text: /**
   * @file
   *
   * Reference implementation of the TCP ISN algorithm standardized in RFC 6528.
   * Produce TCP Initial Sequence Numbers by combining an MD5-generated hash
   * based on the new TCP connection's identity and a stable secret, with the
   * current time at 4-microsecond granularity.
   *
   * Specifically, the implementation uses MD5 to compute a hash of the input
   * buffer, which contains both the four-tuple of the new TCP connection (local
   * and remote IP address and port), as well as a 16-byte secret to make the
   * results unpredictable to external parties.  The secret must be given at
   * initialization time and should ideally remain the same across system
   * reboots.  To be sure: the spoofing-resistance of the resulting ISN depends
   * mainly on the strength of the supplied secret!
   *
   * The implementation takes 32 bits from the computed hash, and adds to it the
   * current time, in 4-microsecond units.  The current time is computed from a
   * boot time given at initialization, and the current uptime as provided by
   * sys_now().  Thus, it assumes that sys_now() returns a time value that is
   * relative to the boot time, i.e., that it starts at 0 at system boot, and
   * only ever increases monotonically.
   *
   * For efficiency reasons, a single MD5 input buffer is used, and partially
   * filled in at initialization time.  Specifically, of this 64-byte buffer, the
   * first 36 bytes are used for the four-way TCP tuple data, followed by the
   * 16-byte secret, followed by 12-byte zero padding.  The 64-byte size of the
   * buffer should achieve the best performance for the actual MD5 computation.
   *
   * Basic usage:
   *
   * 1. in your lwipopts.h, add the following lines:
   *
   *    #include <lwip/arch.h>
   *    struct ip_addr;
   *    u32_t lwip_hook_tcp_isn(const struct ip_addr *local_ip, u16_t local_port,
   *      const struct ip_addr *remote_ip, u16_t remote_port);
   *   "#define LWIP_HOOK_TCP_ISN lwip_hook_tcp_isn";
   *
   * 2. from your own code, call lwip_init_tcp_isn() at initialization time, with
   *    appropriate parameters.
   */
  
  /*
   * Copyright (c) 2016 The MINIX 3 Project.
   * 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.
   * 3. The name of the author may not be used to endorse or promote products
   *    derived from this software without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 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.
   *
   * Author: David van Moolenbroek <david@minix3.org>
   */
  
  #include "lwip_default_hooks.h"
  #include "lwip/ip_addr.h"
  #include "lwip/sys.h"
  #include <string.h>
  #include "esp_rom_md5.h"
  
  #ifdef CONFIG_LWIP_HOOK_TCP_ISN_DEFAULT
  
  static u8_t input[64];
  static u32_t base_time;
  
  /**
   * Initialize the TCP ISN module, with the boot time and a secret.
   *
   * @param boot_time Wall clock boot time of the system, in seconds.
   * @param secret_16_bytes A 16-byte secret used to randomize the TCP ISNs.
   */
  void
  lwip_init_tcp_isn(u32_t boot_time, const u8_t *secret_16_bytes)
  {
    /* Initialize the input buffer with the secret and trailing zeroes. */
    memset(input, 0, sizeof(input));
  
    MEMCPY(&input[36], secret_16_bytes, 16);
  
    /* Save the boot time in 4-us units. Overflow is no problem here. */
    base_time = boot_time * 250000;
  }
  
  /**
   * Hook to generate an Initial Sequence Number (ISN) for a new TCP connection.
   *
   * @param local_ip The local IP address.
   * @param local_port The local port number, in host-byte order.
   * @param remote_ip The remote IP address.
   * @param remote_port The remote port number, in host-byte order.
   * @return The ISN to use for the new TCP connection.
   */
  u32_t
  lwip_hook_tcp_isn(const ip_addr_t *local_ip, u16_t local_port,
      const ip_addr_t *remote_ip, u16_t remote_port)
  {
    u8_t output[16];
    u32_t isn;
  
  #if LWIP_IPV4 && LWIP_IPV6
    if (IP_IS_V6(local_ip))
  #endif /* LWIP_IPV4 && LWIP_IPV6 */
  #if LWIP_IPV6
    {
      const ip6_addr_t *local_ip6, *remote_ip6;
  
      local_ip6  = ip_2_ip6(local_ip);
      remote_ip6 = ip_2_ip6(remote_ip);
  
      SMEMCPY(&input[0],  &local_ip6->addr,  16);
      SMEMCPY(&input[16], &remote_ip6->addr, 16);
    }
  #endif /* LWIP_IPV6 */
  #if LWIP_IPV4 && LWIP_IPV6
    else
  #endif /* LWIP_IPV4 && LWIP_IPV6 */
  #if LWIP_IPV4
    {
      const ip4_addr_t *local_ip4, *remote_ip4;
  
      local_ip4  = ip_2_ip4(local_ip);
      remote_ip4 = ip_2_ip4(remote_ip);
  
      /* Represent IPv4 addresses as IPv4-mapped IPv6 addresses, to ensure that
       * the IPv4 and IPv6 address spaces are completely disjoint. */
      memset(&input[0], 0, 10);
      input[10] = 0xff;
      input[11] = 0xff;
      SMEMCPY(&input[12], &local_ip4->addr, 4);
      memset(&input[16], 0, 10);
      input[26] = 0xff;
      input[27] = 0xff;
      SMEMCPY(&input[28], &remote_ip4->addr, 4);
    }
  #endif /* LWIP_IPV4 */
  
    input[32] = (u8_t)(local_port >> 8);
    input[33] = (u8_t)(local_port & 0xff);
    input[34] = (u8_t)(remote_port >> 8);
    input[35] = (u8_t)(remote_port & 0xff);
  
    /* The secret and padding are already filled in. */
  
    /*
     * Generate the hash using ROM MD5 APIs
     * This hook is invoked in the context of TCP/IP (tiT) task and
     * it is unlikely that its stack would be placed in SPIRAM. Hence
     * even with SPIRAM enabled case and ESP32 revision < 3, using ROM
     * APIs should not create any issues.
     */
  #if CONFIG_SPIRAM_ALLOW_STACK_EXTERNAL_MEMORY
    assert(!esp_ptr_external_ram(esp_cpu_get_sp()));
  #endif
  
    md5_context_t ctx;
    esp_rom_md5_init(&ctx);
    esp_rom_md5_update(&ctx, input, sizeof(input));
    esp_rom_md5_final(output, &ctx);
  
  
    /* Arbitrarily take the first 32 bits from the generated hash. */
    MEMCPY(&isn, output, sizeof(isn));
  
    /* Add the current time in 4-microsecond units. */
    return isn + base_time + sys_now() * 250;
  }
  
  #endif /* LWIP_HOOK_TCP_ISN */
  