What he probably means by this is that lot of NSA-designed crypto contains somewhat weird design choices and it is interesting to try to find out what is ration

What he probably means by this is that lot of NSA-designed crypto contains somewhat weird design choices and it is interesting to try to find out what is rationale behind such choices. One well-known example of this are DES S-boxes, while the somewhat complex key-schedule of Speck/Simon is also interesting (both algorithms use essentially same feistel network for both key schedule and main cipher). Another interesting thing is NSA's tendency to design cryptographic primitives with explicit key checksums byked into them (parity bits in DES, BATON with 320bit key out of which 160bits (!) apparently are checksum...).