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=pod
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=head1 NAME
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EVP_KDF-HKDF - The HKDF EVP_KDF implementation
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=head1 DESCRIPTION
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Support for computing the B<HKDF> KDF through the B<EVP_KDF> API.
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The EVP_KDF-HKDF algorithm implements the HKDF key derivation function.
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HKDF follows the "extract-then-expand" paradigm, where the KDF logically
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consists of two modules. The first stage takes the input keying material
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and "extracts" from it a fixed-length pseudorandom key K. The second stage
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"expands" the key K into several additional pseudorandom keys (the output
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of the KDF).
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=head2 Identity
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"HKDF" 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 B<OSSL_KDF_PARAM_PROPERTIES> ("properties") <UTF8 string>
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=item B<OSSL_KDF_PARAM_DIGEST> ("digest") <UTF8 string>
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=item B<OSSL_KDF_PARAM_KEY> ("key") <octet string>
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=item B<OSSL_KDF_PARAM_SALT> ("salt") <octet string>
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These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
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=item B<OSSL_KDF_PARAM_INFO> ("info") <octet string>
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This parameter sets the info value.
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The length of the context info buffer cannot exceed 1024 bytes;
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this should be more than enough for any normal use of HKDF.
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=item B<OSSL_KDF_PARAM_MODE> ("mode") <UTF8 string> or <int>
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This parameter sets the mode for the HKDF operation.
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There are three modes that are currently defined:
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=over 4
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=item B<EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND> "EXTRACT_AND_EXPAND"
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This is the default mode. Calling L<EVP_KDF-derive(3)> on an EVP_KDF_CTX set
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up for HKDF will perform an extract followed by an expand operation in one go.
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The derived key returned will be the result after the expand operation. The
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intermediate fixed-length pseudorandom key K is not returned.
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In this mode the digest, key, salt and info values must be set before a key is
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derived otherwise an error will occur.
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=item B<EVP_KDF_HKDF_MODE_EXTRACT_ONLY> "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 C<keylen> parameter must match the size of K, which can be looked
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up by calling EVP_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 B<EVP_KDF_HKDF_MODE_EXPAND_ONLY> "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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A context for HKDF can be obtained by calling:
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EVP_KDF *kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
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EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
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The output length of an HKDF expand operation is specified via the C<keylen>
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parameter to the L<EVP_KDF-derive(3)> function. When using
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EVP_KDF_HKDF_MODE_EXTRACT_ONLY the C<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 EVP_KDF_size()
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after setting the mode and digest on the C<EVP_KDF_CTX>.
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=head1 EXAMPLES
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This example derives 10 bytes using SHA-256 with the secret key "secret",
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salt value "salt" and info value "label":
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EVP_KDF *kdf;
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EVP_KDF_CTX *kctx;
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unsigned char out[10];
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OSSL_PARAM params[5], *p = params;
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kdf = EVP_KDF_fetch(NULL, "HKDF", NULL);
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kctx = EVP_KDF_CTX_new(kdf);
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EVP_KDF_free(kdf);
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*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
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SN_sha256, strlen(SN_sha256));
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*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
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"secret", (size_t)6);
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*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
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"label", (size_t)5);
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*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
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"salt", (size_t)4);
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*p = OSSL_PARAM_construct_end();
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if (EVP_KDF_CTX_set_params(kctx, params) <= 0) {
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error("EVP_KDF_CTX_set_params");
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}
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if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) {
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error("EVP_KDF_derive");
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}
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EVP_KDF_CTX_free(kctx);
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=head1 CONFORMING TO
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RFC 5869
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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_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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=head1 COPYRIGHT
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Copyright 2016-2019 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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