Blocks of k characters each

The RSA algorithm can encrypt and decrypt numbers from the range of values 0,1,2, ...., N-1. A message M can thus not be encrypted if the bit length of the message is greater or equal to the bit length of the RSA key. Therefore the message is split up into k blocks each of bit length k*8, whereby k*8 must be smaller than the bit length of the RSA modulus. The number 8 comes from the fact that 8 bits are needed to present ASCII characters.