Abstract: A coding transformation method includes the steps of dividing an input data into blocks with the length of 13 bits, where the original data set X includes all 8192 13-bit symbols and 12 filling-bit symbols, and uses 91 printable characters in the output representation of the input data. The output set Y is a sub-set of the direct product R91×R91, with R91 being a set of 91 printable ASCII characters. The reversible coding mapping relationship is Base91[x]=(ch1, ch2)=(R91—CH[x/91], R91—CH[x%91]) where x?X, ch1, ch2?R91. For an input data block less than 13-bits long, n bits are added to the specified side to make it become a complete block with the image of the block of data in the direct product R91×R91.