1=pod 2 3=head1 NAME 4 5EVP_KDF-KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation 6 7=head1 DESCRIPTION 8 9Support for computing the B<KRB5KDF> KDF through the B<EVP_KDF> API. 10 11The EVP_KDF-KRB5KDF algorithm implements the key derivation function defined 12in RFC 3961, section 5.1 and is used by Krb5 to derive session keys. 13Three inputs are required to perform key derivation: a cipher, (for example 14AES-128-CBC), the initial key, and a constant. 15 16=head2 Identity 17 18"KRB5KDF" is the name for this implementation; 19it can be used with the EVP_KDF_fetch() function. 20 21=head2 Supported parameters 22 23The supported parameters are: 24 25=over 4 26 27=item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string> 28 29=item "cipher" (B<OSSL_KDF_PARAM_CIPHER>) <UTF8 string> 30 31=item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string> 32 33These parameters work as described in L<EVP_KDF(3)/PARAMETERS>. 34 35=item "constant" (B<OSSL_KDF_PARAM_CONSTANT>) <octet string> 36 37This parameter sets the constant value for the KDF. 38If a value is already set, the contents are replaced. 39 40=back 41 42=head1 NOTES 43 44A context for KRB5KDF can be obtained by calling: 45 46 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL); 47 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf); 48 49The output length of the KRB5KDF derivation is specified via the I<keylen> 50parameter to the L<EVP_KDF_derive(3)> function, and MUST match the key 51length for the chosen cipher or an error is returned. Moreover, the 52constant's length must not exceed the block size of the cipher. 53Since the KRB5KDF output length depends on the chosen cipher, calling 54L<EVP_KDF_CTX_get_kdf_size(3)> to obtain the requisite length returns the correct length 55only after the cipher is set. Prior to that B<EVP_MAX_KEY_LENGTH> is returned. 56The caller must allocate a buffer of the correct length for the chosen 57cipher, and pass that buffer to the L<EVP_KDF_derive(3)> function along 58with that length. 59 60=head1 EXAMPLES 61 62This example derives a key using the AES-128-CBC cipher: 63 64 EVP_KDF *kdf; 65 EVP_KDF_CTX *kctx; 66 unsigned char key[16] = "01234..."; 67 unsigned char constant[] = "I'm a constant"; 68 unsigned char out[16]; 69 size_t outlen = sizeof(out); 70 OSSL_PARAM params[4], *p = params; 71 72 kdf = EVP_KDF_fetch(NULL, "KRB5KDF", NULL); 73 kctx = EVP_KDF_CTX_new(kdf); 74 EVP_KDF_free(kdf); 75 76 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_CIPHER, 77 SN_aes_128_cbc, 78 strlen(SN_aes_128_cbc)); 79 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY, 80 key, (size_t)16); 81 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_CONSTANT, 82 constant, strlen(constant)); 83 *p = OSSL_PARAM_construct_end(); 84 if (EVP_KDF_derive(kctx, out, outlen, params) <= 0) 85 /* Error */ 86 87 EVP_KDF_CTX_free(kctx); 88 89=head1 CONFORMING TO 90 91RFC 3961 92 93=head1 SEE ALSO 94 95L<EVP_KDF(3)>, 96L<EVP_KDF_CTX_free(3)>, 97L<EVP_KDF_CTX_get_kdf_size(3)>, 98L<EVP_KDF_derive(3)>, 99L<EVP_KDF(3)/PARAMETERS> 100 101=head1 HISTORY 102 103This functionality was added in OpenSSL 3.0. 104 105=head1 COPYRIGHT 106 107Copyright 2016-2021 The OpenSSL Project Authors. All Rights Reserved. 108 109Licensed under the Apache License 2.0 (the "License"). You may not use 110this file except in compliance with the License. You can obtain a copy 111in the file LICENSE in the source distribution or at 112L<https://www.openssl.org/source/license.html>. 113 114=cut 115 116