1=pod 2 3=head1 NAME 4 5EVP_KDF-HKDF - The HKDF EVP_KDF implementation 6 7=head1 DESCRIPTION 8 9Support for computing the B<HKDF> KDF through the B<EVP_KDF> API. 10 11The EVP_KDF-HKDF algorithm implements the HKDF key derivation function. 12HKDF follows the "extract-then-expand" paradigm, where the KDF logically 13consists of two modules. The first stage takes the input keying material 14and "extracts" from it a fixed-length pseudorandom key K. The second stage 15"expands" the key K into several additional pseudorandom keys (the output 16of the KDF). 17 18=head2 Identity 19 20"HKDF" is the name for this implementation; it 21can be used with the EVP_KDF_fetch() function. 22 23=head2 Supported parameters 24 25The supported parameters are: 26 27=over 4 28 29=item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string> 30 31=item "digest" (B<OSSL_KDF_PARAM_DIGEST>) <UTF8 string> 32 33=item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string> 34 35=item "salt" (B<OSSL_KDF_PARAM_SALT>) <octet string> 36 37These parameters work as described in L<EVP_KDF(3)/PARAMETERS>. 38 39=item "info" (B<OSSL_KDF_PARAM_INFO>) <octet string> 40 41This parameter sets the info value. 42The length of the context info buffer cannot exceed 1024 bytes; 43this should be more than enough for any normal use of HKDF. 44 45=item "mode" (B<OSSL_KDF_PARAM_MODE>) <UTF8 string> or <integer> 46 47This parameter sets the mode for the HKDF operation. 48There are three modes that are currently defined: 49 50=over 4 51 52=item "EXTRACT_AND_EXPAND" or B<EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND> 53 54This is the default mode. Calling L<EVP_KDF_derive(3)> on an EVP_KDF_CTX set 55up for HKDF will perform an extract followed by an expand operation in one go. 56The derived key returned will be the result after the expand operation. The 57intermediate fixed-length pseudorandom key K is not returned. 58 59In this mode the digest, key, salt and info values must be set before a key is 60derived otherwise an error will occur. 61 62=item "EXTRACT_ONLY" or B<EVP_KDF_HKDF_MODE_EXTRACT_ONLY> 63 64In this mode calling L<EVP_KDF_derive(3)> will just perform the extract 65operation. The value returned will be the intermediate fixed-length pseudorandom 66key K. The I<keylen> parameter must match the size of K, which can be looked 67up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest. 68 69The digest, key and salt values must be set before a key is derived otherwise 70an error will occur. 71 72=item "EXPAND_ONLY" or B<EVP_KDF_HKDF_MODE_EXPAND_ONLY> 73 74In this mode calling L<EVP_KDF_derive(3)> will just perform the expand 75operation. The input key should be set to the intermediate fixed-length 76pseudorandom key K returned from a previous extract operation. 77 78The digest, key and info values must be set before a key is derived otherwise 79an error will occur. 80 81=back 82 83=back 84 85=head1 NOTES 86 87A context for HKDF can be obtained by calling: 88 89 EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL); 90 EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf); 91 92The output length of an HKDF expand operation is specified via the I<keylen> 93parameter to the L<EVP_KDF_derive(3)> function. When using 94EVP_KDF_HKDF_MODE_EXTRACT_ONLY the I<keylen> parameter must equal the size of 95the intermediate fixed-length pseudorandom key otherwise an error will occur. 96For that mode, the fixed output size can be looked up by calling EVP_KDF_CTX_get_kdf_size() 97after setting the mode and digest on the B<EVP_KDF_CTX>. 98 99=head1 EXAMPLES 100 101This example derives 10 bytes using SHA-256 with the secret key "secret", 102salt value "salt" and info value "label": 103 104 EVP_KDF *kdf; 105 EVP_KDF_CTX *kctx; 106 unsigned char out[10]; 107 OSSL_PARAM params[5], *p = params; 108 109 kdf = EVP_KDF_fetch(NULL, "HKDF", NULL); 110 kctx = EVP_KDF_CTX_new(kdf); 111 EVP_KDF_free(kdf); 112 113 *p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST, 114 SN_sha256, strlen(SN_sha256)); 115 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY, 116 "secret", (size_t)6); 117 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO, 118 "label", (size_t)5); 119 *p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT, 120 "salt", (size_t)4); 121 *p = OSSL_PARAM_construct_end(); 122 if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) { 123 error("EVP_KDF_derive"); 124 } 125 126 EVP_KDF_CTX_free(kctx); 127 128=head1 CONFORMING TO 129 130RFC 5869 131 132=head1 SEE ALSO 133 134L<EVP_KDF(3)>, 135L<EVP_KDF_CTX_new(3)>, 136L<EVP_KDF_CTX_free(3)>, 137L<EVP_KDF_CTX_get_kdf_size(3)>, 138L<EVP_KDF_CTX_set_params(3)>, 139L<EVP_KDF_derive(3)>, 140L<EVP_KDF(3)/PARAMETERS>, 141L<EVP_KDF-TLS13_KDF(7)> 142 143=head1 HISTORY 144 145This functionality was added in OpenSSL 3.0. 146 147=head1 COPYRIGHT 148 149Copyright 2016-2021 The OpenSSL Project Authors. All Rights Reserved. 150 151Licensed under the Apache License 2.0 (the "License"). You may not use 152this file except in compliance with the License. You can obtain a copy 153in the file LICENSE in the source distribution or at 154L<https://www.openssl.org/source/license.html>. 155 156=cut 157