ctr_mode.h File Reference

Interface to CTR mode. More...

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

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