Source? Wikipedia says exactly the opposite: https://en.m.wikipedia.org/wiki/Data_Encryption_Standard
NSA made DES more resistant.
NSA made DES more resistant.
They reduced the key space down to 56 bits. Which was small enough for government computers to hack even decades ago. The original proposal by IBM was for 128 bits IIRC.
https://en.m.wikipedia.org/wiki/56-bit_encryption
That is interesting that the Wikipedia page you linked claims the contrary and makes me suspect tampering.
Because this story has been told over and over again, even in textbooks. See Springer's Understanding Cryptography chapter on DES for instance.
EDIT: In fact, the Wikipedia page you linked states
DES, as stated above, is insecure. This is mainly due to the 56-bit key size being too small.
So where is this nonsense about the NSA making DES more secure?
The original cipher proposed by IBM had a key length of 128 bits and it is suspicious that it was reduced to 56 bits.
The official statement that a cipher with a shorter key length made it easier to implement the DES algorithm on a single chip in 1974 does not sound too convincing
"It took the academic community two decades to figure out that the NSA "tweaks" actually improved the security of DES. This means that back in the '70s, the National Security Agency was two decades ahead of the state of the art."
By the mid-1990s, it became widely believed that the NSA was able to break DES by trying every possible key. This ability was demonstrated in 1998, when a $220,000 machine was built that could brute-force a DES key in a few days
(1) Keeping the key size within brute-forcing range. Agreed, they did that and surely wittingly. By the mid-90s nobody doubted they had the computational strength for it; the 1998 demonstration was that any modestly-funded organisation could too.
(2) Improving the resistance to differential analysis by changing the S-boxes. This is what the Schneier quote is about: the NSA making a change and refusing to disclose the basis, causing suspicion that this installed a backdoor for them, whereas in fact it was (as far as any public cryptographer has disclosed) strengthening it against a cryptanalytic technique that the NSA didn't want to reveal.
But the NSA purposely dictated that DES use 56 bit keys (a strange choice of key length to begin with), so that it would be possible for brute force attacks using government mainframes. The original proposal was for 128 bits IIRC.
This was way back before many people studied cryptanalysis, which is why no one cared at the time.