Interface to CTR mode. More...

#include "tc_aes.h"
#include "tc_constants.h"

Functions
int	tc_ctr_mode (uint8_t out, unsigned int outlen, const uint8_t in, unsigned int inlen, uint8_t *ctr, const TCAesKeySched_t sched)
	CTR mode encryption/decryption procedure. More...

Detailed Description

Interface to CTR mode.

Overview: CTR (pronounced "counter") mode is a NIST approved mode of operation defined in SP 800-38a. It can be used with any block cipher to provide confidentiality of strings of any length. TinyCrypt hard codes AES128 as the block cipher.

Security: CTR mode achieves confidentiality only if the counter value is never reused with a same encryption key. If the counter is repeated, than an adversary might be able to defeat the scheme.

A usual method to ensure different counter values refers to initialize the counter in a given value (0, for example) and increases it every time a new block is enciphered. This naturally leaves to a limitation on the number q of blocks that can be enciphered using a same key: q < 2^(counter size).

TinyCrypt uses a counter of 32 bits. This means that after 2^32 block encryptions, the counter will be reused (thus losing CBC security). 2^32 block encryptions should be enough for most of applications targeting constrained devices. Applications intended to encrypt a larger number of blocks must replace the key after 2^32 block encryptions.

CTR mode provides NO data integrity.

Requires: AES-128

Usage: 1) call tc_ctr_mode to process the data to encrypt/decrypt.

Function Documentation

◆ tc_ctr_mode()

int tc_ctr_mode	(	uint8_t *	out,
		unsigned int	outlen,
		const uint8_t *	in,
		unsigned int	inlen,
		uint8_t *	ctr,
		const TCAesKeySched_t	sched
	)

CTR mode encryption/decryption procedure.

CTR mode encrypts (or decrypts) inlen bytes from in buffer into out buffer

Returns: returns TC_CRYPTO_SUCCESS (1) returns TC_CRYPTO_FAIL (0) if: out == NULL or in == NULL or ctr == NULL or sched == NULL or inlen == 0 or outlen == 0 or inlen != outlen

Note

Assumes:- The current value in ctr has NOT been used with sched

out points to inlen bytes
in points to inlen bytes
ctr is an integer counter in littleEndian format
sched was initialized by aes_set_encrypt_key

Parameters

out	OUT – produced ciphertext (plaintext)
outlen	IN – length of ciphertext buffer in bytes
in	IN – data to encrypt (or decrypt)
inlen	IN – length of input data in bytes
ctr	IN/OUT – the current counter value
sched	IN – an initialized AES key schedule

Functions

Detailed Description

Function Documentation

◆ tc_ctr_mode()