EVP_MAC-KMAC(7ossl)

📦 version 14.5, in freebsd 14.5

✨ NAME ✨

Okay bestie, welcome to the EVP_MAC-KMAC(7ossl) page from OpenSSL. We’re talking EVP_MAC-KMAC, EVP_MAC-KMAC128, and EVP_MAC-KMAC256 — the KMAC EVP_MAC implementations. 💅

🌸 DESCRIPTION 🌸

This is support for computing KMAC MACs through the EVP_MAC API. Periodt. ✨

🪪 Identity

These implementations get identified with one of these names and properties, and that’s what you use with EVP_MAC_fetch():

  • "KMAC-128" with "provider=default" or "provider=fips"
  • "KMAC-256" with "provider=default" or "provider=fips"

🎛️ Supported parameters

The general description of these parameters lives in "PARAMETERS" in EVP_MAC(3). Not me making it up — that’s the spot. 💖

All these parameters except for "block-size" can be set with EVP_MAC_CTX_set_params(). Also, the "size" parameter can be retrieved with EVP_MAC_CTX_get_params(), or with EVP_MAC_CTX_get_mac_size(). The length of the "size" parameter should not exceed that of a size_t. Likewise, the "block-size" parameter can be retrieved with EVP_MAC_CTX_get_params(), or with EVP_MAC_CTX_get_block_size().

  • "key" (OSSL_MAC_PARAM_KEY) <octet string> — Sets the MAC key. Setting this parameter is identical to passing a key to EVP_MAC_init(3). The length of the key (in bytes) must be in the range 4...512.
  • "custom" (OSSL_MAC_PARAM_CUSTOM) <octet string> — Sets the customization string. It’s an optional value with a length of at most 512 bytes, and it’s empty by default.
  • "size" (OSSL_MAC_PARAM_SIZE) <unsigned integer> — Sets the MAC size. By default, it’s 32 for "KMAC-128" and 64 for "KMAC-256".
  • "block-size" (OSSL_MAC_PARAM_BLOCK_SIZE) <unsigned integer> — Gets the MAC block size. It’s 168 for "KMAC-128" and 136 for "KMAC-256".
  • "xof" (OSSL_MAC_PARAM_XOF) <integer> — The "xof" parameter value is expected to be 1 or 0. Use 1 to enable XOF mode. The default value is 0.

Timing check, bestie: the "custom" parameter must be set as part of or before the EVP_MAC_init() call. The "xof" and "size" parameters can be set at any time before EVP_MAC_final(). The "key" parameter is set as part of the EVP_MAC_init() call, but can be set before it instead. 🫶

🔥 EXAMPLES 🔥

Here’s the example exactly as the page gives it — full code, no skipping. 😭✨

#include <openssl/evp.h>
#include <openssl/params.h>

static int do_kmac(const unsigned char *in, size_t in_len,
                   const unsigned char *key, size_t key_len,
                   const unsigned char *custom, size_t custom_len,
                   int xof_enabled, unsigned char *out, int out_len)
{
    EVP_MAC_CTX *ctx = NULL;
    EVP_MAC *mac = NULL;
    OSSL_PARAM params[4], *p;
    int ret = 0;
    size_t l = 0;

    mac = EVP_MAC_fetch(NULL, "KMAC-128", NULL);
    if (mac == NULL)
        goto err;
    ctx = EVP_MAC_CTX_new(mac);
    /* The mac can be freed after it is used by EVP_MAC_CTX_new */
    EVP_MAC_free(mac);
    if (ctx == NULL)
        goto err;

    /*
     * Setup parameters required before calling EVP_MAC_init()
     * The parameters OSSL_MAC_PARAM_XOF and OSSL_MAC_PARAM_SIZE may also be
     * used at this point.
     */
    p = params;
    *p++ = OSSL_PARAM_construct_octet_string(OSSL_MAC_PARAM_KEY,
                                             (void *)key, key_len);
    if (custom != NULL && custom_len != 0)
      *p++ = OSSL_PARAM_construct_octet_string(OSSL_MAC_PARAM_CUSTOM,
                                               (void *)custom, custom_len);
    *p = OSSL_PARAM_construct_end();
    if (!EVP_MAC_CTX_set_params(ctx, params))
        goto err;

    if (!EVP_MAC_init(ctx))
        goto err;

    /*
     * Note: the following optional parameters can be set any time
     * before EVP_MAC_final().
     */
    p = params;
    *p++ = OSSL_PARAM_construct_int(OSSL_MAC_PARAM_XOF, &xof_enabled);
    *p++ = OSSL_PARAM_construct_int(OSSL_MAC_PARAM_SIZE, &out_len);
    *p = OSSL_PARAM_construct_end();
    if (!EVP_MAC_CTX_set_params(ctx, params))
        goto err;

    /* The update may be called multiple times here for streamed input */
    if (!EVP_MAC_update(ctx, in, in_len))
        goto err;
    if (!EVP_MAC_final(ctx, out, &l, out_len))
        goto err;
    ret = 1;
err:
    EVP_MAC_CTX_free(ctx);
    return ret;
}

👀 SEE ALSO 👀

Copyright 2018-2021 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

And that’s the whole page, bestie — OpenSSL 3.0.21, dated 2026-06-09, manual EVP_MAC-KMAC(7ossl). Slay. 💖

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