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Modular Multiplication

  1. Dec 29, 2006 #1
    I want to write a C/C++ program and encounter a problem.If I have three 64 bit numbers and need to manipulate
    "a * b mod c"

    Is there any well-known, efficient and simple algorithm to implement it?

    I only know it can be calculated sth. like this..
    __int64 a, b, c
    a = x1 * 2^32 + x0
    b = y1 * 2^32 + y0

    then a * b = x1y1 * 2^64 + (x0y1 + x1y0) * 2^32 + x0y0

    but I dunno what to do in the next step

    Note that this cannot be calculated using some classes like BigInt, gmp

    Thank you very much!!!
  2. jcsd
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