This question was originally posted here on Crypto StackExchange. As suggested by an answer I am posting it here to help get a better perspective on the math side.
Public-key cryptography was not invented until the 1970's. Apart from the idea not existing earlier (as talked about here), is there any reason it could not have been used earlier? For example, are there forms that are easy enough to perform by hand but complicated enough to not be solved (easily) by hand?