LIBRESSL-EVP_PKEY_CTX_SET_HKDF_MD(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

Bestie, welcome to the HKDF page — the EVP_PKEY_CTX_SET_HKDF_MD(3) one from the Library Functions Manual 💅. It’s giving “key derivation but make it two-stage,” and honestly? Iconic.

🎀 NAME 🎀

EVP_PKEY_CTX_set_hkdf_md, EVP_PKEY_CTX_set1_hkdf_salt, EVP_PKEY_CTX_set1_hkdf_key, EVP_PKEY_CTX_add1_hkdf_info, EVP_PKEY_CTX_hkdf_mode — HKDF, the HMAC-based Extract-and-Expand key derivation algorithm 🌈.

The whole squad:

  • EVP_PKEY_CTX_set_hkdf_md — sets the digest for HKDF.
  • EVP_PKEY_CTX_set1_hkdf_salt — sets/replaces the salt.
  • EVP_PKEY_CTX_set1_hkdf_key — sets/replaces the key.
  • EVP_PKEY_CTX_add1_hkdf_info — sets or appends the info value.
  • EVP_PKEY_CTX_hkdf_mode — picks the HKDF mode.

✨ SYNOPSIS ✨

You’re linking against libcrypto, bestie 💖. Here’s the exact synopsis vibe:

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

int
EVP_PKEY_CTX_hkdf_mode(EVP_PKEY_CTX *pctx, int mode);

int
EVP_PKEY_CTX_set_hkdf_md(EVP_PKEY_CTX *pctx, const EVP_MD *md);

int
EVP_PKEY_CTX_set1_hkdf_salt(EVP_PKEY_CTX *pctx, unsigned char *salt,
    int saltlen);

int
EVP_PKEY_CTX_set1_hkdf_key(EVP_PKEY_CTX *pctx, unsigned char *key,
    int keylen);

int
EVP_PKEY_CTX_add1_hkdf_info(EVP_PKEY_CTX *pctx, unsigned char *info,
    int infolen);

🦋 DESCRIPTION 🦋

The EVP_PKEY_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 — that’s the output of the KDF 👑

EVP_PKEY_CTX_hkdf_mode() sets the mode for the HKDF operation. There are three modes that are currently defined:

  • EVP_PKEY_HKDEF_MODE_EXTRACT_AND_EXPAND — This is the default mode. Calling EVP_PKEY_derive(3) on an EVP_PKEY_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.

In this mode the digest, key, salt and info values must be set before a key is derived or an error occurs.

  • EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY — In this mode calling EVP_PKEY_derive(3) will just perform the extract operation. The value returned will be the intermediate fixed-length pseudorandom key K.

The digest, key and salt values must be set before a key is derived or an error occurs.

  • EVP_PKEY_HKDEF_MODE_EXPAND_ONLY — In this mode calling EVP_PKEY_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.

The digest, key and info values must be set before a key is derived or an error occurs.

EVP_PKEY_CTX_set_hkdf_md() sets the message digest associated with the HKDF.

EVP_PKEY_CTX_set1_hkdf_salt() sets the salt to saltlen bytes of the buffer salt. Any existing value is replaced.

EVP_PKEY_CTX_set1_hkdf_key() sets the key to keylen bytes of the buffer key. Any existing value is replaced.

EVP_PKEY_CTX_add1_hkdf_info() sets the info value to infolen bytes of the buffer info. If a value is already set, it is appended to the existing value 🫶.

🌸 STRING CTRLS 🌸

HKDF also supports string based control operations via EVP_PKEY_CTX_ctrl_str(3).

  • The type parameter "md" uses the supplied value as the name of the digest algorithm to use.
  • The type parameter "mode" accepts "EXTRACT_AND_EXPAND", "EXTRACT_ONLY" and "EXPAND_ONLY" as value to determine the mode to use.
  • The type parameters "salt", "key" and "info" use the supplied value parameter as a seed, key, or info.
  • The names "hexsalt", "hexkey" and "hexinfo" are similar except they take a hex string which is converted to binary.

🧚 NOTES 🧚

All these functions are implemented as macros. Yes, macros — don’t @ me 💅

A context for HKDF can be obtained by calling:

EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);

The total length of the info buffer cannot exceed 1024 bytes in length: this should be more than enough for any normal use of HKDF.

The output length of an HKDF expand operation is specified via the length parameter to the EVP_PKEY_derive(3) function. Since the HKDF output length is variable, passing a NULL buffer as a means to obtain the requisite length is not meaningful with HKDF in any mode that performs an expand operation. Instead, the caller must allocate a buffer of the desired length, and pass that buffer to EVP_PKEY_derive(3) along with (a pointer initialized to) the desired length. Passing a NULL buffer to obtain the length is allowed when using EVP_PKEY_HKDEF_MODE_EXTRACT_ONLY.

💜 RETURN VALUES 💜

All these functions return 1 for success and 0 or a negative value for failure. In particular a return value of -2 indicates the operation is not supported by the public key algorithm.

🔥 EXAMPLES 🔥

This example derives 10 bytes using SHA-256 with the secret key "secret", salt value "salt" and info value "label":

EVP_PKEY_CTX *pctx;
unsigned char out[10];
size_t outlen = sizeof(out);

if ((pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL)) == NULL)
        /* Error */

if (EVP_PKEY_derive_init(pctx) <= 0)
        /* Error */
if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha256()) <= 0)
        /* Error */
if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, "salt", 4) <= 0)
        /* Error */
if (EVP_PKEY_CTX_set1_hkdf_key(pctx, "secret", 6) <= 0)
        /* Error */
if (EVP_PKEY_CTX_add1_hkdf_info(pctx, "label", 5) <= 0)
        /* Error */
if (EVP_PKEY_derive(pctx, out, &outlen) <= 0)
        /* Error */

👑 SEE ALSO 👑

🌈 STANDARDS 🌈

RFC 5869: HMAC-based Extract-and-Expand Key Derivation Function (HKDF)

⭐ HISTORY ⭐

EVP_PKEY_CTX_set_hkdf_md(), EVP_PKEY_CTX_set1_hkdf_salt(), EVP_PKEY_CTX_set1_hkdf_key(), and EVP_PKEY_CTX_add1_hkdf_info() first appeared in OpenSSL 1.1.0 and EVP_PKEY_CTX_hkdf_mode() in OpenSSL 1.1.1. These functions have been available since OpenBSD 7.2.

That’s the whole page, bestie! Footer vibe check: source/version Debian, date June 8, 2025, and the page is EVP_PKEY_CTX_SET_HKDF_MD(3) 💖🎀

📦 where this page comes from