SUBMIT.
Of course, 2-block ciphers can be cracked using a frequency analysis on pairs of letters
rather than on individual letters. (As an example, Q is almost always followed by U in
English.) The idea of a 2-block cipher easily extends to blocks of greater length, using
larger matrices. One difficulty with a block cipher is that knowledge of the encryption and
decryption matrices are essentially equivalent: anyone who discovers the encryption matrix
can compute its inverse (the decryption matrix) using row reduction or other methods of
computing matrix inverses. This means the encryption matrix must be kept secret from
your enemies. Until the 1970s, everyone (outside US and British intelligence) thought that
encryption and decryption using any cipher would be more or less symmetric processes. But
this is false: there are cryptographic algorithms where the encryption process can be made
public to the whole world and decryption is still secure. Number theory is a source of such
algorithms which are used in e-commerce, ATM transactions, cell phone communications,
etc.