func_scramble.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2021, Naveen Albert
 *
 * Naveen Albert <[email protected]>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*! \file
 *
 * \brief Frequency inverter
 *
 * \author Naveen Albert <[email protected]>
 *
 * \ingroup functions
 *
 */

/*** MODULEINFO
   <support_level>extended</support_level>
 ***/

/*** DOCUMENTATION
   <function name="SCRAMBLE" language="en_US">
      <synopsis>
         Scrambles audio on a channel.
      </synopsis>
      <syntax>
         <parameter name="direction" required="false">
            <para>Must be <literal>TX</literal> or <literal>RX</literal>
            to limit to a specific direction, or <literal>both</literal>
            for both directions. <literal>remove</literal>
            will remove an existing scrambler.</para>
         </parameter>
      </syntax>
      <description>
         <para>Scrambles audio on a channel using whole spectrum inversion.
         This is not intended to be used for securely scrambling
         audio. It merely renders audio unintelligible on a channel
         as a privacy enhancement.</para>
      </description>
      <see-also>
         <ref type="application">ChanSpy</ref>
      </see-also>
   </function>
 ***/

#include "asterisk.h"

#include "asterisk/module.h"
#include "asterisk/channel.h"
#include "asterisk/pbx.h"
#include "asterisk/utils.h"
#include "asterisk/audiohook.h"
#include "asterisk/app.h"

#include <stdio.h>
#include <string.h>

struct scramble_information {
   struct ast_audiohook audiohook;
   unsigned short int tx;
   unsigned short int rx;
   unsigned short int state;
};

static void destroy_callback(void *data)
{
   struct scramble_information *ni = data;

   /* Destroy the audiohook, and destroy ourselves */
   ast_audiohook_lock(&ni->audiohook);
   ast_audiohook_detach(&ni->audiohook);
   ast_audiohook_unlock(&ni->audiohook);
   ast_audiohook_destroy(&ni->audiohook);
   ast_free(ni);

   return;
}

/*! \brief Static structure for datastore information */
static const struct ast_datastore_info scramble_datastore = {
   .type = "scramble",
   .destroy = destroy_callback
};

/* return 0 if nothing detected, 1 if lack of tone, 2 if presence of tone */
/* modifies buffer pointed to by 'amp' with inverted values */
static inline void freq_invert(short *amp, int samples)
{
   int i;
   /* invert every other sample by 1 */
   for (i = 0; i < samples; i += 2)
      amp[i] = -amp[i];
}

static int scramble_callback(struct ast_audiohook *audiohook, struct ast_channel *chan, struct ast_frame *frame, enum ast_audiohook_direction direction)
{
   struct ast_datastore *datastore = NULL;
   struct scramble_information *ni = NULL;

   /* If the audiohook is stopping it means the channel is shutting down.... but we let the datastore destroy take care of it */
   if (audiohook->status == AST_AUDIOHOOK_STATUS_DONE)
      return 0;

   /* Grab datastore which contains our gain information */
   if (!(datastore = ast_channel_datastore_find(chan, &scramble_datastore, NULL)))
      return 0;

   if (frame->frametype == AST_FRAME_VOICE) { /* only invert voice frequencies */
      /* Based on direction of frame, and confirm it is applicable */
      if (!(direction == AST_AUDIOHOOK_DIRECTION_READ ? &ni->rx : &ni->tx))
         return 0;
      /* Scramble the sample now */
      freq_invert(frame->data.ptr, frame->samples);
   }
   return 0;
}

/*! \internal \brief Disable scrambling on the channel */
static int remove_scrambler(struct ast_channel *chan)
{
   struct ast_datastore *datastore = NULL;
   struct scramble_information *data;
   SCOPED_CHANNELLOCK(chan_lock, chan);

   datastore = ast_channel_datastore_find(chan, &scramble_datastore, NULL);
   if (!datastore) {
      ast_log(AST_LOG_WARNING, "Cannot remove SCRAMBLE from %s: SCRAMBLE not currently enabled\n",
              ast_channel_name(chan));
      return -1;
   }
   data = datastore->data;

   if (ast_audiohook_remove(chan, &data->audiohook)) {
      ast_log(AST_LOG_WARNING, "Failed to remove SCRAMBLE audiohook from channel %s\n", ast_channel_name(chan));
      return -1;
   }

   if (ast_channel_datastore_remove(chan, datastore)) {
      ast_log(AST_LOG_WARNING, "Failed to remove SCRAMBLE datastore from channel %s\n",
              ast_channel_name(chan));
      return -1;
   }
   ast_datastore_free(datastore);

   return 0;
}

static int scramble_write(struct ast_channel *chan, const char *cmd, char *data, const char *value)
{
   char *parse;
   struct ast_datastore *datastore = NULL;
   struct scramble_information *ni = NULL;
   int is_new = 0;

   AST_DECLARE_APP_ARGS(args,
      AST_APP_ARG(direction);
   );

   if (!chan) {
      ast_log(LOG_WARNING, "No channel was provided to %s function.\n", cmd);
      return -1;
   }

   parse = ast_strdupa(value);
   AST_STANDARD_APP_ARGS(args, parse);

   ast_channel_lock(chan);
   if (!(datastore = ast_channel_datastore_find(chan, &scramble_datastore, NULL))) {
      ast_channel_unlock(chan);
      /* Allocate a new datastore to hold the reference to this audiohook information */
      if (!(datastore = ast_datastore_alloc(&scramble_datastore, NULL)))
         return 0;
      if (!(ni = ast_calloc(1, sizeof(*ni)))) {
         ast_datastore_free(datastore);
         return 0;
      }
      ast_audiohook_init(&ni->audiohook, AST_AUDIOHOOK_TYPE_MANIPULATE, "Voice scrambler", AST_AUDIOHOOK_MANIPULATE_ALL_RATES);
      ni->audiohook.manipulate_callback = scramble_callback;
      is_new = 1;
   } else {
      ast_channel_unlock(chan);
      ni = datastore->data;
   }

   if (!strcasecmp(args.direction, "tx")) {
      ni->tx = 1;
      ni->rx = 0;
   } else if (!strcasecmp(args.direction, "rx")) {
      ni->rx = 0;
      ni->tx = 1;
   } else if (!strcasecmp(args.direction, "both")) {
      ni->rx = 1;
      ni->tx = 1;
   } else if (!strcasecmp(args.direction, "remove")) {
      return remove_scrambler(chan);
   } else {
      ast_log(LOG_ERROR, "Direction must be either RX, TX or remove\n");
      return -1;
   }

   if (is_new) {
      datastore->data = ni;
      ast_channel_lock(chan);
      ast_channel_datastore_add(chan, datastore);
      ast_channel_unlock(chan);
      ast_audiohook_attach(chan, &ni->audiohook);
   }

   return 0;
}

static struct ast_custom_function scramble_function = {
   .name = "SCRAMBLE",
   .write = scramble_write,
};

static int unload_module(void)
{
   return ast_custom_function_unregister(&scramble_function);
}

static int load_module(void)
{
   return ast_custom_function_register(&scramble_function);
}

AST_MODULE_INFO_STANDARD(ASTERISK_GPL_KEY, "Frequency inverting voice scrambler");