New Directions in Cryptography

Diffie and Hellman ask how two people who have never exchanged a secret can agree on one over a public network. Read it to see how private exponents and exchanged modular powers meet at the same shared value, why reversing a public value is meant to be difficult, and why authentication must still stop an active intermediary from substituting keys.

Reading focus: Why advance pairwise secret delivery grows poorly as a network adds users, even before anyone sends an encrypted message. How each party keeps one exponent private, publishes one modular power, and combines the other public value with its own secret to derive the same result. Why the algebra blocks a passive eavesdropper only under hard-problem assumptions, while authentication, key derivation, parameter validation, and symmetric encryption remain separate protocol duties.

IEEE Transactions on Information Theory 1976. Whitfield Diffie and Martin E. Hellman. 30 min read, easy difficulty.