There was error checking, but not security. Each bit was expanded to a 5-bit sequence, so bit errors could be detected. The message started with a 3-bit vehicle address, so separate messages could be delivered to the Command Module and the Lunar Module.

For the range safety officer's destruct control, NASA did use cryptographic security because they wanted to make sure the Russians couldn't blow up the spacecraft. This was an entirely separate system. The control unit used a secret plug that encoded the "key", that was installed just before launch so nobody could spoof the command.