[aliakbarkhan]

You forgot to mention that the NSA's S-Box tampering actually made it stronger against differential cryptanalysis, which was unknown to the public at that time. It seems like the key size reduction was a compromise for that enhancement. It's an interesting move because it shifts the power from the smartest adversary to the one with the most brute force, perhaps with the assumption that only the U.S. and its allies had access to systems capable of cracking a 56-bit key at the time. That would have made the cipher more secure against other countries that had to rely on cryptanalysis, while still giving them the access they needed if they so desired it.