File: yarrow256.c

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/* yarrow256.c

   The yarrow pseudo-randomness generator.

   Copyright (C) 2001, 2008, 2013 Niels Möller

   This file is part of GNU Nettle.

   GNU Nettle is free software: you can redistribute it and/or
   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free
       Software Foundation; either version 3 of the License, or (at your
       option) any later version.

   or

     * the GNU General Public License as published by the Free
       Software Foundation; either version 2 of the License, or (at your
       option) any later version.

   or both in parallel, as here.

   GNU Nettle 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
   General Public License for more details.

   You should have received copies of the GNU General Public License and
   the GNU Lesser General Public License along with this program.  If
   not, see http://www.gnu.org/licenses/.
*/

#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <assert.h>
#include <stdlib.h>
#include <string.h>

#include "yarrow.h"

#include "macros.h"

#ifndef YARROW_DEBUG
#define YARROW_DEBUG 0
#endif

#if YARROW_DEBUG
#include <stdio.h>
#endif

/* Parameters */

/* An upper limit on the entropy (in bits) in one octet of sample
 * data. */
#define YARROW_MULTIPLIER 4

/* Entropy threshold for reseeding from the fast pool */
#define YARROW_FAST_THRESHOLD 100

/* Entropy threshold for reseeding from the fast pool */
#define YARROW_SLOW_THRESHOLD 160

/* Number of sources that must exceed the threshold for slow reseed */
#define YARROW_SLOW_K 2

/* The number of iterations when reseeding, P_t in the yarrow paper.
 * Should be chosen so that reseeding takes on the order of 0.1-1
 * seconds. */
#define YARROW_RESEED_ITERATIONS 1500

/* Entropy estimates sticks to this value, it is treated as infinity
 * in calculations. It should fit comfortably in an uint32_t, to avoid
 * overflows. */
#define YARROW_MAX_ENTROPY 0x100000

/* Forward declarations */
static void
yarrow_gate(struct yarrow256_ctx *ctx);

void
yarrow256_init(struct yarrow256_ctx *ctx,
	       unsigned n,
	       struct yarrow_source *s)
{
  unsigned i;

  sha256_init(&ctx->pools[0]);
  sha256_init(&ctx->pools[1]);
  
  ctx->seeded = 0;

  /* Not strictly necessary, but it makes it easier to see if the
   * values are sane. */
  memset(ctx->counter, 0, sizeof(ctx->counter));
  
  ctx->nsources = n;
  ctx->sources = s;

  for (i = 0; i<n; i++)
    {
      ctx->sources[i].estimate[YARROW_FAST] = 0;
      ctx->sources[i].estimate[YARROW_SLOW] = 0;
      ctx->sources[i].next = YARROW_FAST;
    }
}

void
yarrow256_seed(struct yarrow256_ctx *ctx,
	       size_t length,
	       const uint8_t *seed_file)
{
  assert(length > 0);

  sha256_update(&ctx->pools[YARROW_FAST], length, seed_file);
  yarrow256_fast_reseed(ctx);
}

/* FIXME: Generalize so that it generates a few more blocks at a
 * time. */
static void
yarrow_generate_block(struct yarrow256_ctx *ctx,
		      uint8_t *block)
{
  unsigned i;

  aes256_encrypt(&ctx->key, sizeof(ctx->counter), block, ctx->counter);

  /* Increment counter, treating it as a big-endian number. This is
   * machine independent, and follows appendix B of the NIST
   * specification of cipher modes of operation.
   *
   * We could keep a representation of the counter as 4 32-bit values,
   * and write entire words (in big-endian byteorder) into the counter
   * block, whenever they change. */
  for (i = sizeof(ctx->counter); i--; )
    {
      if (++ctx->counter[i])
	break;
    }
}

static void
yarrow_iterate(uint8_t *digest)
{
  uint8_t v0[SHA256_DIGEST_SIZE];
  unsigned i;
  
  memcpy(v0, digest, SHA256_DIGEST_SIZE);
  
  /* When hashed inside the loop, i should run from 1 to
   * YARROW_RESEED_ITERATIONS */
  for (i = 0; ++i < YARROW_RESEED_ITERATIONS; )
    {
      uint8_t count[4];
      struct sha256_ctx hash;
  
      sha256_init(&hash);

      /* Hash v_i | v_0 | i */
      WRITE_UINT32(count, i);
      sha256_update(&hash, SHA256_DIGEST_SIZE, digest);
      sha256_update(&hash, sizeof(v0), v0);
      sha256_update(&hash, sizeof(count), count);

      sha256_digest(&hash, SHA256_DIGEST_SIZE, digest);
    }
}

/* NOTE: The SHA-256 digest size equals the AES key size, so we need
 * no "size adaptor". */

void
yarrow256_fast_reseed(struct yarrow256_ctx *ctx)
{
  uint8_t digest[SHA256_DIGEST_SIZE];
  unsigned i;
  
#if YARROW_DEBUG
  fprintf(stderr, "yarrow256_fast_reseed\n");
#endif
  
  /* We feed two block of output using the current key into the pool
   * before emptying it. */
  if (ctx->seeded)
    {
      uint8_t blocks[AES_BLOCK_SIZE * 2];
      
      yarrow_generate_block(ctx, blocks);
      yarrow_generate_block(ctx, blocks + AES_BLOCK_SIZE);
      sha256_update(&ctx->pools[YARROW_FAST], sizeof(blocks), blocks);
    }
  
  sha256_digest(&ctx->pools[YARROW_FAST], sizeof(digest), digest);

