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📛 NAME

EVP_KDF-SS - The Single Step / One Step EVP_KDF implementation

🚀 Quick Reference

Use Case	Command	Description
🔑 SHA‑256 derivation	EVP_KDF_derive(kctx, out, 10, params)	Digest=SHA‑256, key="secret", info="label"
🔐 HMAC‑SHA‑256 derivation	EVP_KDF_derive(kctx, out, 10, params)	MAC=HMAC, digest=SHA‑256, key="secret", salt="salt", info="label"
🛡️ KMAC128 derivation	EVP_KDF_derive(kctx, out, 10, params)	MAC=KMAC128, digest=SHA‑256, key="secret", salt="salt", maclen=20, info="label"

📖 DESCRIPTION

The EVP_KDF-SS algorithm implements the Single Step key derivation function (SSKDF). SSKDF derives a key using input such as a shared secret key (that was generated during the execution of a key establishment scheme) and fixedinfo. SSKDF is also informally referred to as 'Concat KDF'.

🧩 Auxiliary function

The implementation uses a selectable auxiliary function H, which can be one of:

• H(x) = hash(x, digest=md)
• H(x) = HMAC_hash(x, key=salt, digest=md)
• H(x) = KMACxxx(x, key=salt, custom="KDF", outlen=mac_size)

Both the HMAC and KMAC implementations set the key using the 'salt' value. The hash and HMAC also require the digest to be set.

🆔 Identity

"SSKDF" is the name for this implementation; it can be used with the EVP_KDF_fetch() function.

⚙️ Supported parameters

• 🏷️ "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
• 🧬 "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>
• 🔐 "mac" (OSSL_KDF_PARAM_MAC) <UTF8 string>
• 📏 "maclen" (OSSL_KDF_PARAM_MAC_SIZE) <unsigned integer>
• 🧂 "salt" (OSSL_KDF_PARAM_SALT) <octet string>

These parameters work as described in "PARAMETERS" in EVP_KDF(3).

• 🔑 "key" (EVP_KDF_CTRL_SET_KEY) <octet string> — This parameter set the shared secret that is used for key derivation.
• ℹ️ "info" (OSSL_KDF_PARAM_INFO) <octet string> — This parameter sets an optional value for fixedinfo, also known as otherinfo.

📝 NOTES

A context for SSKDF can be obtained by calling:

EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

The output length of an SSKDF is specified via the keylen parameter to the EVP_KDF_derive(3) function.

💡 EXAMPLES

💡 This example derives 10 bytes using H(x) = SHA-256, with the secret key "secret" and fixedinfo value "label":

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char out[10];
OSSL_PARAM params[4], *p = params;

kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                        SN_sha256, strlen(SN_sha256));
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                         "secret", (size_t)6);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                         "label", (size_t)5);
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    error("EVP_KDF_derive");
}

EVP_KDF_CTX_free(kctx);

💡 This example derives 10 bytes using H(x) = HMAC(SHA-256), with the secret key "secret", fixedinfo value "label" and salt "salt":

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char out[10];
OSSL_PARAM params[6], *p = params;

kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_MAC,
                                        SN_hmac, strlen(SN_hmac));
*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                        SN_sha256, strlen(SN_sha256));
*p++ = OSSL_PARAM_construct_octet_string(EVP_KDF_CTRL_SET_KEY,
                                         "secret", (size_t)6);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                         "label", (size_t)5);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                         "salt", (size_t)4);
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    error("EVP_KDF_derive");
}

EVP_KDF_CTX_free(kctx);

💡 This example derives 10 bytes using H(x) = KMAC128(x,salt,outlen), with the secret key "secret" fixedinfo value "label", salt of "salt" and KMAC outlen of 20:

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char out[10];
OSSL_PARAM params[7], *p = params;

kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_MAC,
                                        SN_kmac128, strlen(SN_kmac128));
*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                        SN_sha256, strlen(SN_sha256));
*p++ = OSSL_PARAM_construct_octet_string(EVP_KDF_CTRL_SET_KEY,
                                         "secret", (size_t)6);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                         "label", (size_t)5);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                         "salt", (size_t)4);
*p++ = OSSL_PARAM_construct_size_t(OSSL_KDF_PARAM_MAC_SIZE, (size_t)20);
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    error("EVP_KDF_derive");
}

EVP_KDF_CTX_free(kctx);

📜 CONFORMING TO

NIST SP800-56Cr1.

🔗 SEE ALSO

📘 EVP_KDF(3), 📘 EVP_KDF_CTX_new(3), 📘 EVP_KDF_CTX_free(3), 📘 EVP_KDF_CTX_set_params(3), 📘 EVP_KDF_CTX_get_kdf_size(3), 📘 EVP_KDF_derive(3), 📘 "PARAMETERS" in EVP_KDF(3)

🕰️ HISTORY

This functionality was added to OpenSSL 3.0.

©️ COPYRIGHT

Copyright 2019-2021 The OpenSSL Project Authors. All Rights Reserved. Copyright (c) 2019, Oracle and/or its affiliates. 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>.

📛 NAME 🚀 Quick Reference 📖 DESCRIPTION 📝 NOTES 💡 EXAMPLES 📜 CONFORMING TO 🔗 SEE ALSO 🕰️ HISTORY ©️ COPYRIGHT