EVP_KDF-KRB5KDF(7ossl)

💅 EVP_KDF-KRB5KDF (the 7ossl girlies' manual) ✨

okay so bestie, welcome to the OpenSSL corner of the group chat — this one's about EVP_KDF-KRB5KDF, and ngl it's lowkey iconic if you're into Krb5. 🎀

🌸 NAME 🌸

EVP_KDF-KRB5KDF — the RFC3961 Krb5 KDF EVP_KDF implementation, periodt.

✨ DESCRIPTION ✨

So basically there's support for computing the KRB5KDF KDF through the EVP_KDF API. Slay. 💖

The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined in RFC 3961, section 5.1, and it's what Krb5 uses to derive session keys. Three inputs are required to perform key derivation: a cipher (for example AES-128-CBC), the initial key, and a constant. That's the whole trio, no cap. 🫶

🦋 Identity 🦋

"KRB5KDF" is the name for this implementation — it can be used with the EVP_KDF_fetch() function. iykyk. ⭐

🧚 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). literally just go read that section, it ate. 💜
• "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. 🎀

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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 it MUST match the key length for the chosen cipher or an error is returned. Like, that's not a suggestion bestie, it's giving hard requirement. 💀

Moreover, the constant's length must not exceed the block size of the cipher.

And 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. Main character memory management energy only. 👑🦋

bestie pls don't skip the "set the cipher first" step, ok? the size function is literally not psychic. 😭

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🌈 EXAMPLES 🌈

This example derives a key using the AES-128-CBC cipher:

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);

look at her go. 🍓🫶

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🎀 CONFORMING TO 🎀

RFC 3961. that's it, that's the flex. ⭐

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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)

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💜 HISTORY 💜

This functionality was added in OpenSSL 3.0. she's new-ish, she's fresh, she's serving. 💅

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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>.

fr fr read the license before you go copying things, it's giving legally binding 💀

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and that's the whole page bestie!! 🧚✨ this lil rewrite was brought to you by the 3.5.9 manual, dated 2026-09-29 — mwah, go derive some keys responsibly 😭💖🌈