The structure of Square is a Substitution-Permutation network with eight rounds, operating on 128-bit blocks and using a 128-bit key.
Square is not patented.
Joan Daemen, Lars Knudsen, Vincent Rijmen, "The Block Cipher Square" (PDF) (http://www.esat.kuleuven.ac.be/~cosicart/pdf/VR-9700.PDF), Fast Software Encryption, Volume 1267 of Lecture Notes in Computer Science (E. Biham, ed.), pp. 149–165. Springer-Verlag, 1997.
Square home page (http://www.esat.kuleuven.ac.be/~rijmen/square/)
The four-square cipher uses four 5 by 5 matrices arranged in a square.
To generate the ciphertext squares, one would first fill in the spaces in the matrix with the letters of a keyword or phrase (dropping any duplicate letters), then fill the remaining spaces with the rest of the letters of the alphabet in order (again omitting "Q" to reduce the alphabet to fit).
When only the ciphertext is known, brute forcecryptanalysis of the cipher involves searching through the key space for matches between the frequency of occurrence of digrams (pairs of letters) and the known frequency of occurrence of digrams in the assumed language of the original message.
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