EVP_KEYEXCH-DH(7ossl)

📦
version 15.1 · 3 versions
, in freebsd 15.1

✨ EVP_KEYEXCH-DH ✨

okay so bestie, welcome to the EVP_KEYEXCH-DH page from the 7ossl manual — like, OpenSSL's own little corner of the docs 💅 this one is literally all about DH Key Exchange algorithm support, and ngl it's kind of iconic if you're into that sort of thing 🦋

💖 NAME 💖

EVP_KEYEXCH-DH — DH Key Exchange algorithm support, periodt ✨

🌸 DESCRIPTION 🌸

Key exchange support for the DH and DHX key types. That's the whole vibe 🔥

⚠️ okay important thing, listen up: even though both key types support the same key exchange operations, they cannot be used together in a single key exchange. Not possible. Like, you literally can't use a private key of the DH type in key exchange with the public key of the DHX type, and vice versa. They don't mix, bestie, it's giving incompatible friend groups 💀

🎀 DH and DHX key exchange parameters 🎀

  • "pad" (OSSL_EXCHANGE_PARAM_PAD) <unsigned integer> — sets the padding mode for the associated key exchange ctx 📐 set it to 1 and padding is on, set it to 0 and padding is off. If padding is off then the derived shared secret may be smaller than the largest possible secret size (so, like, shorter than expected 😭). If padding is on then the derived shared secret will have its first bytes filled with zeros where necessary to make the shared secret the same size as the largest possible secret size — very "make it fit" of it honestly 💖 also! the padding mode parameter is ignored (and padding is implicitly enabled) when the KDF type is set to "X942KDF-ASN1" (OSSL_KDF_NAME_X942KDF_ASN1). Just so you know 👑
  • "kdf-type" (OSSL_EXCHANGE_PARAM_KDF_TYPE) <UTF8 string>
  • "kdf-digest" (OSSL_EXCHANGE_PARAM_KDF_DIGEST) <UTF8 string>
  • "kdf-digest-props" (OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS) <UTF8 string>
  • "kdf-outlen" (OSSL_EXCHANGE_PARAM_KDF_OUTLEN) <unsigned integer>
  • "kdf-ukm" (OSSL_EXCHANGE_PARAM_KDF_UKM) <octet string>
  • "fips-indicator" (OSSL_EXCHANGE_PARAM_FIPS_APPROVED_INDICATOR) <integer>
  • "key-check" (OSSL_EXCHANGE_PARAM_FIPS_KEY_CHECK) <integer>
  • "digest-check" (OSSL_EXCHANGE_PARAM_FIPS_DIGEST_CHECK) <integer> — all of these cuties? see "Common Key Exchange parameters" in provider-keyexch(7) 🫶
  • "cekalg" (OSSL_KDF_PARAM_CEK_ALG) <octet string ptr> — and for this one, see "KDF Parameters" in provider-kdf(7) 🌈

⭐ EXAMPLES ⭐

The examples assume a host and peer both generate keys using the same named group (or domain parameters). See "Examples" in EVP_PKEY-DH(7) 👀 Both the host and peer transfer their public key to each other — it's a whole mutual exchange moment, we love to see it 🎀

🦋 converting the peer's key pair to DER 🦋

To convert the peer's generated key pair to a public key in DER format in order to transfer to the host:

EVP_PKEY *peer_key; /* It is assumed this contains the peers generated key */
unsigned char *peer_pub_der = NULL;
int peer_pub_der_len;

peer_pub_der_len = i2d_PUBKEY(peer_key, &peer_pub_der);
...
OPENSSL_free(peer_pub_der);

🌸 converting the received public key from DER on the host 🌸

To convert the received peer's public key from DER format on the host:

const unsigned char *pd = peer_pub_der;
EVP_PKEY *peer_pub_key = d2i_PUBKEY(NULL, &pd, peer_pub_der_len);
...
EVP_PKEY_free(peer_pub_key);

💜 deriving the shared secret 💜

To derive a shared secret on the host using the host's key and the peer's public key:

/* It is assumed that the host_key and peer_pub_key are set up */
void derive_secret(EVP_KEY *host_key, EVP_PKEY *peer_pub_key)
{
    unsigned int pad = 1;
    OSSL_PARAM params[2];
    unsigned char *secret = NULL;
    size_t secret_len = 0;
    EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);

    EVP_PKEY_derive_init(dctx);

    /* Optionally set the padding */
    params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
    params[1] = OSSL_PARAM_construct_end();
    EVP_PKEY_CTX_set_params(dctx, params);

    EVP_PKEY_derive_set_peer(dctx, peer_pub_key);

    /* Get the size by passing NULL as the buffer */
    EVP_PKEY_derive(dctx, NULL, &secret_len);
    secret = OPENSSL_zalloc(secret_len);

    EVP_PKEY_derive(dctx, secret, &secret_len);
    ...
    OPENSSL_clear_free(secret, secret_len);
    EVP_PKEY_CTX_free(dctx);
}

Very similar code can be used by the peer to derive the same shared secret using the host's public key and the peer's generated key pair — like, mirror it, it's giving symmetry 💅✨

🔥 SEE ALSO 🔥

EVP_PKEY-DH(7), EVP_PKEY-FFC(7), EVP_PKEY(3), provider-keyexch(7), provider-keymgmt(7), OSSL_PROVIDER-default(7), OSSL_PROVIDER-FIPS(7),

(yes, that trailing comma is literally in the original, we don't gatekeep 🫶)

Copyright 2020-2024 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>.

aaaand that's the page bestie, you ate that 🍓 this was OpenSSL 3.5.6, dated 2026-04-07, straight from the EVP_KEYEXCH-DH(7ossl) manual — go forth and derive those secrets responsibly, mwah 💖🧚‍♀️

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