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CMAC_INIT(3) Library Functions Manual CMAC_INIT(3)

CMAC_CTX_new, CMAC_Init, CMAC_Update, CMAC_Final, CMAC_resume, CMAC_CTX_copy, CMAC_CTX_get0_cipher_ctx, CMAC_CTX_cleanup, CMAC_CTX_free — Cipher-based message authentication code

#include <openssl/cmac.h>

CMAC_CTX *
CMAC_CTX_new(void);

int
CMAC_Init(CMAC_CTX *ctx, const void *key, size_t key_len, const EVP_CIPHER *cipher, ENGINE *impl);

int
CMAC_Update(CMAC_CTX *ctx, const void *in_data, size_t in_len);

int
CMAC_Final(CMAC_CTX *ctx, unsigned char *out_mac, size_t *out_len);

int
CMAC_resume(CMAC_CTX *ctx);

int
CMAC_CTX_copy(CMAC_CTX *out_ctx, CMAC_CTX *in_ctx);

EVP_CIPHER_CTX *
CMAC_CTX_get0_cipher_ctx(CMAC_CTX *ctx);

void
CMAC_CTX_cleanup(CMAC_CTX *ctx);

void
CMAC_CTX_free(CMAC_CTX *ctx);

CMAC is a message authentication code algorithm that can employ an arbitrary block cipher using a symmetric key.

The present manual page describes low-level functions implementing CMAC. Instead of using these functions directly, application programs normally call EVP_PKEY_CTX_new_id(3) with an argument of EVP_PKEY_CMAC and then pass the resulting EVP_MD_CTX object to EVP_DigestInit_ex(3).

The CMAC API is object-oriented. Calculating a message authentication code requires a CMAC_CTX object. Usually, the functions (), CMAC_Init(), CMAC_Update(), CMAC_Final(), and CMAC_CTX_free() need to be called in this order.

() allocates a new CMAC_CTX object, initializes the embedded EVP_CIPHER_CTX object, and marks the object itself as uninitialized.

() selects the given block cipher for use by ctx. Functions to obtain suitable EVP_CIPHER objects are listed in the CIPHER LISTING section of the EVP_Cipher(3) manual page. Unless key is NULL, CMAC_Init() also initializes ctx for use with the given symmetric key that is key_len bytes long. In particular, it calculates and internally stores the two subkeys and initializes ctx for subsequently feeding in data with CMAC_Update(). To use the default cipher implementations provided by the library, pass NULL as the impl argument.

If ctx is already initialized, () can be called again with key, cipher, and impl all set to NULL and key_len set to 0. In that case, any data already processed is discarded and ctx is re-initialized to start reading data anew.

() processes in_len bytes of input data pointed to by in_data. Depending on the number of input bytes already cached in ctx, on in_len, and on the block size, this may encrypt zero or more blocks. Unless in_len is zero, this function leaves at least one byte and at most one block of input cached but unprocessed inside the ctx object. CMAC_Update() can be called multiple times to concatenate several chunks of input data of varying sizes.

() stores the length of the message authentication code in bytes, which equals the cipher block size, into *out_len. Unless out_mac is NULL, it encrypts the last block, padding it if required, and copies the resulting message authentication code to out_mac. The caller is responsible for providing a buffer of sufficient size.

Calling () after CMAC_Final() allows the user to subsequently append additional data with CMAC_Update(). Otherwise, unless CMAC_Init() is called to start from scratch, CMAC_Update() can no longer be used after CMAC_Final().

() performs a deep copy of the already initialized in_ctx into out_ctx.

() zeros out both subkeys and all temporary data in ctx and in the embedded EVP_CIPHER_CTX object, frees all allocated memory associated with it, except for ctx itself, and marks it as uninitialized, such that it can be reused for subsequent CMAC_Init().

() calls CMAC_CTX_cleanup(), then frees ctx itself. If ctx is NULL, no action occurs.

CMAC_CTX_new() returns the new context object or NULL in case of failure. It succeeds unless memory is exhausted.

CMAC_Init(), CMAC_Update(), CMAC_Final(), CMAC_resume(), and CMAC_CTX_copy() return 1 on success or 0 on failure. CMAC_Init() fails if initializing the embedded EVP_CIPHER_CTX object fails. The others fail if in_ctx is uninitialized. CMAC_Update() and CMAC_Final() also fail if encrypting a block fails, and CMAC_CTX_copy() if copying the embedded EVP_CIPHER_CTX object fails, which can for example happen when memory is exhausted.

CMAC_CTX_get0_cipher_ctx() returns an internal pointer to the EVP_CIPHER_CTX object that is embedded in ctx.

The CMAC code itself does not use the <openssl/err.h> framework, so in general, the reasons for failure cannot be found out with ERR_get_error(3). However, since the EVP_Cipher(3) functions are used internally, entries may still get pushed onto the error stack in some cases of failure.

EVP_aes_128_cbc(3), EVP_Cipher(3), EVP_DigestInit(3), EVP_PKEY_CTX_new_id(3), HMAC(3)

Morris Dworkin, Recommendation for Block Cipher Modes of Operation: The CMAC Mode for Authentication, National Institute of Standards and Technology, NIST Special Publication 800-38B, https://doi.org/10.6028/NIST.SP.800-38B, Gaithersburg, Maryland, May 2005, updated October 6, 2016.

These functions first appeared in OpenSSL 1.0.1 and have been available since OpenBSD 5.3.

August 6, 2020 OpenBSD-7.3