[#HMAC]
= HMAC
:mrdocs:

Compute an HMAC over `data` using digest `evp_md` and key `key.`

== Synopsis

Declared in `<openssl/hmac.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
unsigned char*
HMAC(
    xref:EVP_MD.adoc[EVP_MD] const* evp_md,
    void const* key,
    int key_len,
    unsigned char const* data,
    size_t data_len,
    unsigned char* md,
    unsigned int* md_len);
----

== Description

Uses the default OSSL_LIB_CTX; prefer EVP_Q_mac() when a library context is required.

== Return Value

Pointer to the MAC bytes (`md` or the static buffer), or NULL on error.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *evp_md*
| Message digest (e.g. EVP_sha256()); variable‐length digests such as SHAKE are not supported.
| *key*
| HMAC key material, or NULL when `key_len` is 0.
| *key_len*
| Length of `key` in bytes.
| *data*
| Message bytes to authenticate.
| *data_len*
| Number of bytes at `data.`
| *md*
| Output buffer of at least EVP_MAX_MD_SIZE bytes, or NULL to use a static buffer (not thread‐safe).
| *md_len*
| Receives the MAC length in bytes; may be NULL.
|===


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