WW2 didn't have the computing technology to use long encryption keys on the fly. We do now.
Computers now are fast enough that we can use 512-bit keys and robust algorithms to encrypt/decrypt large amounts of data in real time.
The way to crack a code is to use HUMINT to narrow the set of possible keys into a manageable set and iterate brute force through them. There are potential technologies using quantum computers, but AFAIK they are all in the lab stage.
So what do all the SIGINT guys in militaries do these days?