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Title "HMAC 3"
way too many mistakes in technical documents.
.nh
Deprecated:
.Vb 4 #if OPENSSL_API_COMPAT < 0x10100000L int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len, const EVP_MD *md); #endif .Ve
\s-1HMAC\s0() computes the message authentication code of the n bytes at \fBd using the hash function evp_md and the key key which is \fBkey_len bytes long.
It places the result in md (which must have space for the output of the hash function, which is no more than \s-1EVP_MAX_MD_SIZE\s0 bytes). If md is \s-1NULL,\s0 the digest is placed in a static array. The size of the output is placed in md_len, unless it is \s-1NULL\s0. Note: passing a \s-1NULL\s0 value for md to use the static array is not thread safe.
\fBevp_md can be EVP_sha1(), EVP_ripemd160() etc.
\fIHMAC_CTX_new() creates a new \s-1HMAC_CTX\s0 in heap memory.
\fIHMAC_CTX_reset() zeroes an existing \s-1HMAC_CTX\s0 and associated resources, making it suitable for new computations as if it was newly created with HMAC_CTX_new().
\fIHMAC_CTX_free() erases the key and other data from the \s-1HMAC_CTX\s0, releases any associated resources and finally frees the \s-1HMAC_CTX\s0 itself.
The following functions may be used if the message is not completely stored in memory:
\fIHMAC_Init() initializes a \s-1HMAC_CTX\s0 structure to use the hash function evp_md and the key key which is key_len bytes long. It is deprecated and only included for backward compatibility with OpenSSL 0.9.6b.
\fIHMAC_Init_ex() initializes or reuses a \s-1HMAC_CTX\s0 structure to use the hash function evp_md and key key. If both are \s-1NULL \s0(or evp_md is the same as the previous digest used by ctx and key is \s-1NULL\s0) the existing key is reused. ctx must have been created with HMAC_CTX_new() before the first use of an \s-1HMAC_CTX\s0 in this function. N.B. \f(BIHMAC_Init() had this undocumented behaviour in previous versions of OpenSSL - failure to switch to \f(BIHMAC_Init_ex() in programs that expect it will cause them to stop working.
\fB\s-1NOTE:\s0 If HMAC_Init_ex() is called with key \s-1NULL\s0 and evp_md is not the same as the previous digest used by ctx then an error is returned because reuse of an existing key with a different digest is not supported.
\fIHMAC_Update() can be called repeatedly with chunks of the message to be authenticated (len bytes at data).
\fIHMAC_Final() places the message authentication code in md, which must have space for the hash function output.
\fIHMAC_CTX_copy() copies all of the internal state from sctx into dctx.
\fIHMAC_CTX_set_flags() applies the specified flags to the internal EVP_MD_CTXs. These flags have the same meaning as for EVP_MD_CTX_set_flags\|(3).
\fIHMAC_CTX_get_md() returns the \s-1EVP_MD\s0 that has previously been set for the supplied \s-1HMAC_CTX.\s0
\fIHMAC_CTX_new() returns a pointer to a new \s-1HMAC_CTX\s0 on success or \fB\s-1NULL\s0 if an error occurred.
\fIHMAC_CTX_reset(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() and \fIHMAC_CTX_copy() return 1 for success or 0 if an error occurred.
\fIHMAC_CTX_get_md() return the \s-1EVP_MD\s0 previously set for the supplied \s-1HMAC_CTX\s0 or \s-1NULL\s0 if no \s-1EVP_MD\s0 has been set.
\fIHMAC_CTX_cleanup() existed in OpenSSL versions before 1.1.0.
\fIHMAC_CTX_new(), HMAC_CTX_free() and HMAC_CTX_get_md() are new in OpenSSL version 1.1.0.
\fIHMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in versions of OpenSSL before 1.0.0.
Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.
Indexes created Wed Apr 22 00:22:44 UTC 2026