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NAME

RSA_security_bits, DSA_security_bits, DH_security_bits, BN_security_bits — get security strength

SYNOPSIS

#include <openssl/rsa.h>

int
RSA_security_bits(const RSA *rsa);

#include <openssl/dsa.h>

int
DSA_security_bits(const DSA *dsa);

#include <openssl/dh.h>

int
DH_security_bits(const DH *dh);

#include <openssl/bn.h>

int
BN_security_bits(int pubbits, int privbits);

DESCRIPTION

These functions return the security strength of some specific types of cryptographic keys, measured in bits. It is approximately the binary logarithm of the number of operations an attacker has to perform in order to break the key.

RSA_security_bits() uses only the number of significant bits in the public modulus of rsa as returned by RSA_bits(3). It returns

or 0 otherwise.

DSA_security_bits() uses the number of significant bits in the public domain parameter p contained in the dsa object, which is equal to the size of the public key, in the same way as RSA_security_bits(). In addition, the public domain parameter q contained in the dsa object, which is equal to the size of the private key, is inspected. The return value is either the security strength according to the above table or half the size of the private key, whichever is smaller. If the return value would be smaller than 80, 0 is returned instead.

DH_security_bits() uses the number of significant bits in the shared secret contained in the dh object as returned by DH_bits(3) in the same way as RSA_security_bits(). If dh contains the domain parameter q, its number of significant bits is used in the same way as for DSA_security_bits() to limit the return value. Otherwise, if dh contains the length of the secret exponent in bits, that number is used. If neither is available, only the above table is used without calculating a minimum.

BN_security_bits() is a combined function. If -1 is passed for the privbits argument, it behaves like RSA_security_bits(). Otherwise, it behaves like DSA_security_bits().

RETURN VALUES

All these functions return numbers in the range from 0 to 256 inclusive.

DSA_security_bits() fails and returns -1 unless both of the p and q domain parameters are present.

SEE ALSO

BN_num_bits(3), DH_bits(3), DH_get0_pqg(3), DSA_get0_pqg(3), RSA_bits(3), SSL_CTX_set_security_level(3)

Elaine Barker, Recommendation for Key Management, U.S. National Institute of Standards and Technology, NIST Special Publication 800-57 Part 1 Revision 5, https://doi.org/10.6028/NIST.SP.800-57pt1r5, Gaithersburg, MD, May 2020.

HISTORY

These functions first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 7.2.