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>.

3.5.0                                              2025-06-04                              EVP_KDF-KRB5KDF(7SSL)