fortuna.c

Go to the documentation of this file.

/*
 * fortuna.c
 *      Fortuna-like PRNG.
 *
 * Copyright (c) 2005 Marko Kreen
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * contrib/pgcrypto/fortuna.c
 */

#include "postgres.h"

#include <sys/time.h>
#include <time.h>

#include "rijndael.h"
#include "sha2.h"
#include "fortuna.h"


/*
 * Why Fortuna-like: There does not seem to be any definitive reference
 * on Fortuna in the net.  Instead this implementation is based on
 * following references:
 *
 * http://en.wikipedia.org/wiki/Fortuna_(PRNG)
 *   - Wikipedia article
 * http://jlcooke.ca/random/
 *   - Jean-Luc Cooke Fortuna-based /dev/random driver for Linux.
 */

/*
 * There is some confusion about whether and how to carry forward
 * the state of the pools.  Seems like original Fortuna does not
 * do it, resetting hash after each request.  I guess expecting
 * feeding to happen more often that requesting.   This is absolutely
 * unsuitable for pgcrypto, as nothing asynchronous happens here.
 *
 * J.L. Cooke fixed this by feeding previous hash to new re-initialized
 * hash context.
 *
 * Fortuna predecessor Yarrow requires ability to query intermediate
 * 'final result' from hash, without affecting it.
 *
 * This implementation uses the Yarrow method - asking intermediate
 * results, but continuing with old state.
 */


/*
 * Algorithm parameters
 */

/*
 * How many pools.
 *
 * Original Fortuna uses 32 pools, that means 32'th pool is
 * used not earlier than in 13th year.  This is a waste in
 * pgcrypto, as we have very low-frequancy seeding.  Here
 * is preferable to have all entropy usable in reasonable time.
 *
 * With 23 pools, 23th pool is used after 9 days which seems
 * more sane.
 *
 * In our case the minimal cycle time would be bit longer
 * than the system-randomness feeding frequency.
 */
#define NUM_POOLS       23

/* in microseconds */
#define RESEED_INTERVAL 100000  /* 0.1 sec */

/* for one big request, reseed after this many bytes */
#define RESEED_BYTES    (1024*1024)

/*
 * Skip reseed if pool 0 has less than this many
 * bytes added since last reseed.
 */
#define POOL0_FILL      (256/8)

/*
 * Algorithm constants
 */

/* Both cipher key size and hash result size */
#define BLOCK           32

/* cipher block size */
#define CIPH_BLOCK      16

/* for internal wrappers */
#define MD_CTX          SHA256_CTX
#define CIPH_CTX        rijndael_ctx

struct fortuna_state
{
    uint8       counter[CIPH_BLOCK];
    uint8       result[CIPH_BLOCK];
    uint8       key[BLOCK];
    MD_CTX      pool[NUM_POOLS];
    CIPH_CTX    ciph;
    unsigned    reseed_count;
    struct timeval last_reseed_time;
    unsigned    pool0_bytes;
    unsigned    rnd_pos;
    int         tricks_done;
};
typedef struct fortuna_state FState;


/*
 * Use our own wrappers here.
 * - Need to get intermediate result from digest, without affecting it.
 * - Need re-set key on a cipher context.
 * - Algorithms are guaranteed to exist.
 * - No memory allocations.
 */

static void
ciph_init(CIPH_CTX * ctx, const uint8 *key, int klen)
{
    rijndael_set_key(ctx, (const uint32 *) key, klen, 1);
}

static void
ciph_encrypt(CIPH_CTX * ctx, const uint8 *in, uint8 *out)
{
    rijndael_encrypt(ctx, (const uint32 *) in, (uint32 *) out);
}

static void
md_init(MD_CTX * ctx)
{
    SHA256_Init(ctx);
}

static void
md_update(MD_CTX * ctx, const uint8 *data, int len)
{
    SHA256_Update(ctx, data, len);
}

static void
md_result(MD_CTX * ctx, uint8 *dst)
{
    SHA256_CTX  tmp;

    memcpy(&tmp, ctx, sizeof(*ctx));
    SHA256_Final(dst, &tmp);
    memset(&tmp, 0, sizeof(tmp));
}

/*
 * initialize state
 */
static void
init_state(FState *st)
{
    int         i;

    memset(st, 0, sizeof(*st));
    for (i = 0; i < NUM_POOLS; i++)
        md_init(&st->pool[i]);
}

/*
 * Endianess does not matter.
 * It just needs to change without repeating.
 */
static void
inc_counter(FState *st)
{
    uint32     *val = (uint32 *) st->counter;

    if (++val[0])
        return;
    if (++val[1])
        return;
    if (++val[2])
        return;
    ++val[3];
}

/*
 * This is called 'cipher in counter mode'.
 */
static void
encrypt_counter(FState *st, uint8 *dst)
{
    ciph_encrypt(&st->ciph, st->counter, dst);
    inc_counter(st);
}


/*
 * The time between reseed must be at least RESEED_INTERVAL
 * microseconds.
 */
static int
enough_time_passed(FState *st)
{
    int         ok;
    struct timeval tv;
    struct timeval *last = &st->last_reseed_time;

    gettimeofday(&tv, NULL);

