Communication Theory of Secrecy Systems
Communication theory is intimately related to the theory of secrecy systems.
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Secrecy as information
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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.
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The one-time pad
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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.
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Read with the 1948 paper
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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.