- NAME
- DESCRIPTION
  - Auxiliary function
  - Identity
  - Supported parameters
- NOTES
- EXAMPLES
- CONFORMING TO
- SEE ALSO
- HISTORY
- COPYRIGHT

# 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>

  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" (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) 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[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(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), 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 2019-2023 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.