REM Modular Exponentiation using Russian Peasant Method
REM Robert Campbell, 970629

DECLARE FUNCTION powmod& (number&, exponent&, modulus&)

PRINT "Enter numbers for powmod computation:"
INPUT "    Base: ", number&
INPUT "    Exponent: ", exponent&
INPUT "    Modulus: ", modulus&
temp& = powmod&(number&, exponent&, modulus&)
PRINT "powmod(", number&, ", ", exponent&, ", ", modulus&, ") = ", temp&
END

FUNCTION powmod& (number&, exponent&, modulus&)
e& = exponent&
accum& = 1
pow2& = number&
WHILE (e& > 0)
        IF ((e& MOD 2) = 1) THEN accum& = (accum& * pow2&) MOD modulus&
        pow2& = (pow2& * pow2&) MOD modulus&
        e& = e& \ 2
WEND
powmod& = accum&
EXIT FUNCTION
END FUNCTION