---
name: "EVP_KDF-SS"
section: "7ssl"
platform: "linux"
package: "openssl"
version: "3.6.5-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/EVP_KDF-SS"
---

# EVP\_KDF-SS(7ssl)

The Single Step / One Step EVP\_KDF implementation

## 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\_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

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_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

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 2019-2026 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\>.