3 * A "lagged fibonacci" pseudorandomness generator.
5 * Described in Knuth, TAOCP, 3.6
8 /* nettle, low-level cryptographics library
10 * Copyright (C) 2002 Niels Möller
12 * Includes code copied verbatim from Knuth's TAOCP.
14 * The nettle library is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU Lesser General Public License as published by
16 * the Free Software Foundation; either version 2.1 of the License, or (at your
17 * option) any later version.
19 * The nettle library is distributed in the hope that it will be useful, but
20 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
21 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
22 * License for more details.
24 * You should have received a copy of the GNU Lesser General Public License
25 * along with the nettle library; see the file COPYING.LIB. If not, write to
26 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
30 /* NOTE: This generator is totally inappropriate for cryptographic
31 * applications. It is useful for generating deterministic but
32 * random-looking test data, and is used by the Nettle testsuite. */
41 #include "knuth-lfib.h"
45 #define KK _KNUTH_LFIB_KK
47 #define MM (1UL << 30)
51 knuth_lfib_init(struct knuth_lfib_ctx *ctx, uint32_t seed)
55 uint32_t ss = (seed + 2) & (MM-2);
57 for (j = 0; j<KK; j++)
60 ss <<= 1; if (ss >= MM) ss -= (MM-2);
70 for (j = KK-1; j>0; j--)
72 for (j = 2*KK-2; j > KK-LL; j-= 2)
73 x[2*KK-1-j] = x[j] & ~1;
74 for (j = 2*KK-2; j>=KK; j--)
77 x[j-(KK-LL)] = (x[j - (KK-LL)] - x[j]) & (MM-1);
78 x[j-KK] = (x[j-KK] - x[j]) & (MM-1);
86 x[LL] = (x[LL] - x[KK]) & (MM-1);
94 ctx->x[j+KK-LL] = x[j];
101 /* Get's a single number in the range 0 ... 2^30-1 */
103 knuth_lfib_get(struct knuth_lfib_ctx *ctx)
106 assert(ctx->index < KK);
108 value = ctx->x[ctx->index];
109 ctx->x[ctx->index] -= ctx->x[(ctx->index + KK - LL) % KK];
110 ctx->x[ctx->index] &= (MM-1);
112 ctx->index = (ctx->index + 1) % KK;
117 /* NOTE: Not at all optimized. */
119 knuth_lfib_get_array(struct knuth_lfib_ctx *ctx,
120 unsigned n, uint32_t *a)
124 for (i = 0; i<n; i++)
125 a[i] = knuth_lfib_get(ctx);
128 /* NOTE: Not at all optimized. */
130 knuth_lfib_random(struct knuth_lfib_ctx *ctx,
131 unsigned n, uint8_t *dst)
133 /* Use 24 bits from each number, xoring together some of the
136 for (; n >= 3; n-=3, dst += 3)
138 uint32_t value = knuth_lfib_get(ctx);
140 /* Xor the most significant octet (containing 6 significant bits)
141 * into the lower octet. */
142 value ^= (value >> 24);
144 WRITE_UINT24(dst, value);
148 /* We need one or two octets more */
149 uint32_t value = knuth_lfib_get(ctx);
153 *dst++ = value & 0xff;
156 WRITE_UINT16(dst, value);