EVP_KEYEXCH-DH(7SSL)

📦
version 4.0.3 · 13 versions
, in ubuntu stonking

💖 EVP_KEYEXCH-DH — the DH key exchange glow-up ✨

okay bestie, gather round, because this is the EVP_KEYEXCH-DH page from the OpenSSL manual set (it lives in section 7SSL, so yes it's one of the provider-flavoured ones 🫶). It's giving "key exchange support, but make it documentation."

🌸 NAME 🌸

EVP_KEYEXCH-DH — DH Key Exchange algorithm support. that's it, that's the name, short and slay 💅

✨ DESCRIPTION ✨

So like, this is the key exchange support for the DH and DHX key types. two key types, one pretty little page 🦋

no cap, important: even though both key types support the same key exchange operations, they absolutely cannot be used together in a single key exchange. it is not possible to use a private key of the DH type in key exchange with the public key of DHX type, and vice versa. they're like two friend groups that simply do not mix. periodt 💀

🔐 DH and DHX key exchange parameters 🔐

okay here's the parameter menu, and every single one is staying, no skipping, we don't do that here 👑

  • "pad" (OSSL_EXCHANGE_PARAM_PAD) <unsigned integer> — sets the padding mode for the associated key exchange ctx. set the value 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 😭. if padding is on, then the derived shared secret will have its first bytes filled with zeros where necessary, so the shared secret ends up the same size as the largest possible secret size. and listen — this padding mode parameter gets ignored (and padding is implicitly enabled) when the KDF type is set to "X942KDF-ASN1" (OSSL_KDF_NAME_X942KDF_ASN1). so like, don't fight it, it's happening anyway 💅
  • "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 the above? they're covered over in "Common Key Exchange parameters" in provider-keyexch(7) — go read that bestie, iykyk 📖✨

  • "cekalg" (OSSL_KDF_PARAM_CEK_ALG) <octet string ptr> — this one's detailed in "KDF Parameters" in provider-kdf(7) 🎀

🌈 EXAMPLES 🌈

okay the examples here assume a host and a peer, and both of them generate keys using the same named group (or domain parameters). see "Examples" in EVP_PKEY-DH(7) for that part. and both the host and the peer transfer their public key to each other, like a little handshake friendship bracelet exchange 🫶

🦋 converting the peer's key pair to a public key in DER format 🦋

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 on the host 💜

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);
}

and bestie — very similar code can be used by the peer to derive that exact same shared secret, using the host's public key and the peer's generated key pair. it's giving symmetry, it's giving mutual, it's giving ✨iconic✨ 😭

👀 SEE ALSO 👀

(yes the comma at the end is staying, it's part of the vibe, we don't edit the ancestors 💅)

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>.

okay that's the whole page, you ate it, you understood it, you're basically a cryptography main character now 👑💖

version 4.0.3 · dated 2026-09-29 · EVP_KEYEXCH-DH(7SSL) — mwah, go derive something safely bestie 🎀🦋✨

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