EVP_KEYEXCH-DH(7ossl)

💅 NAME 💅

EVP_KEYEXCH-DH — it's the DH Key Exchange algorithm support page, bestie. This is the 7ossl manual, and yes we're gonna read the whole thing together 🫶

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✨ DESCRIPTION ✨

So basically this is the key exchange support for the DH and DHX key types. That's the whole vibe.

⚠️ Okay but listen — this is important. Even though both key types support the same key exchange operations, they literally cannot be used together in a single key exchange. Like, no. 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 don't mix, periodt. 💀

🌸 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. And what does that actually mean? If padding is off, the derived shared secret may be smaller than the largest possible secret size. If padding is on, the derived shared secret will have its first bytes filled with zeros where necessary, so it comes out the same size as the largest possible secret size. Slay. 🎀

Heads up: 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). So don't come for it, it's just like that.

• "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 eight of those cuties? See "Common Key Exchange parameters" in provider-keyexch(7). Iykyk 💜

• "cekalg" (OSSL_KDF_PARAM_CEK_ALG) — <octet string ptr>. See "KDF Parameters" in provider-kdf(7).

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🔥 EXAMPLES 🔥

Okay so these examples assume a host and peer both generate keys using the same named group (or domain parameters). Peep "Examples" in EVP_PKEY-DH(7) for that part. And both the host and peer transfer their public key to each other. Main character energy from both sides, honestly. ✨

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);

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);

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 the same shared secret using the host's public key and the peer's generated key pair. Symmetry, we love to see it 🦋

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👑 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),

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💖 COPYRIGHT 💖

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>.

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And that's the whole page, bestie 🎀 — this was OpenSSL 3.5.9, dated 2026-09-29, page EVP_KEYEXCH-DH(7ossl). Go derive some secrets responsibly, mwah 💋