1HMAC(3ossl)                         OpenSSL                        HMAC(3ossl)
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NAME

6       HMAC, HMAC_CTX_new, HMAC_CTX_reset, HMAC_CTX_free, HMAC_Init,
7       HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_copy,
8       HMAC_CTX_set_flags, HMAC_CTX_get_md, HMAC_size - HMAC message
9       authentication code
10

SYNOPSIS

12        #include <openssl/hmac.h>
13
14        unsigned char *HMAC(const EVP_MD *evp_md, const void *key, int key_len,
15                            const unsigned char *data, size_t data_len,
16                            unsigned char *md, unsigned int *md_len);
17
18       The following functions have been deprecated since OpenSSL 3.0, and can
19       be hidden entirely by defining OPENSSL_API_COMPAT with a suitable
20       version value, see openssl_user_macros(7):
21
22        HMAC_CTX *HMAC_CTX_new(void);
23        int HMAC_CTX_reset(HMAC_CTX *ctx);
24
25        int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
26                         const EVP_MD *md, ENGINE *impl);
27        int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, size_t len);
28        int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
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30        void HMAC_CTX_free(HMAC_CTX *ctx);
31
32        int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx);
33        void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags);
34        const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);
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36        size_t HMAC_size(const HMAC_CTX *e);
37
38       The following function has been deprecated since OpenSSL 1.1.0, and can
39       be hidden entirely by defining OPENSSL_API_COMPAT with a suitable
40       version value, see openssl_user_macros(7):
41
42        int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
43                      const EVP_MD *md);
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DESCRIPTION

46       HMAC is a MAC (message authentication code), i.e. a keyed hash function
47       used for message authentication, which is based on a hash function.
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49       HMAC() computes the message authentication code of the data_len bytes
50       at data using the hash function evp_md and the key key which is key_len
51       bytes long. The key may also be NULL with key_len being 0.
52
53       It places the result in md (which must have space for the output of the
54       hash function, which is no more than EVP_MAX_MD_SIZE bytes).  If md is
55       NULL, the digest is placed in a static array.  The size of the output
56       is placed in md_len, unless it is NULL. Note: passing a NULL value for
57       md to use the static array is not thread safe.
58
59       evp_md is a message digest such as EVP_sha1(), EVP_ripemd160() etc.
60       HMAC does not support variable output length digests such as
61       EVP_shake128() and EVP_shake256().
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63       All of the functions described below are deprecated.  Applications
64       should instead use EVP_MAC_CTX_new(3), EVP_MAC_CTX_free(3),
65       EVP_MAC_init(3), EVP_MAC_update(3) and EVP_MAC_final(3) or the 'quick'
66       single-shot MAC function EVP_Q_mac(3).
67
68       HMAC_CTX_new() creates a new HMAC_CTX in heap memory.
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70       HMAC_CTX_reset() clears an existing HMAC_CTX and associated resources,
71       making it suitable for new computations as if it was newly created with
72       HMAC_CTX_new().
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74       HMAC_CTX_free() erases the key and other data from the HMAC_CTX,
75       releases any associated resources and finally frees the HMAC_CTX
76       itself.
77
78       The following functions may be used if the message is not completely
79       stored in memory:
80
81       HMAC_Init_ex() initializes or reuses a HMAC_CTX structure to use the
82       hash function evp_md and key key. If both are NULL, or if key is NULL
83       and evp_md is the same as the previous call, then the existing key is
84       reused. ctx must have been created with HMAC_CTX_new() before the first
85       use of an HMAC_CTX in this function.
86
87       If HMAC_Init_ex() is called with key NULL and evp_md is not the same as
88       the previous digest used by ctx then an error is returned because reuse
89       of an existing key with a different digest is not supported.
90
91       HMAC_Init() initializes a HMAC_CTX structure to use the hash function
92       evp_md and the key key which is key_len bytes long.
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94       HMAC_Update() can be called repeatedly with chunks of the message to be
95       authenticated (len bytes at data).
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97       HMAC_Final() places the message authentication code in md, which must
98       have space for the hash function output.
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100       HMAC_CTX_copy() copies all of the internal state from sctx into dctx.
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102       HMAC_CTX_set_flags() applies the specified flags to the internal
103       EVP_MD_CTXs.  These flags have the same meaning as for
104       EVP_MD_CTX_set_flags(3).
105
106       HMAC_CTX_get_md() returns the EVP_MD that has previously been set for
107       the supplied HMAC_CTX.
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109       HMAC_size() returns the length in bytes of the underlying hash function
110       output.
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RETURN VALUES

113       HMAC() returns a pointer to the message authentication code or NULL if
114       an error occurred.
115
116       HMAC_CTX_new() returns a pointer to a new HMAC_CTX on success or NULL
117       if an error occurred.
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119       HMAC_CTX_reset(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() and
120       HMAC_CTX_copy() return 1 for success or 0 if an error occurred.
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122       HMAC_CTX_get_md() return the EVP_MD previously set for the supplied
123       HMAC_CTX or NULL if no EVP_MD has been set.
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125       HMAC_size() returns the length in bytes of the underlying hash function
126       output or zero on error.
127

CONFORMING TO

129       RFC 2104
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SEE ALSO

132       SHA1(3), EVP_Q_mac(3), evp(7)
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HISTORY

135       All functions except for HMAC() were deprecated in OpenSSL 3.0.
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137       HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL 1.1.0.
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139       HMAC_CTX_cleanup() existed in OpenSSL before version 1.1.0.
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141       HMAC_CTX_new(), HMAC_CTX_free() and HMAC_CTX_get_md() are new in
142       OpenSSL 1.1.0.
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144       HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
145       OpenSSL before version 1.0.0.
146
148       Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.
149
150       Licensed under the Apache License 2.0 (the "License").  You may not use
151       this file except in compliance with the License.  You can obtain a copy
152       in the file LICENSE in the source distribution or at
153       <https://www.openssl.org/source/license.html>.
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1573.0.5                             2022-07-05                       HMAC(3ossl)
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