EVP_PKEY_DERIVE(3ossl)

✨ EVP_PKEY_DERIVE(3ossl) ✨

okay so bestie, we're doing EVP_PKEY_DERIVE(3ossl) today — it's the OpenSSL manual page for deriving a shared secret with public key algorithms, and it's literally so useful. no cap. 💅

🌸 NAME 🌸

• EVP_PKEY_derive_init, EVP_PKEY_derive_init_ex, EVP_PKEY_derive_set_peer_ex, EVP_PKEY_derive_set_peer, EVP_PKEY_derive — derive public key algorithm shared secret ✨

🦋 SYNOPSIS 🦋

#include <openssl/evp.h>

int EVP_PKEY_derive_init(EVP_PKEY_CTX *ctx);
int EVP_PKEY_derive_init_ex(EVP_PKEY_CTX *ctx, const OSSL_PARAM params[]);
int EVP_PKEY_derive_set_peer_ex(EVP_PKEY_CTX *ctx, EVP_PKEY *peer,
                                int validate_peer);
int EVP_PKEY_derive_set_peer(EVP_PKEY_CTX *ctx, EVP_PKEY *peer);
int EVP_PKEY_derive(EVP_PKEY_CTX *ctx, unsigned char *key, size_t *keylen);

💖 DESCRIPTION 💖

Okay so let's break it down. 💫

EVP_PKEY_derive_init() initializes a public key algorithm context ctx for shared secret derivation using the algorithm given when the context was created using EVP_PKEY_CTX_new(3) or variants thereof. The algorithm is used to fetch a EVP_KEYEXCH method implicitly, see "Implicit fetch" in provider(7) for more information about implicit fetches. slay. 💅

EVP_PKEY_derive_init_ex() is the same as EVP_PKEY_derive_init() but additionally sets the passed parameters params on the context before returning. lowkey extra, highkey useful. 🎀

EVP_PKEY_derive_set_peer_ex() sets the peer key: this will normally be a public key. The validate_peer will validate the public key if this value is non zero. so if validate_peer is non-zero, it's doing a whole background check on that peer key, no cap. 🔍

EVP_PKEY_derive_set_peer() is similar to EVP_PKEY_derive_set_peer_ex() with validate_peer set to 1. it's the default vibe. 🫶

EVP_PKEY_derive() derives a shared secret using ctx. If key is NULL then the maximum size of the output buffer is written to the keylen parameter. If key is not NULL then before the call the keylen parameter should contain the length of the key buffer, if the call is successful the shared secret is written to key and the amount of data written to keylen. so you literally pass NULL first to get the size, then allocate, then call again. iconic two-step. 🌈

🔥 NOTES 🔥

After the call to EVP_PKEY_derive_init(), algorithm specific control operations can be performed to set any appropriate parameters for the operation. like, you can tweak stuff before the actual derivation. ⚙️

The function EVP_PKEY_derive() can be called more than once on the same context if several operations are performed using the same parameters. so you can reuse the ctx, it's not a one-time thing. periodt. 💀

👑 RETURN VALUES 👑

EVP_PKEY_derive_init() and EVP_PKEY_derive() 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. so if you see -2, it's giving "not supported", and that's not a vibe. 😭

🍓 EXAMPLES 🍓

Derive shared secret (for example DH or EC keys):

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

EVP_PKEY_CTX *ctx;
ENGINE *eng;
unsigned char *skey;
size_t skeylen;
EVP_PKEY *pkey, *peerkey;
/* NB: assumes pkey, eng, peerkey have been already set up */

ctx = EVP_PKEY_CTX_new(pkey, eng);
if (!ctx)
    /* Error occurred */
if (EVP_PKEY_derive_init(ctx) <= 0)
    /* Error */
if (EVP_PKEY_derive_set_peer(ctx, peerkey) <= 0)
    /* Error */

/* Determine buffer length */
if (EVP_PKEY_derive(ctx, NULL, &skeylen) <= 0)
    /* Error */

skey = OPENSSL_malloc(skeylen);

if (!skey)
    /* malloc failure */

if (EVP_PKEY_derive(ctx, skey, &skeylen) <= 0)
    /* Error */

/* Shared secret is skey bytes written to buffer skey */

🧚 SEE ALSO 🧚

• EVP_PKEY_CTX_new(3)
• EVP_PKEY_encrypt(3)
• EVP_PKEY_decrypt(3)
• EVP_PKEY_sign(3)
• EVP_PKEY_verify(3)
• EVP_PKEY_verify_recover(3)
• EVP_KEYEXCH_fetch(3)

🌟 HISTORY 🌟

The EVP_PKEY_derive_init(), EVP_PKEY_derive_set_peer() and EVP_PKEY_derive() functions were originally added in OpenSSL 1.0.0. 📜

The EVP_PKEY_derive_init_ex() and EVP_PKEY_derive_set_peer_ex() functions were added in OpenSSL 3.0. so the _ex versions are newer, iykyk. ✨

💜 COPYRIGHT 💜

Copyright 2006-2022 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! this page is from OpenSSL 3.5.6, dated 2026-04-07, and it's literally iconic. bye bestie! 💖✨🦋