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=pod
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=head1 NAME
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EVP_KDF_KRB5KDF - The RFC3961 Krb5 KDF EVP_KDF implementation
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=head1 DESCRIPTION
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Support for computing the B<KRB5KDF> KDF through the B<EVP_KDF> API.
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The B<EVP_KDF_KRB5KDF> algorithm implements the key derivation function defined
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in RFC 3961, section 5.1 and is used by Krb5 to derive session keys.
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Three inputs are required to perform key derivation: a cipher, (for example
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AES-128-CBC), the initial key, and a constant.
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=head2 Numeric identity
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B<EVP_KDF_KRB5KDF> is the numeric identity for this implementation; it can be used with the
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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_CIPHER>
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=item B<EVP_KDF_CTRL_SET_KEY>
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These controls work as described in L<EVP_KDF_CTX(3)/CONTROLS>.
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=item B<EVP_KDF_CTRL_SET_KRB5KDF_CONSTANT>
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This control expects two arguments: C<unsigned char *constant>, C<size_t constantlen>
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This control sets the I<constant> value for the KDF.
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If a value is already set, the contents are replaced.
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=back
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=head1 NOTES
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A context for KRB5KDF can be obtained by calling:
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EVP_KDF_CTX *kctx = EVP_KDF_CTX_new_id(EVP_KDF_KRB5KDF);
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The output length of the KRB5KDF derivation is specified via the I<keylen>
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parameter to the L<EVP_KDF_derive(3)> function, and MUST match the key
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length for the chosen cipher or an error is returned. Moreover the
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I<constant>'s length must not exceed the block size of the cipher.
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Since the KRB5KDF output length depends on the chosen cipher, calling
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L<EVP_KDF_size()> to obtain the requisite length returns the correct length
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only after the cipher is set. Prior to that B<EVP_MAX_KEY_LENGTH> is returned.
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The caller must allocate a buffer of the correct length for the chosen
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cipher, and pass that buffer to the L<EVP_KDF_derive(3)> function along
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with that length.
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=head1 EXAMPLES
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This example derives a key using the AES-128-CBC cipher:
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EVP_KDF_CTX *kctx;
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unsigned char key[16] = "01234...";
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unsigned char constant[] = "I'm a constant";
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unsigned char out[16];
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size_t outlen = sizeof(out);
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kctx = EVP_KDF_CTX_new_id(EVP_KDF_KRB5KDF);
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EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_CIPHER, EVP_aes_128_cbc());
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EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KEY, key, (size_t)16);
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EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_KRB5KDF_CONSTANT, constant, strlen(constant));
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if (EVP_KDF_derive(kctx, out, outlen) <= 0)
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/* Error */
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EVP_KDF_CTX_free(kctx);
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=head1 CONFORMING TO
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RFC 3961
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=head1 SEE ALSO
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L<EVP_KDF_CTX(3)>,
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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.
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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 OpenSSL license (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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