Big MATH is a calculator which go well beyond the typical 12 or even 16 digits of general purpose calculators. It can handle over 600 digits values in base 10 (2048 bits in binary). All of the math is integer math though, no factions. Special functions, useful for computer scientist and cryptographer, like 'Mod', Power: y^x, logic operators and several bases (you can even make your own). You can also do Modulo math to any number.
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Checkout this Easy, pencil & paper implemented, Cipher: E-Cipher Overview