"New Directions in Cryptography," by Whitfield Diffie and Martin Hellman

In 1976, Whitfield Diffie and Martin Hellman published New Directions in Cryptography, a paper which introduced public key encryption and what would be later termed Diffie Hellman Key Exchange to the general public. For their efforts, they were rewarded with an immediate threat of jail, and, later, the ACM Turing Award.

You can read the original paper as well as [this article about the NSA's attempt to suppress this paper.

There's a bit of math in the middle of the paper, but feel free to skip it if you like. The rest of the paper is quite interesting on its own.