EVP_KDF-SS(7) | OpenSSL | EVP_KDF-SS(7)

# NAME

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

# 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

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

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.

# 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(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);

# 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 in OpenSSL 3.0.

# COPYRIGHT

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>.

2026-06-09 | 3.5.7