tweak

LIBTOMCRYPT(3)						     Library Functions Manual						    LIBTOMCRYPT(3)

NAME

       libtomcrypt - public domain open source crypthographic toolkit

SYNOPSIS

       #include <tomcrypt.h>

       Link with -ltomcrypt (use pkg-config --libs libtomcrypt)

DESCRIPTION

       libtomcrypt is documented in /usr/share/doc/libtomcrypt-dev/crypt.pdf. To give you a very brief introduction, the following example is pro-
       vided.

EXAMPLE

	 /* AES-XTS example for libtomcrypt. (c) 2008 Michael Stapelberg, Public Domain */
	 #include <stdint.h>
	 #include <string.h>
	 #include <tomcrypt.h>

	 static symmetric_xts xts;

	 /*
	  * Initializes AES-XTS for use with encrypt(). Key must be at least 32 bytes long, only
	  * the first 32 bytes will be used.
	  *
	  */
	 void initialize_xts(unsigned char *key) {
	   int idx, err;
	   unsigned char aeskey1[16], aeskey2[16];

	   /* You can use 32 different ciphers simultaneously. Before using a cipher, you must register it. */
	   register_cipher(&aes_desc);

	   /* Get the index of the cipher registered before */
	   if ((idx = find_cipher("aes")) == -1) {
		   fprintf(stderr, "ERROR: AES not available in libtomcrypt. Please upgrade/fix libtomcrypt.
");
		   exit(EXIT_FAILURE);
	   }

	   /* Set up the two private keys required by AES-XTS (see 3.4.10 of crypt.pdf) */
	   strncpy((char*)aeskey1, key, 16);
	   strncpy((char*)aeskey2, key+16, 16);

	   printf("Initializing with keys "%.16s" and "%.16s" (AES-XTS)
", aeskey1, aeskey2);

	   /* Initialize AES-XTS */
	   if ((err = xts_start(idx, aeskey1, aeskey2, 16, 0, &xts)) != CRYPT_OK) {
		   fprintf(stderr, "ERROR starting XTS: %s
", error_to_string(err));
		   exit(EXIT_FAILURE);
	   }
	 }

	 /*
	  * Encrypts the input (of input_size) and stores the result in output. The piece index
	  * is required because XTS wants a tweak for each block so that it doesn't generate
	  * patterns which would be visible in the encrypted output.
	  *
	  */
	 void encrypt(const uint8_t *input, uint8_t *output, int input_size, int piece_idx) {
	   unsigned char tweak[256];
	   int err;

	   memset(tweak, '', 256);
	   snprintf((char*)tweak, 256, "%d", piece_idx);

	   if ((err = xts_encrypt(input, input_size, output, tweak, &xts)) != CRYPT_OK) {
		   fprintf(stderr, "ERROR in AES encryption: %d: %s
", err, error_to_string(err));
		   exit(EXIT_FAILURE);
	   }
	 }

SEE ALSO

       pkg-config(1)

AUTHOR

       libtomcrypt was written by Tom St Denis.

       This manual page was written by Michael Stapelberg <michael@stapelberg.de>, for the Debian project (and may be used by others).

								     June 2009							    LIBTOMCRYPT(3)