No, you don't.   Nobody ever cracked DES.  Yes, the key length was too short and it ended up being bruteforced.  But nobody ever CRACKED it.  Same for many other cryptographic protocols. 

If I give you the monero block chain, and nothing else, then chances are VERY VERY low that you will find a way to descrable it, if the crypto was correctly implemented. 

That's something entirely different from breaking into a computer system.  We're talking about mathematics here.  If the mathematics are sound, and if they were correctly implemented, which is not too hard to verify, then this is something entirely different than finding a buffer overflow.  You're not looking at a running system, you're looking at an encrypted piece of data.  Most of the time, this cannot be undone.  Of course, it is not unthinkable that the cryptography used has a hole in it, or is erroneously implemented.   The implementation is usually rather easy to check, because, contrary to a computing system with a very complicated state space, the real cryptographic state space is relatively simple to verify.

You'll find rarely an erroneous implementation of AES or of a public key cryptosystem.  These are usually correctly implemented, especially if it is done by someone aware of cryptography.  Once this code has FINISHED its calculation, and you are only left with the output data, "cracking" is of a more difficult nature than penetrating a running system.

Yes, you can maybe enter a running computer that is encrypting a file with AES.  However, once you have the output, and you only have the output, good luck in "cracking" it.  Totally different story.