  /* Iterate */
  yarrow_iterate(digest);

  aes256_set_encrypt_key(&ctx->key, digest);
  ctx->seeded = 1;

  /* Derive new counter value */
  memset(ctx->counter, 0, sizeof(ctx->counter));
  aes256_encrypt(&ctx->key, sizeof(ctx->counter), ctx->counter, ctx->counter);
  
  /* Reset estimates. */
  for (i = 0; i<ctx->nsources; i++)
    ctx->sources[i].estimate[YARROW_FAST] = 0;
}

void
yarrow256_slow_reseed(struct yarrow256_ctx *ctx)
{
  uint8_t digest[SHA256_DIGEST_SIZE];
  unsigned i;

#if YARROW_DEBUG
  fprintf(stderr, "yarrow256_slow_reseed\n");
#endif

  /* Get digest of the slow pool*/
  sha256_digest(&ctx->pools[YARROW_SLOW], sizeof(digest), digest);

  /* Feed it into the fast pool */
  sha256_update(&ctx->pools[YARROW_FAST], sizeof(digest), digest);

  yarrow256_fast_reseed(ctx);
  
  /* Reset estimates. */
  for (i = 0; i<ctx->nsources; i++)
    ctx->sources[i].estimate[YARROW_SLOW] = 0;
}

int
yarrow256_update(struct yarrow256_ctx *ctx,
		 unsigned source_index, unsigned entropy,
		 size_t length, const uint8_t *data)
{
  enum yarrow_pool_id current;
  struct yarrow_source *source;
  
  assert(source_index < ctx->nsources);

  if (!length)
    /* Nothing happens */
    return 0;

  source = &ctx->sources[source_index];
  
  if (!ctx->seeded)
    /* While seeding, use the slow pool */
    current = YARROW_SLOW;
  else
    {
      current = source->next;
      source->next = !source->next;
    }

  sha256_update(&ctx->pools[current], length, data);
 
  /* NOTE: We should be careful to avoid overflows in the estimates. */
  if (source->estimate[current] < YARROW_MAX_ENTROPY)
    {
      if (entropy > YARROW_MAX_ENTROPY)
	entropy = YARROW_MAX_ENTROPY;

      if ( (length < (YARROW_MAX_ENTROPY / YARROW_MULTIPLIER))
	   && (entropy > YARROW_MULTIPLIER * length) )
	entropy = YARROW_MULTIPLIER * length;

      entropy += source->estimate[current];
      if (entropy > YARROW_MAX_ENTROPY)
	entropy = YARROW_MAX_ENTROPY;

      source->estimate[current] = entropy;
    }

  /* Check for seed/reseed */
  switch(current)
    {
    case YARROW_FAST:
#if YARROW_DEBUG
      fprintf(stderr,
              "yarrow256_update: source_index = %d,\n"
              "            fast pool estimate = %d\n",
              source_index, source->estimate[YARROW_FAST]);
#endif
      if (source->estimate[YARROW_FAST] >= YARROW_FAST_THRESHOLD)
	{
	  yarrow256_fast_reseed(ctx);
	  return 1;
	}
      else
	return 0;

    case YARROW_SLOW:
      {
        if (!yarrow256_needed_sources(ctx))
	  {
	    yarrow256_slow_reseed(ctx);
	    return 1;
	  }
	else
	  return 0;
      }
    default:
      abort();
    }
}

static void
yarrow_gate(struct yarrow256_ctx *ctx)
{
  uint8_t key[AES256_KEY_SIZE];
  unsigned i;

  for (i = 0; i < sizeof(key); i+= AES_BLOCK_SIZE)
    yarrow_generate_block(ctx, key + i);

  aes256_set_encrypt_key(&ctx->key, key);
}

void
yarrow256_random(struct yarrow256_ctx *ctx, size_t length, uint8_t *dst)
{
  assert(ctx->seeded);

  while (length >= AES_BLOCK_SIZE)
    {
      yarrow_generate_block(ctx, dst);
      dst += AES_BLOCK_SIZE;
      length -= AES_BLOCK_SIZE;
    }
  if (length)
    {
      uint8_t buffer[AES_BLOCK_SIZE];
      
      assert(length < AES_BLOCK_SIZE);
      yarrow_generate_block(ctx, buffer);
      memcpy(dst, buffer, length);
    }
  yarrow_gate(ctx);
}

int
yarrow256_is_seeded(struct yarrow256_ctx *ctx)
{
  return ctx->seeded;
}

unsigned
yarrow256_needed_sources(struct yarrow256_ctx *ctx)
{
  /* FIXME: This is somewhat inefficient. It would be better to
   * either maintain the count, or do this loop only if the
   * current source just crossed the threshold. */
  unsigned k, i;

  for (i = k = 0; i < ctx->nsources; i++)
    if (ctx->sources[i].estimate[YARROW_SLOW] >= YARROW_SLOW_THRESHOLD)
      k++;

#if YARROW_DEBUG
  fprintf(stderr,
          "yarrow256_needed_sources: source_index = %d,\n"
          "                    slow pool estimate = %d,\n"
          "     number of sources above threshold = %d\n",
          source_index, source->estimate[YARROW_SLOW], k);
#endif
  
  return (k < YARROW_SLOW_K) ? (YARROW_SLOW_K - k) : 0;
}