If you have some you should be able to crack them.
2^256 = 115792089237316195423570985008687907853269984665640564039457584007913129639936 tries to brute force
If you have half:
2^128 = 340282366920938463463374607431768211456 tries to brute force
See the difference? #1 is impossible, #2 actually would take the bitcoin global hashing network like two days.
Wrong... #2 is also vastly out of reach of any current computational resources.