.\" $OpenBSD: CMAC_Init.3,v 1.9 2024/11/12 00:42:28 schwarze Exp $
.\"
.\" Copyright (c) 2020 Ingo Schwarze <schwarze@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd $Mdocdate: November 12 2024 $
.Dt CMAC_INIT 3
.Os
.Sh NAME
.Nm CMAC_CTX_new ,
.Nm CMAC_Init ,
.Nm CMAC_Update ,
.Nm CMAC_Final ,
.Nm CMAC_CTX_copy ,
.Nm CMAC_CTX_get0_cipher_ctx ,
.Nm CMAC_CTX_cleanup ,
.Nm CMAC_CTX_free
.Nd Cipher-based message authentication code
.Sh SYNOPSIS
.In openssl/cmac.h
.Ft CMAC_CTX *
.Fn CMAC_CTX_new void
.Ft int
.Fo CMAC_Init
.Fa "CMAC_CTX *ctx"
.Fa "const void *key"
.Fa "size_t key_len"
.Fa "const EVP_CIPHER *cipher"
.Fa "ENGINE *engine"
.Fc
.Ft int
.Fo CMAC_Update
.Fa "CMAC_CTX *ctx"
.Fa "const void *in_data"
.Fa "size_t in_len"
.Fc
.Ft int
.Fo CMAC_Final
.Fa "CMAC_CTX *ctx"
.Fa "unsigned char *out_mac"
.Fa "size_t *out_len"
.Fc
.Ft int
.Fo CMAC_CTX_copy
.Fa "CMAC_CTX *out_ctx"
.Fa "CMAC_CTX *in_ctx"
.Fc
.Ft EVP_CIPHER_CTX *
.Fn CMAC_CTX_get0_cipher_ctx "CMAC_CTX *ctx"
.Ft void
.Fn CMAC_CTX_cleanup "CMAC_CTX *ctx"
.Ft void
.Fn CMAC_CTX_free "CMAC_CTX *ctx"
.Sh DESCRIPTION
CMAC is a message authentication code algorithm that can employ an
arbitrary block cipher using a symmetric key.
.Pp
The present manual page describes low-level functions implementing CMAC.
Instead of using these functions directly,
application programs normally call
.Xr EVP_PKEY_new_CMAC_key 3
and then pass the resulting
.Vt EVP_PKEY
object to
.Xr EVP_DigestSignInit 3 .
.Pp
The CMAC API is object-oriented.
Calculating a message authentication code requires a
.Vt CMAC_CTX
object.
Usually, the functions
.Fn CMAC_CTX_new ,
.Fn CMAC_Init ,
.Fn CMAC_Update ,
.Fn CMAC_Final ,
and
.Fn CMAC_CTX_free
need to be called in this order.
.Pp
.Fn CMAC_CTX_new
allocates a new
.Vt CMAC_CTX
object, initializes the embedded
.Vt EVP_CIPHER_CTX
object, and marks the object itself as uninitialized.
.Pp
.Fn CMAC_Init
selects the given block
.Fa cipher
for use by
.Fa ctx .
Functions to obtain suitable
.Vt EVP_CIPHER
objects are listed in the CIPHER LISTING section of the
.Xr EVP_EncryptInit 3
manual page.
Unless
.Fa key
is
.Dv NULL ,
.Fn CMAC_Init
also initializes
.Fa ctx
for use with the given symmetric
.Fa key
that is
.Fa key_len
bytes long.
In particular, it calculates and internally stores the two subkeys
and initializes
.Fa ctx
for subsequently feeding in data with
.Fn CMAC_Update .
The
.Fa engine
argument is ignored; passing
.Dv NULL
is recommended.
.Pp
If
.Fa ctx
is already initialized,
.Fn CMAC_Init
can be called again with
.Fa key
and
.Fa cipher
both set to
.Dv NULL
and
.Fa key_len
set to 0.
In that case, any data already processed is discarded and
.Fa ctx
is re-initialized to start reading data anew.
.Pp
.Fn CMAC_Update
processes
.Fa in_len
bytes of input data pointed to by
.Fa in_data .
Depending on the number of input bytes already cached in
.Fa ctx ,
on
.Fa in_len ,
and on the block size, this may encrypt zero or more blocks.
Unless
.Fa in_len
is zero, this function leaves at least one byte and at most one
block of input cached but unprocessed inside the
.Fa ctx
object.
.Fn CMAC_Update
can be called multiple times
to concatenate several chunks of input data of varying sizes.
.Pp
.Fn CMAC_Final
stores the length of the message authentication code in bytes,
which equals the cipher block size, into
.Pf * Fa out_len .
Unless
.Fa out_mac
is
.Dv NULL ,
it encrypts the last block, padding it if required, and copies the
resulting message authentication code to
.Fa out_mac .
The caller is responsible for providing a buffer of sufficient size.
.Pp
.Fn CMAC_CTX_copy
performs a deep copy of the already initialized
.Fa in_ctx
into
.Fa out_ctx .
.Pp
.Fn CMAC_CTX_cleanup
zeros out both subkeys and all temporary data in
.Fa ctx
and in the embedded
.Vt EVP_CIPHER_CTX
object, frees all allocated memory associated with it,
except for
.Fa ctx
itself, and marks it as uninitialized,
such that it can be reused for subsequent
.Fn CMAC_Init .
.Pp
.Fn CMAC_CTX_free
calls
.Fn CMAC_CTX_cleanup ,
then frees
.Fa ctx
itself.
If
.Fa ctx
is
.Dv NULL ,
no action occurs.
.Sh RETURN VALUES
.Fn CMAC_CTX_new
returns the new context object or
.Dv NULL
in case of failure.
It succeeds unless memory is exhausted.
.Pp
.Fn CMAC_Init ,
.Fn CMAC_Update ,
.Fn CMAC_Final ,
and
.Fn CMAC_CTX_copy
return 1 on success or 0 on failure.
.Fn CMAC_Init
fails if initializing the embedded
.Vt EVP_CIPHER_CTX
object fails.
The others fail if
.Fa in_ctx
is uninitialized.
.Fn CMAC_Update
and
.Fn CMAC_Final
also fail if encrypting a block fails, and
.Fn CMAC_CTX_copy
if copying the embedded
.Vt EVP_CIPHER_CTX
object fails, which can for example happen when memory is exhausted.
.Pp
.Fn CMAC_CTX_get0_cipher_ctx
returns an internal pointer to the
.Vt EVP_CIPHER_CTX
object that is embedded in
.Fa ctx .
.Sh ERRORS
The CMAC code itself does not use the
.In openssl/err.h
framework, so in general, the reasons for failure cannot be found out with
.Xr ERR_get_error 3 .
However, since the
.Xr EVP_EncryptInit 3
functions are used internally, entries may still get pushed onto
the error stack in some cases of failure.
.Sh SEE ALSO
.Xr EVP_aes_128_cbc 3 ,
.Xr EVP_DigestSignInit 3 ,
.Xr EVP_EncryptInit 3 ,
.Xr EVP_PKEY_new_CMAC_key 3 ,
.Xr HMAC 3
.Sh STANDARDS
.Rs
.%A Morris Dworkin
.%T "Recommendation for Block Cipher Modes of Operation:\
 The CMAC Mode for Authentication"
.%I National Institute of Standards and Technology
.%R NIST Special Publication 800-38B
.%U https://doi.org/10.6028/NIST.SP.800-38B
.%C Gaithersburg, Maryland
.%D May 2005, updated October 6, 2016
.Re
.Sh HISTORY
These functions first appeared in OpenSSL 1.0.1
and have been available since
.Ox 5.3 .