EVP_MAC-KMAC(7ossl)

📦
version 15.1 · 3 versions
, in freebsd 15.1

💅 EVP_MAC-KMAC — the KMAC glow-up, explained

okay so bestie, this is a page from the OpenSSL manual set (section 7ossl), and it's all about EVP_MAC-KMAC, EVP_MAC-KMAC128, and EVP_MAC-KMAC256 — the three KMAC EVP_MAC implementations. pull up a chair, we're getting into it 💖

✨ NAME ✨

EVP_MAC-KMAC, EVP_MAC-KMAC128, EVP_MAC-KMAC256 — the KMAC EVP_MAC implementations, aka the whole squad 💅

🦋 DESCRIPTION 🦋

So basically: this is the support for computing KMAC MACs through the EVP_MAC API. that's the vibe, that's the whole thing 🌸

🎀 Identity 🎀

These implementations are identified with one of these names and properties, to be used with EVP_MAC_fetch():

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

like, that's their whole government name, no cap 🫶

⭐ Supported parameters ⭐

The general description of these parameters can be found in "PARAMETERS" in EVP_MAC(3) — go read that for the full lore, iykyk.

All these parameters (except for "block-size") can be set with EVP_MAC_CTX_set_params(). Furthermore, 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(). okay that's a lot of getters and setters but like, it's fine, you got this 🧚

  • "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. yes, that range exactly, don't try to be cute about it 😭
  • "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. so if you don't set it, you get the empty-string experience 💀
  • "size" (OSSL_MAC_PARAM_SIZE) <unsigned integer> — sets the MAC size. by default, it's 32 for "KMAC-128" and 64 for "KMAC-256". iconic defaults, honestly 👑
  • "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". note the word gets bestie, this one is read-only energy 🙈
  • "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.
  • "fips-indicator" (OSSL_MAC_PARAM_FIPS_APPROVED_INDICATOR) <int> — this settable parameter is described in provider-mac(7). that's it, that's the whole entry, go read the other page ✨
  • "no-short-mac" (OSSL_MAC_PARAM_FIPS_NO_SHORT_MAC) <integer> — this settable parameter is described in provider-mac(7). it is used by the OpenSSL FIPS provider and the minimum length output for KMAC is defined by NIST's SP 800-185 8.4.2. so yes, there's a whole federal document about why your MAC can't be tiny 🫡
  • "key-check" (OSSL_MAC_PARAM_FIPS_KEY_CHECK) <integer> — this settable parameter is described in provider-mac(7). (again, other page, go look 🔥)
⚠️ okay so the timing rules are lowkey the most important part, don't skip this:

The "custom" and "no-short-mac" parameters 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. so, like, know your deadlines 💅

🔥 EXAMPLES 🔥

here's the actual code, verbatim, no edits, she's perfect as she is:

#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 the goto err pattern? that's the classic C cleanup choreography, she ate and left no crumbs 🍓

🌈 SEE ALSO 🌈

go touch these pages, bestie, they're all linked up:

Copyright 2018-2024 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 🫶 this was OpenSSL 3.5.6, dated 2026-04-07 — KMAC squad, signing off 💜✨🎀

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