In particular a return value of -2 indicates the operation is not supported by
the public key algorithm.
-=head1 EXAMPLE
+=head1 EXAMPLES
This example derives 10 bytes using SHA-256 with the secret key "secret",
salt value "salt" and info value "label":
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+Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
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L<https://www.openssl.org/source/license.html>.