In this paper, a variant of the Hill cipher is proposed. In the classical
Hill cipher, an invertible matrix is used for encryption but the scheme is
vulnerable to the known-plaintext attack which can reveal the matrix. In our
proposed cryptosystem, each plaintext block is encrypted by a new invertible
key matrix that thwarts the known-plaintext attack. To generate the invertible
matrices which serve as the dynamic keys we make use of the vector spaces,
randomly generated basis and non-singular linear transformation. Resulting
cipher is secure against the known-plaintext attack.

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