Alice holds her message to Bob as a bitstring and encodes it into a matrix. She starts from the identity matrix
A=(1001)
and reads the bitstring one character at a time, starting from the leftmost bit. For a 0 bit she multiplies A on the right by
(1101)
that is, A←A⋅(1101). For a 1 bit she multiplies A on the right by
(1011)
that is, A←A⋅(1011). After the last bit she transmits the resulting matrix A.
Bob deleted the program that decodes a message from Alice. Given the transmitted matrix, recover the original bitstring.