In the 80s, the NSA was a major customer of supercomputers. https://www.nsa.gov/Portals/70/documents/news-features/declassified-documents/cryptologic-quarterly/NSA_and_the_Supercomputer.pdf discusses this, including the lamentation that the industry was moving from the old vector supercomputers to new massively parallel models, as they had legacy software valued at $3 billion, which was not necessarily designed to run well on massively parallel machines.
What exactly did they use supercomputers for? In particular, those vector machines from vendors like Cray, were designed to crunch floating-point numbers. It doesn't jump out at me that this is useful for cryptography, which would seem to require bit shuffling rather than number crunching. At first glance, it seems like a Cray would be wasted on cryptography, achieving no more performance than a cheaper machine like a DEC.
What am I missing?