1=pod 2 3=head1 NAME 4 5EVP_KDF-TLS13_KDF - The TLS 1.3 EVP_KDF implementation 6 7=head1 DESCRIPTION 8 9Support for computing the TLS 1.3 version of the B<HKDF> KDF through 10the B<EVP_KDF> API. 11 12The EVP_KDF-TLS13_KDF algorithm implements the HKDF key derivation function 13as used by TLS 1.3. 14 15=head2 Identity 16 17"TLS13-KDF" is the name for this implementation; it 18can be used with the EVP_KDF_fetch() function. 19 20=head2 Supported parameters 21 22The supported parameters are: 23 24=over 4 25 26=item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string> 27 28=item "digest" (B<OSSL_KDF_PARAM_DIGEST>) <UTF8 string> 29 30=item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string> 31 32=item "salt" (B<OSSL_KDF_PARAM_SALT>) <octet string> 33 34These parameters work as described in L<EVP_KDF(3)/PARAMETERS>. 35 36=item "prefix" (B<OSSL_KDF_PARAM_PREFIX>) <octet string> 37 38This parameter sets the label prefix on the specified TLS 1.3 KDF context. 39For TLS 1.3 this should be set to the ASCII string "tls13 " without a 40trailing zero byte. Refer to RFC 8446 section 7.1 "Key Schedule" for details. 41 42=item "label" (B<OSSL_KDF_PARAM_LABEL>) <octet string> 43 44This parameter sets the label on the specified TLS 1.3 KDF context. 45Refer to RFC 8446 section 7.1 "Key Schedule" for details. 46 47=item "data" (B<OSSL_KDF_PARAM_DATA>) <octet string> 48 49This parameter sets the context data on the specified TLS 1.3 KDF context. 50Refer to RFC 8446 section 7.1 "Key Schedule" for details. 51 52=item "mode" (B<OSSL_KDF_PARAM_MODE>) <UTF8 string> or <integer> 53 54This parameter sets the mode for the TLS 1.3 KDF operation. 55There are two modes that are currently defined: 56 57=over 4 58 59=item "EXTRACT_ONLY" or B<EVP_KDF_HKDF_MODE_EXTRACT_ONLY> 60 61In this mode calling L<EVP_KDF_derive(3)> will just perform the extract 62operation. The value returned will be the intermediate fixed-length pseudorandom 63key K. The I<keylen> parameter must match the size of K, which can be looked 64up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest. 65 66The digest, key and salt values must be set before a key is derived otherwise 67an error will occur. 68 69=item "EXPAND_ONLY" or B<EVP_KDF_HKDF_MODE_EXPAND_ONLY> 70 71In this mode calling L<EVP_KDF_derive(3)> will just perform the expand 72operation. The input key should be set to the intermediate fixed-length 73pseudorandom key K returned from a previous extract operation. 74 75The digest, key and info values must be set before a key is derived otherwise 76an error will occur. 77 78=back 79 80=back 81 82=head1 NOTES 83 84This KDF is intended for use by the TLS 1.3 implementation in libssl. 85It does not support all the options and capabilities that HKDF does. 86 87The I<OSSL_PARAM> array passed to L<EVP_KDF_derive(3)> or 88L<EVP_KDF_CTX_set_params(3)> must specify all of the parameters required. 89This KDF does not support a piecemeal approach to providing these. 90 91A context for a TLS 1.3 KDF can be obtained by calling: 92 93 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "TLS13-KDF", NULL); 94 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf); 95 96The output length of a TLS 1.3 KDF expand operation is specified via the 97I<keylen> parameter to the L<EVP_KDF_derive(3)> function. When using 98EVP_KDF_HKDF_MODE_EXTRACT_ONLY the I<keylen> parameter must equal the size of 99the intermediate fixed-length pseudorandom key otherwise an error will occur. 100For that mode, the fixed output size can be looked up by calling 101EVP_KDF_CTX_get_kdf_size() after setting the mode and digest on the 102B<EVP_KDF_CTX>. 103 104=head1 CONFORMING TO 105 106RFC 8446 107 108=head1 SEE ALSO 109 110L<EVP_KDF(3)>, 111L<EVP_KDF_CTX_new(3)>, 112L<EVP_KDF_CTX_free(3)>, 113L<EVP_KDF_CTX_get_kdf_size(3)>, 114L<EVP_KDF_CTX_set_params(3)>, 115L<EVP_KDF_derive(3)>, 116L<EVP_KDF(3)/PARAMETERS>, 117L<EVP_KDF-HKDF(7)> 118 119=head1 HISTORY 120 121This functionality was added in OpenSSL 3.0. 122 123=head1 COPYRIGHT 124 125Copyright 2021 The OpenSSL Project Authors. All Rights Reserved. 126 127Licensed under the Apache License 2.0 (the "License"). You may not use 128this file except in compliance with the License. You can obtain a copy 129in the file LICENSE in the source distribution or at 130L<https://www.openssl.org/source/license.html>. 131 132=cut 133