I\'m looking for a c++ class/library that provides 1024 bit and bigger integers and bit operations like: - bit shifting, - bitwise OR/AND, - position first zero bit
There are several, including GMP, but for speed, the best is likely TTmath. TTmath's design decision to use templated fixed lengths at compiletime lets it be quite fast.
try gmp library. It is a C library. Beginning with GMP 4.0 a C++ wrapper is bundled with the release.
Just consider the big integer multiplication, the TTmath Library use Karatsuba algorithm, which has time complexity $O(n^{1.585})$. The fastest algorithm is $O(n log n log log n)$, using FFT.