.\" $OpenBSD: HMAC.3,v 1.6 2017/01/06 17:38:21 schwarze Exp $ .\" OpenSSL a528d4f0 Oct 27 13:40:11 2015 -0400 .\" .\" This file was written by Ulf Moeller . .\" Copyright (c) 2000-2002, 2006, 2008, 2009, 2013, 2016 The OpenSSL Project. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. All advertising materials mentioning features or use of this .\" software must display the following acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" .\" .\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to .\" endorse or promote products derived from this software without .\" prior written permission. For written permission, please contact .\" openssl-core@openssl.org. .\" .\" 5. Products derived from this software may not be called "OpenSSL" .\" nor may "OpenSSL" appear in their names without prior written .\" permission of the OpenSSL Project. .\" .\" 6. Redistributions of any form whatsoever must retain the following .\" acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" .\" .\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY .\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: January 6 2017 $ .Dt HMAC 3 .Os .Sh NAME .Nm HMAC , .Nm HMAC_CTX_init , .Nm HMAC_Init , .Nm HMAC_Init_ex , .Nm HMAC_Update , .Nm HMAC_Final , .Nm HMAC_CTX_cleanup , .Nm HMAC_cleanup .Nd HMAC message authentication code .Sh SYNOPSIS .In openssl/hmac.h .Ft unsigned char * .Fo HMAC .Fa "const EVP_MD *evp_md" .Fa "const void *key" .Fa "int key_len" .Fa "const unsigned char *d" .Fa "int n" .Fa "unsigned char *md" .Fa "unsigned int *md_len" .Fc .Ft void .Fo HMAC_CTX_init .Fa "HMAC_CTX *ctx" .Fc .Ft int .Fo HMAC_Init .Fa "HMAC_CTX *ctx" .Fa "const void *key" .Fa "int key_len" .Fa "const EVP_MD *md" .Fc .Ft int .Fo HMAC_Init_ex .Fa "HMAC_CTX *ctx" .Fa "const void *key" .Fa "int key_len" .Fa "const EVP_MD *md" .Fa "ENGINE *impl" .Fc .Ft int .Fo HMAC_Update .Fa "HMAC_CTX *ctx" .Fa "const unsigned char *data" .Fa "int len" .Fc .Ft int .Fo HMAC_Final .Fa "HMAC_CTX *ctx" .Fa "unsigned char *md" .Fa "unsigned int *len" .Fc .Ft void .Fo HMAC_CTX_cleanup .Fa "HMAC_CTX *ctx" .Fc .Ft void .Fo HMAC_cleanup .Fa "HMAC_CTX *ctx" .Fc .Ft int .Fo HMAC_CTX_copy .Fa "HMAC_CTX *dctx" .Fa "HMAC_CTX *sctx" .Fc .Ft void .Fo HMAC_CTX_set_flags .Fa "HMAC_CTX *ctx" .Fa "unsigned long flags" .Fc .Ft size_t .Fo HMAC_size .Fa "const HMAC_CTX *e" .Fc .Sh DESCRIPTION HMAC is a MAC (message authentication code), i.e. a keyed hash function used for message authentication, which is based on a hash function. .Pp .Fn HMAC computes the message authentication code of the .Fa n bytes at .Fa d using the hash function .Fa evp_md and the key .Fa key which is .Fa key_len bytes long. .Pp It places the result in .Fa md , which must have space for the output of the hash function, which is no more than .Dv EVP_MAX_MD_SIZE bytes. If .Fa md is .Dv NULL , the digest is placed in a static array. The size of the output is placed in .Fa md_len , unless it is .Dv NULL . .Pp .Fa evp_md can be .Xr EVP_sha1 3 , .Xr EVP_ripemd160 3 , etc. .Pp .Fn HMAC_CTX_init initialises a .Vt HMAC_CTX before first use. It must be called. .Pp .Fn HMAC_CTX_cleanup erases the key and other data from the .Vt HMAC_CTX and releases any associated resources. It must be called when an .Vt HMAC_CTX is no longer required. .Pp .Fn HMAC_cleanup is an alias for .Fn HMAC_CTX_cleanup included for backward compatibility with 0.9.6b. It is deprecated and implemented as a macro. .Pp The following functions may be used if the message is not completely stored in memory: .Pp .Fn HMAC_Init initializes a .Vt HMAC_CTX structure to use the hash function .Fa evp_md and the key .Fa key which is .Fa key_len bytes long. It is deprecated and only included for backward compatibility with OpenSSL 0.9.6b. .Pp .Fn HMAC_Init_ex initializes or reuses a .Vt HMAC_CTX structure to use the function .Fa evp_md and key .Fa key . Either can be .Dv NULL , in which case the existing one will be reused. .Fn HMAC_CTX_init must have been called before the first use of an .Vt HMAC_CTX in this function. .Sy N.B. .Fn HMAC_Init had this undocumented behaviour in previous versions of OpenSSL - failure to switch to .Fn HMAC_Init_ex in programs that expect it will cause them to stop working. .Pp .Fn HMAC_Update can be called repeatedly with chunks of the message to be authenticated .Pq Fa len No bytes at Fa data . .Pp .Fn HMAC_Final places the message authentication code in .Fa md , which must have space for the hash function output. .Pp .Fn HMAC_CTX_copy copies all of the internal state from .Fa sctx into .Fa dctx . .Pp .Fn HMAC_CTX_set_flags applies the specified flags to the internal .Vt EVP_MD_CTX objects. Possible flag values .Dv EVP_MD_CTX_FLAG_* are defined in .In openssl/evp.h . .Pp .Fn HMAC_size returns the length in bytes of the underlying hash function output. It is implemented as a macro. .Sh RETURN VALUES .Fn HMAC returns a pointer to the message authentication code or .Dv NULL if an error occurred. .Pp .Fn HMAC_Init_ex , .Fn HMAC_Update , .Fn HMAC_Final , and .Fn HMAC_CTX_copy return 1 for success or 0 if an error occurred. .Pp .Fn HMAC_size returns the length in bytes of the underlying hash function output or 0 on error. .Sh SEE ALSO .Xr evp 3 .Sh STANDARDS RFC 2104 .Sh HISTORY .Fn HMAC , .Fn HMAC_Init , .Fn HMAC_Update , .Fn HMAC_Final , and .Fn HMAC_cleanup are available since SSLeay 0.9.0. .Pp .Fn HMAC_CTX_init , .Fn HMAC_Init_ex , and .Fn HMAC_CTX_cleanup are available since OpenSSL 0.9.7. .Pp .Fn HMAC_Init_ex , .Fn HMAC_Update , and .Fn HMAC_Final did not return values in versions of OpenSSL before 1.0.0.