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