Julius Cæsar is reported to have replaced each letter in the plaintext by the one standing three places further in the alphabet. For instance, when the key has the value 3, the plaintext word cleopatra will be encrypted by the ciphertext word fohrsdwud. Augustus allegedly found this too difficult and always took the next letter. Breaking the Cæsar cipher is almost trivial: there are only 26 possible keys to check (exhaustive key search) and after the first four or five letters are decrypted the solution is usually unique.
The Cæsar cipher is one of the most simple cryptosystems, with a monoalphabetic encryption: by counting down in the cyclically closed ordering of an alphabet, a specified number of steps.
Cæsar encryptions are special linear substitution (see substitutions and permutations) with \(n=1\) and the identity as homogeneous part ϕ. Interesting linear substitutions with \(n\geq 2\) have been patented by Lester S. Hill in 1932.
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References
Bauer, F.L. (1997). “Decrypted secrets.” Methods and Maxims of Cryptology. Springer-Verlag, Berlin.
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Bauer, F.L. (2005). Cæsar cipher. In: van Tilborg, H.C.A. (eds) Encyclopedia of Cryptography and Security. Springer, Boston, MA . https://doi.org/10.1007/0-387-23483-7_43
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