The two primary references, and a book recommendation

1. W. Diffie and M. E. Hellman, New directions in cryptography , IEEE Transactions on Information Theory IT-22 (1976), 644-654.

2. L. M. Adelman, R. L. Rivest and A. Shamir, A method for obtaining digital signatures and public-key cryptosystems , Communications of the ACM , 21 (1978), 120-126. (It made its way into the public domain the previous year)

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If you don't already know of it - or do, but don't yet have a copy - then seek out In Code ( A Mathematical Journey ) by remarkable young Irish woman Sarah Flannery, and her co-author father David Flannery (mathematician, and teacher extraordinaire). Check out (rave) reviews at the MAA and AMS web sites.