man > EVP_KDF-KRB5KDF(7ssl)

EVP_KDF-KRB5KDF(7SSL)                          OpenSSL                         EVP_KDF-KRB5KDF(7SSL)

NAME
       EVP_KDF-KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation

DESCRIPTION
       Support for computing the KRB5KDF KDF through the EVP_KDF API.

       The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined in RFC 3961,
       section 5.1 and is used by Krb5 to derive session keys.  Three inputs are required to perform
       key derivation: a cipher, (for example AES-128-CBC), the initial key, and a constant.

   Identity
       "KRB5KDF" 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>
       "cipher" (OSSL_KDF_PARAM_CIPHER) <UTF8 string>
       "key" (OSSL_KDF_PARAM_KEY) <octet string>
           These parameters work as described in "PARAMETERS" in EVP_KDF(3).

       "constant" (OSSL_KDF_PARAM_CONSTANT) <octet string>
           This  parameter  sets  the  constant  value  for the KDF.  If a value is already set, the
           contents are replaced.

NOTES
       A context for KRB5KDF can be obtained by calling:

        EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
        EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

       The output length of the KRB5KDF derivation is specified via  the  keylen  parameter  to  the
       EVP_KDF_derive(3)  function,  and MUST match the key length for the chosen cipher or an error
       is returned. Moreover, the constant's length must not exceed the block size  of  the  cipher.
       Since    the    KRB5KDF    output    length   depends   on   the   chosen   cipher,   calling
       EVP_KDF_CTX_get_kdf_size(3) to obtain the requisite length returns the  correct  length  only
       after  the  cipher  is  set.  Prior  to that EVP_MAX_KEY_LENGTH is returned.  The caller must
       allocate a buffer of the correct length for the chosen cipher, and pass that  buffer  to  the
       EVP_KDF_derive(3) function along with that length.

EXAMPLES
       This example derives a key using the AES-128-CBC cipher:

        EVP_KDF *kdf;
        EVP_KDF_CTX *kctx;
        unsigned char key[16] = "01234...";
        unsigned char constant[] = "I'm a constant";
        unsigned char out[16];
        size_t outlen = sizeof(out);
        OSSL_PARAM params[4], *p = params;

        kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL);
        kctx = EVP_KDF_CTX_new(kdf);
        EVP_KDF_free(kdf);

        *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER,
                                                SN_aes_128_cbc,
                                                strlen(SN_aes_128_cbc));
        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                                 key, (size_t)16);
        *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT,
                                                 constant, strlen(constant));
        *p = OSSL_PARAM_construct_end();
        if (EVP_KDF_derive(kctx, out, outlen, params) <= 0)
            /* Error */

        EVP_KDF_CTX_free(kctx);

CONFORMING TO
       RFC 3961

SEE ALSO
       EVP_KDF(3), EVP_KDF_CTX_free(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 2016-2021 The OpenSSL Project Authors. 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>.

3.0.13                                       2026-07-29                        EVP_KDF-KRB5KDF(7SSL)