The matrix theory is used by hill cipher.
Explanation:
Th technique which uses matrix theory is hill cipher technique.
Hill cipher technique is a polygraphic substitution cipher and it is based on linear algebra.
For encrypting a message in Hill Cipher technique each block of n letters is multiplied by an invertible matrix of nxn and that to against modulus 26 and Β for the decryption of the message, every block is multiplied by the inverse of the matrix that was used for encryption.