EVP_KDF-KRB5KDF(7ossl)

💖 NAME 💖

Okay bestie, so this one is the <rainbow>EVP_KDF-KRB5KDF</rainbow> page from the OpenSSL 7ossl manual — it's all about The RFC3961 Krb5 KDF EVP_KDF implementation, fr fr. It's giving "crypto key derivation but make it Kerberos" energy and honestly it ate.

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🌸 DESCRIPTION 🌸

So like, this is support for computing the KRB5KDF KDF through the EVP_KDF API — no cap.

The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined in RFC 3961, section 5.1 and is used by Krb5 to derive session keys. To do key derivation you need three inputs, periodt: a cipher (for example AES-128-CBC), the initial key, and a constant. That's the whole vibe check.

🦋 Identity 🦋

"KRB5KDF" is the name for this implementation — it's literally the main character. It can be used with the EVP_KDF_fetch() function. Slay.

✨ Supported parameters ✨

The supported parameters are:

• "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
• "cipher" (OSSL_KDF_PARAM_CIPHER) <UTF8 string>
• "key" (OSSL_KDF_PARAM_KEY) <octet string>

These parameters work as described in "PARAMETERS" in EVP_KDF(3). Iconic behavior, honestly.

• "constant" (OSSL_KDF_PARAM_CONSTANT) <octet string>

This parameter sets the constant value for the KDF. If a value is already set, the contents are replaced. Not me replacing your constant like it's nothing 💅

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🎀 NOTES 🎀

A context for KRB5KDF can be obtained by calling:

EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

The output length of the KRB5KDF derivation is specified via the keylen parameter to the EVP_KDF_derive(3) function, and MUST match the key length for the chosen cipher or an error is returned. Like, it's not negotiable bestie — that's the rule. Moreover, the constant's length must not exceed the block size of the cipher. Since the KRB5KDF output length depends on the chosen cipher, calling EVP_KDF_CTX_get_kdf_size(3) to obtain the requisite length returns the correct length only after the cipher is set. Prior to that EVP_MAX_KEY_LENGTH is returned. So the caller must allocate a buffer of the correct length for the chosen cipher, and pass that buffer to the EVP_KDF_derive(3) function along with that length. It's giving "read the room before you allocate" energy, honestly.

👑 Pro tip (from the man page, not from me): set your cipher first, then ask for the size. The order literally matters here.

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🧚 EXAMPLES 🧚

This example derives a key using the AES-128-CBC cipher — and yes it's giving "I know my C code is cute":

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char key[16] = "01234...";
unsigned char constant[] = "I'm a constant";
unsigned char out[16];
size_t outlen = sizeof(out);
OSSL_PARAM params[4], *p = params;

kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER,
                                        SN_aes_128_cbc,
                                        strlen(SN_aes_128_cbc));
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                         key, (size_t)16);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT,
                                         constant, strlen(constant));
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
    /* Error */

EVP_KDF_CTX_free(kctx);

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💜 CONFORMING TO 💜

RFC 3961 — she follows the rules, periodt.

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🌈 SEE ALSO 🌈

EVP_KDF(3), EVP_KDF_CTX_free(3), EVP_KDF_CTX_get_kdf_size(3), EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3) — go read those too bestie, they're all in the group chat 👑

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🌸 HISTORY 🌸

This functionality was added in OpenSSL 3.0 — so it's not a new drop but it's still iconic.

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🫶 COPYRIGHT 🫶

Copyright 2016-2021 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>.

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And that's the whole vibe, bestie! Page was brought to you by OpenSSL 3.5.6, dated 2026-04-07, manual page EVP_KDF-KRB5KDF(7ossl) — love that for us 💖✨