1. A method of converting data between an unencrypted format and an encrypted format, the data being organized in bit words and being stored in at least one register, the method comprising: using a circuit cooperating with the at least one register to convert the data by at least performing a plurality of transformation rounds, each transformation round having a respective round key and comprising applying at least one transformation to a two-dimensional array of rows and columns of bit words defining a state array, exchanging each of the rows with a respective column of the state array to form a transposed state array for at least one of the transformation rounds so that the at least one transformation is applied to the transposed state array, transposing the respective round key, and applying the respective transposed round key to the state array in at least one of the transformation rounds; and using the circuit to transpose an output of a final round from the plurality of transformation rounds.