NAME

       EVP_PKEY_get_size, EVP_PKEY_get_bits, EVP_PKEY_get_security_bits, EVP_PKEY_bits, EVP_PKEY_security_bits,
       EVP_PKEY_size - EVP_PKEY information functions

SYNOPSIS

        #include <openssl/evp.h>

        int EVP_PKEY_get_size(const EVP_PKEY *pkey);
        int EVP_PKEY_get_bits(const EVP_PKEY *pkey);
        int EVP_PKEY_get_security_bits(const EVP_PKEY *pkey);

        #define EVP_PKEY_bits EVP_PKEY_get_bits
        #define EVP_PKEY_security_bits EVP_PKEY_get_security_bits
        #define EVP_PKEY_size EVP_PKEY_get_size

DESCRIPTION

       EVP_PKEY_get_size() returns the maximum suitable size for the output buffers for almost all operations
       that can be done with pkey.  The primary documented use is with EVP_SignFinal(3) and EVP_SealInit(3), but
       it isn't limited there.  The returned size is also large enough for the output buffer of
       EVP_PKEY_sign(3), EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3), 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_get_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 EVP_PKEY_sign(3) (the siglen argument will
       receive that length), to avoid bugs.

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

       EVP_PKEY_get_security_bits() returns the number of security bits of the given pkey, bits of security is
       defined in NIST SP800-57.

RETURN VALUES

       EVP_PKEY_get_size(), EVP_PKEY_get_bits() and EVP_PKEY_get_security_bits() return a positive number, or 0
       if this size isn't available.

NOTES

       Most functions that have an output buffer and are mentioned with EVP_PKEY_get_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_get_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_get_size() shouldn't be used to get the output size for
       EVP_DigestSignFinal(), according to "NOTES" in EVP_DigestSignFinal(3).

SEE ALSO

       EVP_SignFinal(3), EVP_SealInit(3), EVP_PKEY_sign(3), EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3),
       EVP_PKEY_derive(3)

HISTORY

       The EVP_PKEY_bits(), EVP_PKEY_security_bits(), and EVP_PKEY_size() functions were renamed to include
       "get" in their names in OpenSSL 3.0, respectively.  The old names are kept as non-deprecated alias
       macros.

COPYRIGHT

       Copyright 2020-2021 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
       <https://www.openssl.org/source/license.html>.

3.0.13                                             2025-09-18                            EVP_PKEY_GET_SIZE(3SSL)