    /* check how much time has passed */
    ok = 0;
    if (tv.tv_sec > last->tv_sec + 1)
        ok = 1;
    else if (tv.tv_sec == last->tv_sec + 1)
    {
        if (1000000 + tv.tv_usec - last->tv_usec >= RESEED_INTERVAL)
            ok = 1;
    }
    else if (tv.tv_usec - last->tv_usec >= RESEED_INTERVAL)
        ok = 1;

    /* reseed will happen, update last_reseed_time */
    if (ok)
        memcpy(last, &tv, sizeof(tv));

    memset(&tv, 0, sizeof(tv));

    return ok;
}

/*
 * generate new key from all the pools
 */
static void
reseed(FState *st)
{
    unsigned    k;
    unsigned    n;
    MD_CTX      key_md;
    uint8       buf[BLOCK];

    /* set pool as empty */
    st->pool0_bytes = 0;

    /*
     * Both #0 and #1 reseed would use only pool 0. Just skip #0 then.
     */
    n = ++st->reseed_count;

    /*
     * The goal: use k-th pool only 1/(2^k) of the time.
     */
    md_init(&key_md);
    for (k = 0; k < NUM_POOLS; k++)
    {
        md_result(&st->pool[k], buf);
        md_update(&key_md, buf, BLOCK);

        if (n & 1 || !n)
            break;
        n >>= 1;
    }

    /* add old key into mix too */
    md_update(&key_md, st->key, BLOCK);

    /* now we have new key */
    md_result(&key_md, st->key);

    /* use new key */
    ciph_init(&st->ciph, st->key, BLOCK);

    memset(&key_md, 0, sizeof(key_md));
    memset(buf, 0, BLOCK);
}

/*
 * Pick a random pool.  This uses key bytes as random source.
 */
static unsigned
get_rand_pool(FState *st)
{
    unsigned    rnd;

    /*
     * This slightly prefers lower pools - thats OK.
     */
    rnd = st->key[st->rnd_pos] % NUM_POOLS;

    st->rnd_pos++;
    if (st->rnd_pos >= BLOCK)
        st->rnd_pos = 0;

    return rnd;
}

/*
 * update pools
 */
static void
add_entropy(FState *st, const uint8 *data, unsigned len)
{
    unsigned    pos;
    uint8       hash[BLOCK];
    MD_CTX      md;

    /* hash given data */
    md_init(&md);
    md_update(&md, data, len);
    md_result(&md, hash);

    /*
     * Make sure the pool 0 is initialized, then update randomly.
     */
    if (st->reseed_count == 0)
        pos = 0;
    else
        pos = get_rand_pool(st);
    md_update(&st->pool[pos], hash, BLOCK);

    if (pos == 0)
        st->pool0_bytes += len;

    memset(hash, 0, BLOCK);
    memset(&md, 0, sizeof(md));
}

/*
 * Just take 2 next blocks as new key
 */
static void
rekey(FState *st)
{
    encrypt_counter(st, st->key);
    encrypt_counter(st, st->key + CIPH_BLOCK);
    ciph_init(&st->ciph, st->key, BLOCK);
}

/*
 * Hide public constants. (counter, pools > 0)
 *
 * This can also be viewed as spreading the startup
 * entropy over all of the components.
 */
static void
startup_tricks(FState *st)
{
    int         i;
    uint8       buf[BLOCK];

    /* Use next block as counter. */
    encrypt_counter(st, st->counter);

    /* Now shuffle pools, excluding #0 */
    for (i = 1; i < NUM_POOLS; i++)
    {
        encrypt_counter(st, buf);
        encrypt_counter(st, buf + CIPH_BLOCK);
        md_update(&st->pool[i], buf, BLOCK);
    }
    memset(buf, 0, BLOCK);

    /* Hide the key. */
    rekey(st);

    /* This can be done only once. */
    st->tricks_done = 1;
}

static void
extract_data(FState *st, unsigned count, uint8 *dst)
{
    unsigned    n;
    unsigned    block_nr = 0;

    /* Should we reseed? */
    if (st->pool0_bytes >= POOL0_FILL || st->reseed_count == 0)
        if (enough_time_passed(st))
            reseed(st);

    /* Do some randomization on first call */
    if (!st->tricks_done)
        startup_tricks(st);

    while (count > 0)
    {
        /* produce bytes */
        encrypt_counter(st, st->result);

        /* copy result */
        if (count > CIPH_BLOCK)
            n = CIPH_BLOCK;
        else
            n = count;
        memcpy(dst, st->result, n);
        dst += n;
        count -= n;

        /* must not give out too many bytes with one key */
        block_nr++;
        if (block_nr > (RESEED_BYTES / CIPH_BLOCK))
        {
            rekey(st);
            block_nr = 0;
        }
    }
    /* Set new key for next request. */
    rekey(st);
}

/*
 * public interface
 */

static FState main_state;
static int  init_done = 0;

void
fortuna_add_entropy(const uint8 *data, unsigned len)
{
    if (!init_done)
    {
        init_state(&main_state);
        init_done = 1;
    }
    if (!data || !len)
        return;
    add_entropy(&main_state, data, len);
}

void
fortuna_get_bytes(unsigned len, uint8 *dst)
{
    if (!init_done)
    {
        init_state(&main_state);
        init_done = 1;
    }
    if (!dst || !len)
        return;
    extract_data(&main_state, len, dst);
}