EVP_KEYEXCH-ECDH(7ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 EVP_KEYEXCH-ECDH 💅

okay so this is the EVP_KEYEXCH-ECDH page from the OpenSSL manual, section 7ossl — and bestie, it's literally all about ECDH Key Exchange algorithm support 🦋✨

🌸 NAME 🌸

EVP_KEYEXCH-ECDH — ECDH Key Exchange algorithm support. that's it, that's the whole name, she's concise 💅

✨ DESCRIPTION ✨

so like, this is the key exchange support for the ECDH key type. main character energy for sure 👑

🎀 ECDH Key Exchange parameters 🎀

here's the lineup of params, no cap:

  • "ecdh-cofactor-mode" (OSSL_EXCHANGE_PARAM_EC_ECDH_COFACTOR_MODE) <integer> — this one sets or gets the ECDH mode of operation for the associated key exchange ctx. 🌀

in the context of an Elliptic Curve Diffie-Hellman key exchange, this parameter can be used to select between the plain Diffie-Hellman (DH) or Cofactor Diffie-Hellman (CDH) variants of the key exchange algorithm.

when setting, the value should be 1, 0 or -1, respectively forcing cofactor mode on, off, or resetting it to the default for the private key associated with the given key exchange ctx.

when getting, the value should be either 1 or 0, respectively signaling if the cofactor mode is on or off.

see also provider-keymgmt(7) for the related OSSL_PKEY_PARAM_USE_COFACTOR_ECDH parameter that can be set on a per-key basis. iykyk 💫

  • "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>

the OpenSSL FIPS provider also supports the following parameters, fyi 🧚:

  • "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>

→ see "Common Key Exchange parameters" in provider-keyexch(7) for these three 🫶

  • "ecdh-cofactor-check" (OSSL_EXCHANGE_PARAM_FIPS_ECDH_COFACTOR_CHECK) <integer> — if required, this parameter should before OSSL_FUNC_keyexch_derive(). the default value of 1 causes an error during the OSSL_FUNC_keyexch_derive if the EC curve has a cofactor that is not 1, and the cofactor is not used. setting this to 0 will ignore the error and set the approved "fips-indicator" to 0.
⚠️ ok listen — this option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0. not me being dramatic, that's literally what it says 💀

🔥 EXAMPLES 🔥

examples of key agreement can be found in demos/keyexch. go look, it's giving tutorial slay ✨

keys for the host and peer must be generated as shown in "Examples" in EVP_PKEY-EC(7) using the same curve name. same curve, bestie, otherwise it's a whole mess 😭

the code to generate a shared secret for the normal case is identical to "Examples" in EVP_KEYEXCH-DH(7).

to derive a shared secret on the host using the host's key and the peer's public key but also using X963KDF with a user key material:

/* It is assumed that the host_key, peer_pub_key and ukm are set up */
void derive_secret(EVP_PKEY *host_key, EVP_PKEY *peer_key,
                   unsigned char *ukm, size_t ukm_len)
{
    unsigned char secret[64];
    size_t out_len = sizeof(secret);
    size_t secret_len = out_len;
    unsigned int pad = 1;
    OSSL_PARAM params[6];
    EVP_PKEY_CTX *dctx = EVP_PKEY_CTX_new_from_pkey(NULL, host_key, NULL);

    EVP_PKEY_derive_init(dctx);

    params[0] = OSSL_PARAM_construct_uint(OSSL_EXCHANGE_PARAM_PAD, &pad);
    params[1] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_TYPE,
                                                 "X963KDF", 0);
    params[2] = OSSL_PARAM_construct_utf8_string(OSSL_EXCHANGE_PARAM_KDF_DIGEST,
                                                 "SHA1", 0);
    params[3] = OSSL_PARAM_construct_size_t(OSSL_EXCHANGE_PARAM_KDF_OUTLEN,
                                            &out_len);
    params[4] = OSSL_PARAM_construct_octet_string(OSSL_EXCHANGE_PARAM_KDF_UKM,
                                                  ukm, ukm_len);
    params[5] = OSSL_PARAM_construct_end();
    EVP_PKEY_CTX_set_params(dctx, params);

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

👀 SEE ALSO 👀

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

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>.

and that's the whole page, bestie 🫶✨ this one's version 3.5.0, dated 2026-09-25, and it lives at EVP_KEYEXCH-ECDH(7ossl) — go forth and derive those secrets 💅🎀🌈

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