The RSA patent covers an encryption algorithm which was used in secure communications.
This patent was the result of a lot of research and it was innovative. It was not possible to overturn the patent by any form of re-examination. The RSA patent is thus a good example of why we need to exclude software from patentable subject matter if we want to be free of harmful software patents.
That said, the key technology for the use of RSA is the production of large prime numbers. Fortunately, this was never patented. RSA itself is icing on the cake.
In 1973 Clifford Cocks of GCHQ had developed the same algorithm. But it remained classified until 1997. Suprisingly, this prior art did not seem to invalidate the RSA patent.