THE HILL CRYPTOGRAPHIC SYSTEM WITH UNKNOWN CIPHER ALPHABET BUT KNOWN PLAINTEXT
Jack Levine, Richard E. Chandler · Cryptologia · 1989
The cryptographic equations relating plaintext, ciphertext, and key-matrix elements in the Hill system are nonlinear equations if the cipher alphabet is unknown. In the case where plaintext is known it is possible to reduce these equations to linear equations by the introduction of a larger set of unknowns. These latter equations are analyzed by a method of successive eliminations of unknowns from a series of related row-reduced echelon forms.