The security of a cryptosystem must not depend on keeping the cryptographic algorithm secret.
Kerkhoffs' Principle
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References
Flannery, S. and Flannery, D. In Code: A Mathematical Journey. London: Profile Books, p. 75, 2000.Referenced on Wolfram|Alpha
Kerkhoffs' PrincipleCite this as:
Weisstein, Eric W. "Kerkhoffs' Principle." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/KerkhoffsPrinciple.html