/*	Decrypt XMPP OMEMO Media sharing encrypted AES-GCM-256 files
	Copyright (C) 2024 DiffieHellman

	This program is free software: you can redistribute it and/or modify
	it under the terms of the GNU Affero General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.

	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU Affero General Public License for more details.

	You should have received a copy of the GNU Affero General Public License
	along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

#include <gcrypt.h>
#include <unistd.h>
#include <stdint.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <sys/stat.h>

/* it's unclear when AES-256-GCM support was added, but around 2008 sounds about right */
#define NEED_LIBGCRYPT_VERSION "1.3.0"

void init_gcrypt()
	{
	if (!gcry_check_version (NEED_LIBGCRYPT_VERSION))
		{
		fprintf(stderr, "libgcrypt is too old (need %s, have %s)\n",
		NEED_LIBGCRYPT_VERSION, gcry_check_version (NULL));
		exit(2);
		}

	/* disable secure memory (encryption key is one off and in insecure memory anyway) */
	gcry_control(GCRYCTL_DISABLE_SECMEM, 0);
	/* initialization has completed */
	gcry_control(GCRYCTL_INITIALIZATION_FINISHED, 0);
	}

int main(int argc, char *argv[])
	{
	init_gcrypt();

	if (argc != 3){fprintf(stderr, "Usage: %s <file to decrypt+overwrite> <hex encoded iv+key>\n", argv[0]); exit(1);}
	if (strlen(argv[2]) != 88){fprintf(stderr, "Hex encoded iv+key should be 88 bytes long\n"); exit(1);}

	uint8_t iv[12];
	uint8_t key[32];

	/* decode ASCII encoded hex */
	for (int i = 0, j = 0; i < sizeof(iv); ++i, j += 2){sscanf(argv[2]+j, "%2hhx", &iv[i]);}
	for (int i = 0, j = 24; i < sizeof(key); ++i, j += 2){sscanf(argv[2]+j, "%2hhx", &key[i]);}

	/* cipher handle */
	gcry_cipher_hd_t handle;
	/* open the cipher in AES-256-GCM mode */
	gcry_cipher_open(&handle, GCRY_CIPHER_AES256, GCRY_CIPHER_MODE_GCM, 0);

	gcry_cipher_setkey(handle, key, sizeof(key));
	/* iv can only be set after setting key */
	gcry_cipher_setiv(handle, iv, sizeof(iv));

	int fd = open(argv[1], O_RDWR);
	if (fd == -1){fprintf(stderr,"Could not open file: %s\n", strerror(errno)); exit(1);}

	/* get filesize */
	struct stat sb;
	fstat(fd, &sb);

	/* read the whole file into stack (files are very unlikely to be more than a few MiB in size) */
	char buffer[sb.st_size];
	if (read(fd, buffer, sb.st_size) != sb.st_size){fprintf(stderr,"Reading file failed!\n"); exit(1);}

	/* the next call to gcry_cipher_decrypt will be the last one */
	gcry_cipher_final(handle);

	/* decrypt in place */
	gcry_cipher_decrypt(handle, buffer, sb.st_size, NULL, 0);

	gcry_cipher_close(handle);

	/* seek to the start of file to overwrite */
	lseek(fd, 0, SEEK_SET);

	if (write(fd, buffer, sb.st_size) != sb.st_size){fprintf(stderr,"Writing file failed!\n"); exit(1);}
	close(fd);

	return 0;
	}