EVP_KDF-HKDF(7ossl)

📦

version 3.5.9 · 8 versions

3.5.9 · alpine 3.23, alpine 3.24 (latest)
3.5.9 · centos-stream 9
3.5.9 · centos-stream 10
3.5.9 · alpine 3.22
3.5.8 · fedora 43
3.5.1 · fedora 43
3.5.9 · fedora 43, fedora 44
3.5.5 · fedora 44

💖 EVP_KDF-HKDF(7ossl) 💖

okay so bestie, you're looking at the 7ossl manual page for EVP_KDF-HKDF and honestly? it's giving key-derivation royalty. let's get into it 🫶

✨ NAME ✨

EVP_KDF-HKDF — the HKDF EVP_KDF implementation, and yes it's as iconic as it sounds 💅

🌸 DESCRIPTION 🌸

Support for computing the HKDF KDF through the EVP_KDF API. that's the whole vibe, but like, let me break it down properly 🧚

The EVP_KDF-HKDF algorithm implements the HKDF key derivation function. HKDF follows the "extract-then-expand" paradigm, where the KDF logically consists of two modules. The first stage takes the input keying material and "extracts" from it a fixed-length pseudorandom key K. The second stage "expands" the key K into several additional pseudorandom keys (the output of the KDF). like two little modules doing their own thing and then slaying together 🔥

The output is considered to be keying material.

👑 Identity

"HKDF" is the name for this implementation; it can be used with the EVP_KDF_fetch() function. that's her government name, remember it 💜

🎀 Supported parameters

The supported parameters are:

"properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
"digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
"key" (OSSL_KDF_PARAM_KEY) <octet string>
"salt" (OSSL_KDF_PARAM_SALT) <octet string>

These parameters work as described in "PARAMETERS" in EVP_KDF(3). no extra lore needed, go read that one 💖

"info" (OSSL_KDF_PARAM_INFO) <octet string>

This parameter sets the info value. The length of the context info buffer cannot exceed 1024 bytes; this should be more than enough for any normal use of HKDF. like, 1024 is genuinely generous, don't push it bestie 🙈

"mode" (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>

This parameter sets the mode for the HKDF operation. There are three modes that are currently defined:

"EXTRACT_AND_EXPAND" or EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND

This is the default mode. Calling EVP_KDF_derive(3) on an EVP_KDF_CTX set up for HKDF will perform an extract followed by an expand operation in one go. The derived key returned will be the result after the expand operation. The intermediate fixed-length pseudorandom key K is not returned. main character mode fr, does both steps and doesn't even show you the middle 💅

In this mode the digest, key, salt and info values must be set before a key is derived otherwise an error will occur. no skipping steps, the vibe check will fail 😭

"EXTRACT_ONLY" or EVP_KDF_HKDF_MODE_EXTRACT_ONLY

In this mode calling EVP_KDF_derive(3) will just perform the extract operation. The value returned will be the intermediate fixed-length pseudorandom key K. The keylen parameter must match the size of K, which can be looked up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest. it's giving one-job-only and honestly, respect 🎀

The digest, key and salt values must be set before a key is derived otherwise an error will occur.

"EXPAND_ONLY" or EVP_KDF_HKDF_MODE_EXPAND_ONLY

In this mode calling EVP_KDF_derive(3) will just perform the expand operation. The input key should be set to the intermediate fixed-length pseudorandom key K returned from a previous extract operation. so like, you gotta have done the first part already, no skipping ahead bestie ✨

The digest, key and info values must be set before a key is derived otherwise an error will occur.

The OpenSSL FIPS provider also supports the following parameters:

"fips-indicator" (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer>

A getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This may be used after calling EVP_KDF_derive. It returns 0 if "key-check" is set to 0 and the check fails. she's basically a vibe-check but for FIPS 🔥

"key-check" (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer>

The default value of 1 causes an error during EVP_KDF_CTX_set_params() if the length of used key-derivation key (OSSL_KDF_PARAM_KEY) is shorter than 112 bits. Setting this to zero will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0.

⚠️ bestie please read that last line again. setting "key-check" to 0 and getting a 0 indicator? that's a compliance L. do not do that casually 💀

🦋 NOTES 🦋

A context for HKDF can be obtained by calling:

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

The output length of an HKDF expand operation is specified via the keylen parameter to the EVP_KDF_derive(3) function. When using EVP_KDF_HKDF_MODE_EXTRACT_ONLY the keylen parameter must equal the size of the intermediate fixed-length pseudorandom key otherwise an error will occur. For that mode, the fixed output size can be looked up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest on the EVP_KDF_CTX. get the size first, then derive, in that order, no cap 🌈

💀 EXAMPLES 💀

This example derives 10 bytes using SHA-256 with the secret key "secret", salt value "salt" and info value "label". look at her go 🍓

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char out[10];
OSSL_PARAM params[5], *p = params;

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

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                        SN_sha256, strlen(SN_sha256));
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                         "secret", (size_t)6);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                         "label", (size_t)5);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                         "salt", (size_t)4);
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    error("EVP_KDF_derive");
}

EVP_KDF_CTX_free(kctx);

and that's it, that's the whole slay ✨

🧚 CONFORMING TO 🧚

RFC 5869 — she follows the rules, unlike some people 😌

🌟 SEE ALSO 🌟

EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3), EVP_KDF_CTX_get_kdf_size(3), EVP_KDF_CTX_set_params(3), EVP_KDF_derive(3), "PARAMETERS" in EVP_KDF(3), EVP_KDF-TLS13_KDF(7) — go read the whole friend group 💖

🌈 HISTORY 🌈

This functionality was added in OpenSSL 3.0. she's been here a minute and she's still thriving 👑

💜 COPYRIGHT 💜

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 page bestie!! 🎀 this one's from 3.5.9, dated 2026-09-29, and it's the EVP_KDF-HKDF(7ossl) manual — go forth and derive those keys responsibly 💅✨🫶

📦 where this page comes from

alpine 3.23: openssl-doc 3.5.9-r0 · Apache-2.0
alpine 3.24: openssl-doc 3.5.9-r0 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms