EVP\_KDF-SS(7ossl) OpenSSL EVP\_KDF-SS(7ossl) # [NAME](#NAME) EVP\_KDF-SS - The Single Step / One Step EVP\_KDF implementation # [DESCRIPTION](#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'. The output is considered to be keying material. ## [Auxiliary function](#Auxiliary_function) The implementation uses a selectable auxiliary function H, which can be one of: [**H(x) = hash(x, digest=md)**](#H%28x%29) [**H(x) = HMAC\_hash(x, key=salt, digest=md)**](#H%28x%29~2) [**H(x) = KMACxxx(x, key=salt, custom="KDF", outlen=mac\_size)**](#H%28x%29~3) 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](#Identity) "SSKDF" is the name for this implementation; it can be used with the **EVP\_KDF\_fetch()** function. ## [Supported parameters](#Supported_parameters) 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)](/man/os/openssl-4.0/EVP_KDF.3). "key" (**OSSL\_KDF\_PARAM\_KEY**) <octet string> This parameter sets the shared secret that is used for key derivation. "secret" (**OSSL\_KDF\_PARAM\_SECRET**) <octet string> This parameter sets 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. # [NOTES](#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)](/man/os/openssl-4.0/EVP_KDF_derive.3) function. # [EXAMPLES](#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(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_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_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](#CONFORMING_TO) NIST SP800-56Cr1. # [SEE ALSO](#SEE_ALSO) [**EVP\_KDF**(3)](/man/os/openssl-4.0/EVP_KDF.3), [**EVP\_KDF\_CTX\_new**(3)](/man/os/openssl-4.0/EVP_KDF_CTX_new.3), [**EVP\_KDF\_CTX\_free**(3)](/man/os/openssl-4.0/EVP_KDF_CTX_free.3), [**EVP\_KDF\_CTX\_set\_params**(3)](/man/os/openssl-4.0/EVP_KDF_CTX_set_params.3), [**EVP\_KDF\_CTX\_get\_kdf\_size**(3)](/man/os/openssl-4.0/EVP_KDF_CTX_get_kdf_size.3), [**EVP\_KDF\_derive**(3)](/man/os/openssl-4.0/EVP_KDF_derive.3), "PARAMETERS" in [**EVP\_KDF**(3)](/man/os/openssl-4.0/EVP_KDF.3) # [HISTORY](#HISTORY) This functionality was added in OpenSSL 3.0. # [COPYRIGHT](#COPYRIGHT) Copyright 2019-2026 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>. 2026-09-29 4.0.3 --- Document: EVP_KDF-SS(7ossl) from openssl-4.0 4.0.3-r6 (Chainguard OS)