The attack is &quot;differential analysis&quot; -- essentially, if you have a black box encoder, carefully chosen plaintexts sent through the system can leak information, and substantially weaken DES as described.<p>Academic literature started discussing differential analysis in the early &#x27;90s if memory serves -- NSA helped harden against it in the 1970s!<p>Current wisdom is that the NSA is not nearly so far ahead of the cryptography curve these days. But, it could just be speculation. :)