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

NIST SP800-56Cr1.

Copyright 2019-2024 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>. 3.5.0 2025-06-04 EVP_KDF-SS(7SSL)

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'. The output is considered to be keying material. Auxiliaryfunction 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. Supportedparameters The supported parameters are: "properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string> "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string> This parameter is ignored for KMAC. "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" (OSSL_KDF_PARAM_SECRET) <octet string> This parameter set the shared secret that is used for key derivation. "info" (OSSL_KDF_PARAM_INFO) <octet string> This parameter sets an optional value for fixedinfo, also known as otherinfo. The OpenSSL FIPS provider also supports the following parameters: "fips-indicator" (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer> A getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This may be used after calling EVP_KDF_derive. It returns 0 if "key-check" is set to 0 and the check fails. "key-check" (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer> The default value of 1 causes an error during EVP_KDF_CTX_set_params() if the length of used key- derivation key (OSSL_KDF_PARAM_KEY) is shorter than 112 bits. Setting this to zero will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0.

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(OSSL_KDF_PARAM_SECRET, "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[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_kmac128, strlen(SN_kmac128)); *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SECRET, "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);

This functionality was added in OpenSSL 3.0.

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

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.

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)