(I originally posted this on law.SE and was directed here.)
Patent US4309569, published in 1982 and expiring 20 years later, describes an algorithm for creating hash trees (known as Merkle trees, after its inventor), used for data validation. How is this patentable?
The US has granted "software patents" since the 70s, but my understanding is that this does not extend to obvious/trivial algorithms or mathematical formulae. In my view, hash trees -- and, thereupon, the application of the mathematical formulae presented in the patent -- are entirely obvious and non-novel. I believe this would be the view of any computer scientist or seasoned programmer; it therefore strikes me as unlikely that it was never independently reinvented (before or since), hence invoking a legal challenge.