EVP_PKEY_size, EVP_PKEY_bits, EVP_PKEY_security_bits - EVP_PKEY information functions
#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);
EVP_PKEY_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_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_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_security_bits() returns the number of security bits of the given pkey, bits of security is defined in NIST SP800-57.
EVP_PKEY_size(), EVP_PKEY_bits() and EVP_PKEY_security_bits() return a positive number, or 0 if this size isn't available.
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 "NOTES" in EVP_DigestSignFinal(3).
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