/*      $OpenBSD: tcp_subr.c,v 1.98 2007/06/25 12:17:43 markus Exp $    */
/*      $NetBSD: tcp_rndiss.c,v 1.4 2011/12/17 20:05:39 tls Exp $       */

/*
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* 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
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#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: tcp_rndiss.c,v 1.4 2011/12/17 20:05:39 tls Exp $");

#include <sys/param.h>
#include <sys/cprng.h>

#include <netinet/tcp.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_rndiss.h>

#define TCP_RNDISS_ROUNDS       16
#define TCP_RNDISS_OUT          7200
#define TCP_RNDISS_MAX          30000

u_int8_t tcp_rndiss_sbox[128];
u_int16_t tcp_rndiss_msb;
u_int16_t tcp_rndiss_cnt;
long tcp_rndiss_reseed;

u_int16_t
tcp_rndiss_encrypt(u_int16_t val)
{
       u_int16_t sum = 0, i;

       for (i = 0; i < TCP_RNDISS_ROUNDS; i++) {
               sum += 0x79b9;
               val ^= ((u_int16_t)tcp_rndiss_sbox[(val^sum) & 0x7f]) << 7;
               val = ((val & 0xff) << 7) | (val >> 8);
       }

       return val;
}

void
tcp_rndiss_init(void)
{
       cprng_strong(kern_cprng, tcp_rndiss_sbox, sizeof(tcp_rndiss_sbox), 0);

       tcp_rndiss_reseed = time_second + TCP_RNDISS_OUT;
       tcp_rndiss_msb = tcp_rndiss_msb == 0x8000 ? 0 : 0x8000;
       tcp_rndiss_cnt = 0;
}

tcp_seq
tcp_rndiss_next(void)
{
       if (tcp_rndiss_cnt >= TCP_RNDISS_MAX ||
           time_second > tcp_rndiss_reseed)
               tcp_rndiss_init();

       /* (arc4random() & 0x7fff) ensures a 32768 byte gap between ISS */
       return ((tcp_rndiss_encrypt(tcp_rndiss_cnt++) | tcp_rndiss_msb) <<16) |
               (cprng_fast32() & 0x7fff);
}