openssl/doc/man3/EVP_PKEY_size.pod

=pod

=head1 NAME

EVP_PKEY_size, EVP_PKEY_bits, EVP_PKEY_security_bits
- EVP_PKEY information functions

=head1 SYNOPSIS

 #include <openssl/evp.h>

 int EVP_PKEY_size(const EVP_PKEY *pkey);
 int EVP_PKEY_bits(const EVP_PKEY *pkey);
 int EVP_PKEY_security_bits(const EVP_PKEY *pkey);

=head1 DESCRIPTION

EVP_PKEY_size() returns the maximum suitable size for the output
buffers for almost all operations that can be done with I<pkey>.
The primary documented use is with L<EVP_SignFinal(3)> and
L<EVP_SealInit(3)>, but it isn't limited there.  The returned size is
also large enough for the output buffer of L<EVP_PKEY_sign(3)>,
L<EVP_PKEY_encrypt(3)>, L<EVP_PKEY_decrypt(3)>, L<EVP_PKEY_derive(3)>.

It must be stressed that, unless the documentation for the operation
that's being performed says otherwise, the size returned by
EVP_PKEY_size() is only preliminary and not exact, so the final
contents of the target buffer may be smaller.  It is therefore crucial
to take note of the size given back by the function that performs the
operation, such as L<EVP_PKEY_sign(3)> (the I<siglen> argument will
receive that length), to avoid bugs.

EVP_PKEY_bits() returns the cryptographic length of the cryptosystem
to which the key in I<pkey> belongs, in bits.  Note that the definition
of cryptographic length is specific to the key cryptosystem.

EVP_PKEY_security_bits() returns the number of security bits of the given
I<pkey>, bits of security is defined in NIST SP800-57.

=head1 RETURN VALUES

EVP_PKEY_size(), EVP_PKEY_bits() and EVP_PKEY_security_bits() return a
positive number, or 0 if this size isn't available.

=head1 NOTES

Most functions that have an output buffer and are mentioned with
EVP_PKEY_size() have a functionality where you can pass NULL for the
buffer and still pass a pointer to an integer and get the exact size
that this function call delivers in the context that it's called in.
This allows those functions to be called twice, once to find out the
exact buffer size, then allocate the buffer in between, and call that
function again actually output the data.  For those functions, it
isn't strictly necessary to call EVP_PKEY_size() to find out the
buffer size, but may be useful in cases where it's desirable to know
the upper limit in advance.

It should also be especially noted that EVP_PKEY_size() shouldn't be
used to get the output size for EVP_DigestSignFinal(), according to
L<EVP_DigestSignFinal(3)/NOTES>.

=head1 SEE ALSO

L<EVP_SignFinal(3)>,
L<EVP_SealInit(3)>,
L<EVP_PKEY_sign(3)>,
L<EVP_PKEY_encrypt(3)>,
L<EVP_PKEY_decrypt(3)>,
L<EVP_PKEY_derive(3)>

=head1 COPYRIGHT

Copyright 2020 The OpenSSL Project Authors. All Rights Reserved.

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
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

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