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I was browsing (ok, lets be honest – Goooogling) the web in order to find some Multiple-Precision Modular Arithmetics algorithms. Our lecturer recomended a very good book – “Handbook of Applied Cryptography“. So, I used this book in order to code my algorithms (modular addition, subraction and multiplication)

So, here is my modular addition algorithm implementation in the C programming language.

// Name: int MultiplePrecisionAddition()
// Input:        x        – first integer, size n
//                y        – second integer, size n
//                n        – number of digits in x, y, w
//                base    – modulus
// Output:        w        – result, size n, w[n - 1]…w[0]
// Description: Calculates (x + y mod base)
int MultiplePrecisionAddition(int x[], int y[], int w[], int n, int base) {
int i = 0, c = 0, result = 0, temp = 0;
for (i = 0; i < n – 1; ++i) {
result = x[i] + y[i] + c;
w[i] = result % base;
result < base ?    c = 0 : c = 1;
}
w[n - 1] = c;

// Converts from w[0]w[1]…w[n - 1] to w[n - 1]…w[0]
for (i = 0; i < n / 2; ++i) {
temp = w[i];
w[i] = w[n - 1 - i];
w[n - 1 - i] = temp;
}
return 0;
}

Any suggestions how to improve my code?