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

Author: InterLinked1

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.

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");