1949
Communication Theory of Secrecy Systems
The proof of what unbreakable encryption actually requires — the foundation under all modern cryptography.
“Communication theory is intimately related to the theory of secrecy systems.”
Secrecy as information
Shannon treats ciphertext as a noisy channel. The enemy knows the system except the key. Perfect secrecy means the ciphertext reveals nothing about the plaintext: mutual information is zero.
The one-time pad
The proof implies an uncomfortable fact: perfect secrecy needs key entropy at least equal to message entropy. The one-time pad is not a hack; it is the only way to reach perfection in this model.
Read with the 1948 paper
The 1948 communication paper asks how much can be sent reliably. This one asks how much can be hidden. Together they frame both the internet and modern crypto engineering.
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A Mathematical Theory of Communication
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Computing Machinery and Intelligence