
95 *a = mp_add(*a, *a, MP_ONE);
96 if (MP_CMP(*a, >=, cc->n))
97 *a = mp_sub(*a, *a, cc->n);
100 cc->ops->sqr(x, cc->c);
101 *a = mp_lsl(*a, *a, 1);
102 if (MP_CMP(*a, >=, cc->n))
103 *a = mp_sub(*a, *a, cc->n);
104 *b = mp_lsl(*b, *b, 1);
105 if (MP_CMP(*b, >=, cc->n))
106 *b = mp_sub(*b, *b, cc->n);
109 cc->ops->mul(x, cc->a, cc->c);
110 *b = mp_add(*b, *b, MP_ONE);
111 if (MP_CMP(*b, >=, cc->n))
112 *b = mp_sub(*b, *b, cc->n);
117 mp *rho(rho_ctx *cc, void *x, void *y, mp *a, mp *b)
119 mp *aa = MP_COPY(a), *bb = MP_COPY(b);
122 /* --- Grind through the random walk until we find a collision --- */
126 step(cc, y, &aa, &bb);
127 step(cc, y, &aa, &bb);
128 } while (!cc->ops->eq(x, y));
132 /* --- Now sort out the mess --- */
134 aa = mp_sub(aa, a, aa);
135 bb = mp_sub(bb, bb, b);
137 mp_gcd(&g, &bb, 0, bb, cc->n);
138 if (MP_CMP(g, !=, MP_ONE)) {
142 aa = mp_mul(aa, aa, bb);
143 mp_div(0, &aa, aa, cc->n);
155 /* --- @rho_prime@ --- *
157 * Arguments: @mp *g@ = generator for the group
158 * @mp *a@ = value to find the logarithm of
159 * @mp *n@ = order of the group
160 * @mp *p@ = prime size of the underlying prime field
162 * Returns: The discrete logarithm %$\log_g a$%.
164 * Use: Computes discrete logarithms in a subgroup of a prime field.
167 static void prime_sqr(void *x, void *c)
172 a = mpmont_reduce(c, a, a);
176 static void prime_mul(void *x, void *y, void *c)
180 a = mpmont_mul(c, a, a, y);
184 static int prime_eq(void *x, void *y)
186 return (MP_CMP(*(mp **)x, ==, *(mp **)y));
189 static int prime_split(void *x)
191 /* --- Notes on the splitting function --- *
193 * The objective is to produce a simple pseudorandom mapping from the
194 * underlying field \gf{p} to \{\,0, 1, 2\,\}$%. This is further
195 * constrained by the fact that we must not have %$1 \mapsto 1$% (since
196 * otherwise the stepping function above will loop).
198 * The function we choose is very simple: we take the least significant
199 * word from the integer, add one (to prevent the %$1 \mapsto 1$% property
200 * described above) and reduce modulo 3. This is slightly biased against
201 * the result 2, but this doesn't appear to be relevant.
204 return (((*(mp **)x)->v[0] + 1) % 3);
207 static void prime_drop(void *x)
212 static rho_ops prime_ops = {
213 prime_sqr, prime_mul, prime_eq, prime_split, prime_drop
216 mp *rho_prime(mp *g, mp *a, mp *n, mp *p)
225 /* --- Initialization --- */
227 mpmont_create(&mm, p);
231 cc.g = mpmont_mul(&mm, MP_NEW, g, mm.r2);
232 cc.a = mpmont_mul(&mm, MP_NEW, a, mm.r2);
237 /* --- The main loop --- */
239 while ((l = rho(&cc, &x, &y, aa, bb)) == 0) {
243 r = fibrand_create(0);
244 aa = mprand_range(MP_NEW, n, r, 0);
245 bb = mprand_range(MP_NEW, n, r, 0);
246 f[0].base = g; f[0].exp = aa;
247 f[1].base = a; f[1].exp = bb;
248 x = mpmont_mexpr(&mm, MP_NEW, f, 2);
252 /* --- Throw everything away now --- */
262 /*----- Test rig ----------------------------------------------------------*/
274 grand *r = fibrand_create(0);
279 fputs("rho: ", stdout);
282 dh_gen(&dp, 32, 256, 0, r, pgen_evspin, 0);
283 x = mprand_range(MP_NEW, dp.q, r, 0);
284 mpmont_create(&mm, dp.p);
285 y = mpmont_exp(&mm, MP_NEW, dp.g, x);
287 l = rho_prime(dp.g, y, dp.q, dp.p);
288 if (MP_CMP(x, ==, l)) {
289 fputs(". ok\n", stdout);
292 fputs("\n*** rho (discrete logs) failed\n", stdout);
301 assert(mparena_count(MPARENA_GLOBAL) == 0);
303 return (ok ? 0 : EXIT_FAILURE);
308 /*----- That's all, folks -------------------------------------------------*/