# phpman > man > EVP_KDF-SS(7ssl)

[EVP_KDF-SS(7SSL)](https://www.chedong.com/phpMan.php/man/EVPKDF-SS/7SSL/markdown)                               OpenSSL                              [EVP_KDF-SS(7SSL)](https://www.chedong.com/phpMan.php/man/EVPKDF-SS/7SSL/markdown)



## 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'.

### 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>
       "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)](https://www.chedong.com/phpMan.php/man/KDF/3/markdown).

       "key" (**EVP**___**KDF**___**CTRL**___**SET**___**KEY**) <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.

## 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)](https://www.chedong.com/phpMan.php/man/derive/3/markdown)
       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)](https://www.chedong.com/phpMan.php/man/KDF/3/markdown), **EVP**___**KDF**___**CTX**___**[new**(3)](https://www.chedong.com/phpMan.php/man/new/3/markdown), **EVP**___**KDF**___**CTX**___**[free**(3)](https://www.chedong.com/phpMan.php/man/free/3/markdown), **EVP**___**KDF**___**CTX**___**set**___**[params**(3)](https://www.chedong.com/phpMan.php/man/params/3/markdown),
       **EVP**___**KDF**___**CTX**___**get**___**kdf**___**[size**(3)](https://www.chedong.com/phpMan.php/man/size/3/markdown), **EVP**___**KDF**___**[derive**(3)](https://www.chedong.com/phpMan.php/man/derive/3/markdown), "PARAMETERS" in **EVP**___**[KDF**(3)](https://www.chedong.com/phpMan.php/man/KDF/3/markdown)

## 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>>.



3.0.2                                        2026-06-02                             [EVP_KDF-SS(7SSL)](https://www.chedong.com/phpMan.php/man/EVPKDF-SS/7SSL/markdown)