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=pod
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=head1 NAME
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EVP_KDF_SS - The Single Step / One Step EVP_KDF implementation
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=head1 DESCRIPTION
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The EVP_KDF_SS algorithm implements the Single Step key derivation function (SSKDF).
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SSKDF derives a key using input such as a shared secret key (that was generated
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during the execution of a key establishment scheme) and fixedinfo.
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SSKDF is also informally referred to as 'Concat KDF'.
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=head2 Auxilary function
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The implementation uses a selectable auxiliary function H, which can be in the
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backported version only a:
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=over 4
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=item B<H(x) = hash(x, digest=md)>
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=back
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=head2 Numeric identity
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B<EVP_KDF_SS> is the numeric identity for this implementation; it
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can be used with the EVP_KDF_CTX_new_id() function.
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=head2 Supported controls
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The supported controls are:
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=over 4
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=item B<EVP_KDF_CTRL_SET_MD>
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This control works as described in L<EVP_KDF_CTX(3)/CONTROLS>.
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=item B<EVP_KDF_CTRL_SET_KEY>
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This control expects two arguments: C<unsigned char *secret>, C<size_t secretlen>
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The shared secret used for key derivation. This control sets the secret.
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EVP_KDF_ctrl_str() takes two type strings for this control:
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=over 4
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=item "secret"
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The value string is used as is.
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=item "hexsecret"
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The value string is expected to be a hexadecimal number, which will be
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decoded before being passed on as the control value.
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=back
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=item B<EVP_KDF_CTRL_SET_SSKDF_INFO>
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This control expects two arguments: C<unsigned char *info>, C<size_t infolen>
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An optional value for fixedinfo, also known as otherinfo. This control sets the fixedinfo.
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EVP_KDF_ctrl_str() takes two type strings for this control:
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=over 4
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=item "info"
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The value string is used as is.
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=item "hexinfo"
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The value string is expected to be a hexadecimal number, which will be
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decoded before being passed on as the control value.
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=back
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=back
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=head1 NOTES
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A context for SSKDF can be obtained by calling:
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EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_SS);
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The output length of an SSKDF is specified via the C<keylen>
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parameter to the L<EVP_KDF_derive(3)> function.
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=head1 EXAMPLE
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This example derives 10 bytes using H(x) = SHA-256, with the secret key "secret"
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and fixedinfo value "label":
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EVP_KDF_CTX *kctx;
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unsigned char out[10];
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kctx = EVP_KDF_CTX_new_id(EVP_KDF_SS);
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if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_MD, EVP_sha256()) <= 0) {
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error("EVP_KDF_CTRL_SET_MD");
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}
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if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KEY, "secret", (size_t)6) <= 0) {
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error("EVP_KDF_CTRL_SET_KEY");
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}
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if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SSKDF_INFO, "label", (size_t)5) <= 0) {
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error("EVP_KDF_CTRL_SET_SSKDF_INFO");
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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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NIST SP800-56Cr1.
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=head1 SEE ALSO
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L<EVP_KDF_CTX>,
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L<EVP_KDF_CTX_new_id(3)>,
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L<EVP_KDF_CTX_free(3)>,
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L<EVP_KDF_ctrl(3)>,
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L<EVP_KDF_size(3)>,
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L<EVP_KDF_derive(3)>,
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L<EVP_KDF_CTX(3)/CONTROLS>
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=head1 HISTORY
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This functionality was added to OpenSSL 3.0.0.
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=head1 COPYRIGHT
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Copyright 2019 The OpenSSL Project Authors. All Rights Reserved. Copyright
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(c) 2019, Oracle and/or its affiliates. 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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