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+=pod
+
+=head1 NAME
+
+EVP_KDF-KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation
+
+=head1 DESCRIPTION
+
+Support for computing the B<KRB5KDF> KDF through the B<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.
+
+=head2 Identity
+
+"KRB5KDF" is the name for this implementation;
+it can be used with the EVP_KDF_fetch() function.
+
+=head2 Supported parameters
+
+The supported parameters are:
+
+=over 4
+
+=item "properies" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
+
+=item "cipher" (B<OSSL_KDF_PARAM_CIPHER>) <UTF8 string>
+
+=item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>
+
+These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
+
+=item "constant" (B<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.
+
+=back
+
+=head1 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 I<keylen>
+parameter to the L<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
+L<EVP_KDF_size()> to obtain the requisite length returns the correct length
+only after the cipher is set. Prior to that B<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 L<EVP_KDF_derive(3)> function along
+with that length.
+
+=head1 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_set_params(kctx, params) <= 0)
+     /* Error */
+
+ if (EVP_KDF_derive(kctx, out, &outlen) <= 0)
+     /* Error */
+
+
+=head1 CONFORMING TO
+
+RFC 3961
+
+=head1 SEE ALSO
+
+L<EVP_KDF>,
+L<EVP_KDF_CTX_new_id(3)>,
+L<EVP_KDF_CTX_free(3)>,
+L<EVP_KDF_ctrl(3)>,
+L<EVP_KDF_size(3)>,
+L<EVP_KDF_derive(3)>,
+L<EVP_KDF(3)/PARAMETERS>
+
+=head1 HISTORY
+
+This functionality was added to OpenSSL 3.0.
+
+=head1 COPYRIGHT
+
+Copyright 2016-2019 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (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
+L<https://www.openssl.org/source/license.html>.
+
+=cut
+