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copensslbit-manipulationbignum

OpenSSL BIGNUM — is there a bitwise & function?

I am using the BIGNUM library from OpenSSL and I need to do a calculation in an if statement as follows:

if (expo & 1)

I know for bit shifting there are functions. Is there a function corresponding to this kind of operation (where expo is a BIGNUM*)? If not, is there a way to carry this out?

Solution

  • The function you are looking for is:

    bool BN_is_bit_set( const BIGNUM *a, int n );

    ... it behaves exactly like the bitwise operator AND. For example, to see if the right-most bit is on I would call the function as follows:

    bool is_set = BN_is_bit_set( ptr_my_bignum, 0x1 );

    ... notice my mask is 0x1 (which implies 1 in binary). More information on the BIGNUM library can be found here.