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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" HMAC, HMAC_CTX_new, HMAC_CTX_reset, HMAC_CTX_free, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_copy, HMAC_CTX_set_flags, HMAC_CTX_get_md, HMAC_size \&\- HMAC message authentication code .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 1 \& #include <openssl/hmac.h> \& \& unsigned char *HMAC(const EVP_MD *evp_md, const void *key, \& int key_len, const unsigned char *d, size_t n, \& unsigned char *md, unsigned int *md_len); \& \& HMAC_CTX *HMAC_CTX_new(void); \& int HMAC_CTX_reset(HMAC_CTX *ctx); \& \& int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len, \& const EVP_MD *md, ENGINE *impl); \& int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, size_t len); \& int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len); \& \& void HMAC_CTX_free(HMAC_CTX *ctx); \& \& int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx); \& void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags); \& const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx); \& \& size_t HMAC_size(const HMAC_CTX *e); .Ve .PP Deprecated: .PP .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 .SH "DESCRIPTION" .IX Header "DESCRIPTION" \&\s-1HMAC\s0 is a \s-1MAC\s0 (message authentication code), i.e. a keyed hash function used for message authentication, which is based on a hash function. .PP \&\s-1\fBHMAC\s0()\fR computes the message authentication code of the \fBn\fR bytes at \&\fBd\fR using the hash function \fBevp_md\fR and the key \fBkey\fR which is \&\fBkey_len\fR bytes long. .PP It places the result in \fBmd\fR (which must have space for the output of the hash function, which is no more than \fB\s-1EVP_MAX_MD_SIZE\s0\fR bytes). If \fBmd\fR is \s-1NULL,\s0 the digest is placed in a static array. The size of the output is placed in \fBmd_len\fR, unless it is \fB\s-1NULL\s0\fR. Note: passing a \s-1NULL\s0 value for \fBmd\fR to use the static array is not thread safe. .PP \&\fBevp_md\fR is a message digest such as \fBEVP_sha1()\fR, \fBEVP_ripemd160()\fR etc. \s-1HMAC\s0 does not support variable output length digests such as \fBEVP_shake128()\fR and \&\fBEVP_shake256()\fR. .PP \&\fBHMAC_CTX_new()\fR creates a new \s-1HMAC_CTX\s0 in heap memory. .PP \&\fBHMAC_CTX_reset()\fR zeros an existing \fB\s-1HMAC_CTX\s0\fR and associated resources, making it suitable for new computations as if it was newly created with \fBHMAC_CTX_new()\fR. .PP \&\fBHMAC_CTX_free()\fR erases the key and other data from the \fB\s-1HMAC_CTX\s0\fR, releases any associated resources and finally frees the \fB\s-1HMAC_CTX\s0\fR itself. .PP The following functions may be used if the message is not completely stored in memory: .PP \&\fBHMAC_Init_ex()\fR initializes or reuses a \fB\s-1HMAC_CTX\s0\fR structure to use the hash function \fBevp_md\fR and key \fBkey\fR. If both are \s-1NULL,\s0 or if \fBkey\fR is \s-1NULL\s0 and \fBevp_md\fR is the same as the previous call, then the existing key is reused. \fBctx\fR must have been created with \fBHMAC_CTX_new()\fR before the first use of an \fB\s-1HMAC_CTX\s0\fR in this function. .PP If \fBHMAC_Init_ex()\fR is called with \fBkey\fR \s-1NULL\s0 and \fBevp_md\fR is not the same as the previous digest used by \fBctx\fR then an error is returned because reuse of an existing key with a different digest is not supported. .PP \&\fBHMAC_Init()\fR initializes a \fB\s-1HMAC_CTX\s0\fR structure to use the hash function \fBevp_md\fR and the key \fBkey\fR which is \fBkey_len\fR bytes long. .PP \&\fBHMAC_Update()\fR can be called repeatedly with chunks of the message to be authenticated (\fBlen\fR bytes at \fBdata\fR). .PP \&\fBHMAC_Final()\fR places the message authentication code in \fBmd\fR, which must have space for the hash function output. .PP \&\fBHMAC_CTX_copy()\fR copies all of the internal state from \fBsctx\fR into \fBdctx\fR. .PP \&\fBHMAC_CTX_set_flags()\fR applies the specified flags to the internal EVP_MD_CTXs. These flags have the same meaning as for \fBEVP_MD_CTX_set_flags\fR\|(3). .PP \&\fBHMAC_CTX_get_md()\fR returns the \s-1EVP_MD\s0 that has previously been set for the supplied \s-1HMAC_CTX.\s0 .PP \&\fBHMAC_size()\fR returns the length in bytes of the underlying hash function output. .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\s-1\fBHMAC\s0()\fR returns a pointer to the message authentication code or \s-1NULL\s0 if an error occurred. .PP \&\fBHMAC_CTX_new()\fR returns a pointer to a new \fB\s-1HMAC_CTX\s0\fR on success or \&\fB\s-1NULL\s0\fR if an error occurred. .PP \&\fBHMAC_CTX_reset()\fR, \fBHMAC_Init_ex()\fR, \fBHMAC_Update()\fR, \fBHMAC_Final()\fR and \&\fBHMAC_CTX_copy()\fR return 1 for success or 0 if an error occurred. .PP \&\fBHMAC_CTX_get_md()\fR 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. .PP \&\fBHMAC_size()\fR returns the length in bytes of the underlying hash function output or zero on error. .SH "CONFORMING TO" .IX Header "CONFORMING TO" \&\s-1RFC 2104\s0 .SH "SEE ALSO" .IX Header "SEE ALSO" \&\s-1\fBSHA1\s0\fR\|(3), \fBevp\fR\|(7) .SH "HISTORY" .IX Header "HISTORY" \&\fBHMAC_CTX_init()\fR was replaced with \fBHMAC_CTX_reset()\fR in OpenSSL 1.1.0. .PP \&\fBHMAC_CTX_cleanup()\fR existed in OpenSSL before version 1.1.0. .PP \&\fBHMAC_CTX_new()\fR, \fBHMAC_CTX_free()\fR and \fBHMAC_CTX_get_md()\fR are new in OpenSSL 1.1.0. .PP \&\fBHMAC_Init_ex()\fR, \fBHMAC_Update()\fR and \fBHMAC_Final()\fR did not return values in OpenSSL before version 1.0.0. .SH "COPYRIGHT" .IX Header "COPYRIGHT" Copyright 2000\-2020 The OpenSSL Project Authors. All Rights Reserved. .PP 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>.