% limb manual
% hmac(3)
% limb 0.1.0
% 2023-07-24

# NAME

hmac - compute the HMAC of a given message using given hasher and key

# SYNOPSIS

    #include <limb/hmac.h>

```pre hl
void hmac(char *<em>out</em>, hasher *<em>hr</em>, const void *<em>key</em>, size_t <em>klen</em>, const void *<em>msg</em>, size_t <em>mlen</em>)
```

# DESCRIPTION

The `hmac`() function calculates the HMAC of message `msg` of length `mlen`,
using the secret key `key` of length `klen` and the cryptographic hasher `hr`,
as described in [RFC 2104][rfc2104].

The results if then copied into the buffer pointed by `out` which must be able
to hold at least the length of a message digest from the hash function used,
i.e. `hr->hlen` bytes.

For a list of available hashers, refer to [hasher_hash](3).

[rfc2104]: https://datatracker.ietf.org/doc/html/rfc2104

# EXAMPLE

```c
#include <limb/hmac.h>
#include <limb/hasher_sha1.h>
#include <limb/output.h>

int main(void)
{
    const char *msg = "The quick brown fox jumps over the lazy dog";
    const char *key = "key";
    char out[sha1->hlen];

    hmac(out, sha1, key, 3, msg, strlen(msg));
    out("HMAC-SHA1(\"key\", \"", msg, "\") = ", HEX(out, sizeof(out)));

    return 0;
}
```

# SEE ALSO

[pbkdf2](3), [poly1305_init](3)