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133 lines
3.9 KiB
Plaintext
133 lines
3.9 KiB
Plaintext
=pod
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=head1 NAME
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EVP_KDF-TLS13_KDF - The TLS 1.3 EVP_KDF implementation
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=head1 DESCRIPTION
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Support for computing the TLS 1.3 version of the B<HKDF> KDF through
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the B<EVP_KDF> API.
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The EVP_KDF-TLS13_KDF algorithm implements the HKDF key derivation function
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as used by TLS 1.3.
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=head2 Identity
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"TLS13-KDF" is the name for this implementation; it
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can be used with the EVP_KDF_fetch() function.
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=head2 Supported parameters
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The supported parameters are:
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=over 4
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=item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
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=item "digest" (B<OSSL_KDF_PARAM_DIGEST>) <UTF8 string>
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=item "key" (B<OSSL_KDF_PARAM_KEY>) <octet string>
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=item "salt" (B<OSSL_KDF_PARAM_SALT>) <octet string>
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These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
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=item "prefix" (B<OSSL_KDF_PARAM_PREFIX>) <octet string>
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This parameter sets the label prefix on the specified TLS 1.3 KDF context.
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For TLS 1.3 this should be set to the ASCII string "tls13 " without a
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trailing zero byte. Refer to RFC 8446 section 7.1 "Key Schedule" for details.
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=item "label" (B<OSSL_KDF_PARAM_LABEL>) <octet string>
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This parameter sets the label on the specified TLS 1.3 KDF context.
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Refer to RFC 8446 section 7.1 "Key Schedule" for details.
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=item "data" (B<OSSL_KDF_PARAM_DATA>) <octet string>
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This parameter sets the context data on the specified TLS 1.3 KDF context.
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Refer to RFC 8446 section 7.1 "Key Schedule" for details.
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=item "mode" (B<OSSL_KDF_PARAM_MODE>) <UTF8 string> or <integer>
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This parameter sets the mode for the TLS 1.3 KDF operation.
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There are two modes that are currently defined:
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=over 4
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=item "EXTRACT_ONLY" or B<EVP_KDF_HKDF_MODE_EXTRACT_ONLY>
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In this mode calling L<EVP_KDF_derive(3)> will just perform the extract
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operation. The value returned will be the intermediate fixed-length pseudorandom
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key K. The I<keylen> parameter must match the size of K, which can be looked
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up by calling EVP_KDF_CTX_get_kdf_size() after setting the mode and digest.
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The digest, key and salt values must be set before a key is derived otherwise
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an error will occur.
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=item "EXPAND_ONLY" or B<EVP_KDF_HKDF_MODE_EXPAND_ONLY>
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In this mode calling L<EVP_KDF_derive(3)> will just perform the expand
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operation. The input key should be set to the intermediate fixed-length
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pseudorandom key K returned from a previous extract operation.
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The digest, key and info values must be set before a key is derived otherwise
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an error will occur.
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=back
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=back
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=head1 NOTES
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This KDF is intended for use by the TLS 1.3 implementation in libssl.
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It does not support all the options and capabilities that HKDF does.
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The I<OSSL_PARAM> array passed to L<EVP_KDF_derive(3)> or
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L<EVP_KDF_CTX_set_params(3)> must specify all of the parameters required.
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This KDF does not support a piecemeal approach to providing these.
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A context for a TLS 1.3 KDF can be obtained by calling:
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EVP_KDF *kdf = EVP_KDF_fetch(NULL, "TLS13-KDF", NULL);
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EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
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The output length of a TLS 1.3 KDF expand operation is specified via the
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I<keylen> parameter to the L<EVP_KDF_derive(3)> function. When using
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EVP_KDF_HKDF_MODE_EXTRACT_ONLY the I<keylen> parameter must equal the size of
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the intermediate fixed-length pseudorandom key otherwise an error will occur.
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For that mode, the fixed output size can be looked up by calling
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EVP_KDF_CTX_get_kdf_size() after setting the mode and digest on the
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B<EVP_KDF_CTX>.
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=head1 CONFORMING TO
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RFC 8446
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=head1 SEE ALSO
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L<EVP_KDF(3)>,
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L<EVP_KDF_CTX_new(3)>,
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L<EVP_KDF_CTX_free(3)>,
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L<EVP_KDF_CTX_get_kdf_size(3)>,
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L<EVP_KDF_CTX_set_params(3)>,
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L<EVP_KDF_derive(3)>,
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L<EVP_KDF(3)/PARAMETERS>,
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L<EVP_KDF-HKDF(7)>
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=head1 HISTORY
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This functionality was added in OpenSSL 3.0.
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=head1 COPYRIGHT
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Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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