DH_GENERATE_KEY(3ossl)

✨ DH_GENERATE_KEY(3ossl): OpenSSL Diffie-Hellman key exchange, but make it make sense ✨

okay bestie, welcome to the DH_GENERATE_KEY(3ossl) page 💅 this is the OpenSSL lore for DH_generate_key, DH_compute_key, and DH_compute_key_padded. It’s giving classic Diffie-Hellman key exchange, but also it’s giving “these are deprecated, so pls read the warning before you get attached.”

🎀 NAME 🎀

DH_generate_key, DH_compute_key, DH_compute_key_padded — perform Diffie-Hellman key exchange. Periodt.

🦋 SYNOPSIS 🦋

First, the include, because we love a proper header:

#include <openssl/dh.h>

🚨 The following functions have been deprecated since OpenSSL 3.0, and can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

int DH_generate_key(DH *dh);

int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh);

int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh);

💖 DESCRIPTION 💖

⚠️ All of the functions described on this page are deprecated. Applications should instead use EVP_PKEY_derive_init(3) and EVP_PKEY_derive(3).

DH_generate_key() performs the first step of a Diffie-Hellman key exchange by generating private and public DH values. By calling DH_compute_key() or DH_compute_key_padded(), these are combined with the other party’s public value to compute the shared key. iconic behavior, honestly.

• DH_generate_key() — expects dh to contain the shared parameters dh->p and dh->g. It generates a random private DH value unless dh->priv_key is already set, and computes the corresponding public value dh->pub_key, which can then be published. very main character energy.
• DH_compute_key() — computes the shared secret from the private DH value in dh and the other party’s public value in pub_key and stores it in key. key must point to DH_size(dh) bytes of memory. The padding style is RFC 5246 (8.1.2) that strips leading zero bytes. It is not constant time due to the leading zero bytes being stripped. The return value should be considered public. no cap, that timing detail matters.
• DH_compute_key_padded() — is similar but stores a fixed number of bytes. The padding style is NIST SP 800-56A (C.1) that retains leading zero bytes. It is constant time due to the leading zero bytes being retained. The return value should be considered public. she’s giving fixed-size and constant-time 🫶

🌈 RETURN VALUES 🌈

• DH_generate_key() returns 1 on success, 0 otherwise.
• DH_compute_key() returns the size of the shared secret on success, -1 on error.
• DH_compute_key_padded() returns DH_size(dh) on success, -1 on error.

The error codes can be obtained by ERR_get_error(3). ty bestie 🔥

👑 SEE ALSO 👑

EVP_PKEY_derive(3), DH_new(3), ERR_get_error(3), RAND_bytes(3), DH_size(3)

🦋 HISTORY 🦋

• DH_compute_key_padded() was added in OpenSSL 1.0.2.
• All of these functions were deprecated in OpenSSL 3.0.

💜 COPYRIGHT 💜

Copyright 2000-2021 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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mwah 💋 this page’s footer says OpenSSL version 3.5.6, dated 2026-04-07 — and that’s the whole vibe check, bestie 🎀