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/* knuth-lfib.c
*
* A "lagged fibonacci" pseudorandomness generator.
*
* Described in Knuth, TAOCP, 3.6
*/
/* nettle, low-level cryptographics library
*
* Copyright (C) 2002 Niels Mller
*
* Includes code copied verbatim from Knuth's TAOCP.
*
* The nettle library is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or (at your
* option) any later version.
*
* The nettle library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
* License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with the nettle library; see the file COPYING.LIB. If not, write to
* the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
* MA 02111-1307, USA.
*/
/* NOTE: This generator is totally inappropriate for cryptographic
* applications. It is useful for generating deterministic but
* random-looking test data, and is used by the Nettle testsuite. */
#include "knuth-lfib.h"
#include "macros.h"
#include <assert.h>
#include <stdlib.h>
#define KK _KNUTH_LFIB_KK
#define LL 37
#define MM (1UL << 30)
#define TT 70
void
knuth_lfib_init(struct knuth_lfib_ctx *ctx, uint32_t seed)
{
uint32_t t,j;
uint32_t x[2*KK - 1];
uint32_t ss = (seed + 2) & (MM-2);
for (j = 0; j<KK; j++)
{
x[j] = ss;
ss <<= 1; if (ss >= MM) ss -= (MM-2);
}
for (;j< 2*KK-1; j++)
x[j] = 0;
x[1]++;
ss = seed & (MM-1);
for (t = TT-1; t; )
{
for (j = KK-1; j>0; j--)
x[j+j] = x[j];
for (j = 2*KK-2; j > KK-LL; j-= 2)
x[2*KK-1-j] = x[j] & ~1;
for (j = 2*KK-2; j>=KK; j--)
if (x[j] & 1)
{
x[j-(KK-LL)] = (x[j - (KK-LL)] - x[j]) & (MM-1);
x[j-KK] = (x[j-KK] - x[j]) & (MM-1);
}
if (ss & 1)
{
for (j=KK; j>0; j--)
x[j] = x[j-1];
x[0] = x[KK];
if (x[KK] & 1)
x[LL] = (x[LL] - x[KK]) & (MM-1);
}
if (ss)
ss >>= 1;
else
t--;
}
for (j=0; j<LL; j++)
ctx->x[j+KK-LL] = x[j];
for (; j<KK; j++)
ctx->x[j-LL] = x[j];
ctx->index = 0;
}
/* Get's a single number in the range 0 ... 2^30-1 */
uint32_t
knuth_lfib_get(struct knuth_lfib_ctx *ctx)
{
uint32_t value;
assert(ctx->index < KK);
value = ctx->x[ctx->index];
ctx->x[ctx->index] -= ctx->x[(ctx->index + KK - LL) % KK];
ctx->x[ctx->index] &= (MM-1);
ctx->index = (ctx->index + 1) % KK;
return value;
}
/* NOTE: Not at all optimized. */
void
knuth_lfib_get_array(struct knuth_lfib_ctx *ctx,
unsigned n, uint32_t *a)
{
unsigned i;
for (i = 0; i<n; i++)
a[i] = knuth_lfib_get(ctx);
}
/* NOTE: Not at all optimized. */
void
knuth_lfib_random(struct knuth_lfib_ctx *ctx,
unsigned n, uint8_t *dst)
{
/* Use 24 bits from each number, xoring together some of the
bits. */
for (; n >= 3; n-=3, dst += 3)
{
uint32_t value = knuth_lfib_get(ctx);
/* Xor the most significant octet (containing 6 significant bits)
* into the lower octet. */
value ^= (value >> 24);
WRITE_UINT24(dst, value);
}
if (n)
{
/* We need one or two octets more */
uint32_t value = knuth_lfib_get(ctx);
switch (n)
{
case 1:
*dst++ = value & 0xff;
break;
case 2:
WRITE_UINT16(dst, value);
break;
default:
abort();
}
}
}