modInverse & powMod

[Salih Dincer]

On Friday, 7 June 2024 at 13:43:29 UTC, Salih Dincer wrote:
> 
> SDB at 79
>
> I have only one question: Is there a modInverse() function in 
> the standard library for use in RSA?

Apparently not, it fell to lot :)

I already had such a function...

```d
auto modInverse(T)(T m, T n) pure
{
   T q, ilk = n;
   T y, tmp, x = 1;

   while (m > 1)
   {
     q = m / n;

     tmp = n;
       n = m % n;
       m = tmp;

     tmp = y;
       y = x - q * y;
       x = tmp;
   }
   return x < 0 ? x + ilk : x;
}
```

And in the BigInt module there was divMod() next to powmod():

```d
auto modInverse(BigInt a, BigInt m)
{
   BigInt q, m0 = m;
   BigInt tmp, y, x = 1;

   while (a > 1)
   {
     // m is remainder now
     tmp = m;
     divMod(a, m, q, m);

     // process same as Euclid's algorithm
     a = tmp;
     tmp = y;

     // Update y and x
     y = x - q * y;
     x = tmp;
   }

   // Make x positive
   if (x < 0) x += m0;

   return x;
}
```

Is PR required? Why not modInverse too!

SDB at 79