The core of Hill-cipher is matrix manipulations. It is a multi-letter cipher, for decryption the inverse of matrix requires and inverse of the matrix doesn’t always exist. Then if the matrix is not invertible then encrypted text cannot be decrypted. However, a drawback of this algorithm is overcome by use of self-repetitive matrix. This matrix if multiplied with itself for a given mod value (i.e. mod value of the matrix is taken after every multiplication) will eventually result in an identity matrix after N multiplications. So, after N 1 multiplication the matrix will repeat itself. Hence, it derives its name i.e. self-repetitive matrix. It should be non-singular square matrix.
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