func_scramble: Audio scrambler function

Adds a function to scramble audio on a channel using
whole spectrum frequency inversion. This can be used
as a privacy enhancement with applications like
ChanSpy or other potentially sensitive audio.

ASTERISK-29542

Change-Id: I01020769d91060a1f56a708eb405f87648d1a67e

diff --git a/doc/CHANGES-staging/func_scramble.txt b/doc/CHANGES-staging/func_scramble.txt
new file mode 100644 (file)
index 0000000..4c1ffab
--- /dev/null
+++ b/doc/CHANGES-staging/func_scramble.txt
@@ -0,0 +1,5 @@
+Subject: func_scramble
+
+Adds an audio scrambler function that may be used to
+distort voice audio on a channel as a privacy
+enhancement.

diff --git a/funcs/func_scramble.c b/funcs/func_scramble.c
new file mode 100644 (file)
index 0000000..01756d9
--- /dev/null
+++ b/funcs/func_scramble.c
@@ -0,0 +1,235 @@
+/*
+ * Asterisk -- An open source telephony toolkit.
+ *
+ * Copyright (C) 2021, Naveen Albert
+ *
+ * Naveen Albert <asterisk@phreaknet.org>
+ *
+ * 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 <asterisk@phreaknet.org>
+ *
+ * \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 obfuscates audio on a channel
+                       to render it unintelligible, 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
+};
+
+/* 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 tx = 1, rx = 1;
+
+       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);
+
+       if (!strcasecmp(args.direction, "remove")) {
+               return remove_scrambler(chan);
+       }
+       if (!strcasecmp(args.direction, "tx")) {
+               tx = 1;
+               rx = 0;
+       } else if (!strcasecmp(args.direction, "rx")) {
+               rx = 0;
+               tx = 1;
+       } else if (strcasecmp(args.direction, "both")) {
+               ast_log(LOG_ERROR, "Direction must be either RX, TX, both, or remove\n");
+               return -1;
+       }
+       ast_channel_lock(chan);
+       if (!(datastore = ast_channel_datastore_find(chan, &scramble_datastore, NULL))) {
+               /* 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;
+               datastore->data = ni;
+               ast_channel_datastore_add(chan, datastore);
+               ast_audiohook_attach(chan, &ni->audiohook);
+       } else {
+               ni = datastore->data;
+       }
+       ni->tx = tx;
+       ni->rx = rx;
+       ast_channel_unlock(chan);
+
+       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");