multiplicative inverse finding algorithm

[creeping death]

hey ,
can someone pliz help me with the project m doin..
i want to implement rsa crypto system in c++
but m finding it difficult to calculate d since i dont know how to code the part where we find inverse of e..

pliz i urgently need sm1 help..

here is toy-RSA ... for 32-bit prime integers....but in C

#include<stdio.h>
#include<string.h>

struct keys
{
    unsigned long int n;
    unsigned long int de;
};

int lenp,lend;

int testprime(int no)
{
    int i;
    for(i=2;i<no;i++)
    {
        if((no &#37; i) == 0)
        {
            return 0;            
        }
    }
    return 1;
}

int testgcd(int dividend,int divisor)
{
    int rem=-1;
    while(rem != 0)
    {
        rem=dividend % divisor;
        dividend=divisor;
        if(rem != 0)
           divisor=rem;
    }
    return divisor;
}

unsigned long int expocal(unsigned long int c,unsigned long int d,unsigned long int n)
{
    unsigned long int i=0,ans=1;
     for(i=1;i<=d;i++)
    {
       ans=ans%n;
       ans=ans*c;
    }
    ans=ans%n;
    return ans;    
}

unsigned long int findd(unsigned long int e,unsigned long int phi)
{
    unsigned long int d=1,rem=-1,test;
    while(rem!=0)
    {
        test=e*d;
        test=test-1;
        rem=test%phi;
        if(rem==0)
            return d;
        d++;
    }
    return 0;
}

unsigned long int rsa_encryption(struct keys publickey,unsigned int long m)
{
    return expocal(m,publickey.de,publickey.n);
}

unsigned long int rsa_decryption(struct keys privatekey,unsigned long int c)
{
    return expocal(c,privatekey.de,privatekey.n);
}

void padding(char *p)
{
    int i;
    for(i=0;i<lenp;i++)
    {
        p++;
    }
    for(i=lenp;i<lend;i++)
    {
        *p=(int) NULL;
        p++;
    }
}

void rsa_algorithm(unsigned long int p,unsigned long int q,int pt)
{
    struct keys publickey,privatekey;
    unsigned long int n,phi,e,d,ct;
    register int i=0;
    n=p*q;
    phi=(p-1)*(q-1);    
    for(i=2;i<phi;i++)
    {
        if(testgcd(phi,i)==1)
        {
            printf("\t| %lu |\t",i);
        }
    }
    printf("\n\nSelect a value of e from the above list\n");
    scanf("%lu",&e);
    while(testgcd(phi,e)!=1)
    {
        printf("uncompatible value of e. choose another value for e\n");
        scanf("%lu",&e);
    }
    publickey.n=n;
    publickey.de=e;
    privatekey.n=n;
    d=findd(e,phi);
    printf("d=%lu\n",d);
    privatekey.de=d;
    ct=rsa_encryption(publickey,pt);
    printf("ct=%d\n",ct);
    pt=rsa_decryption(privatekey,ct);
    printf("pt=%d\n",pt);   
}    

int main()
{
    unsigned long int pr1,pr2,pt;
    printf("Enter an integer(plain-text value)");
    scanf("%lu",&pt);
    printf("enter 2 prime numbers");
    scanf("%lu,%lu",&pr1,&pr2);
    while(testprime(pr1)==0)
    {
        printf("the number %lu is not prime\nenter a prime no",pr1);
        scanf("%lu",&pr1);
    }
    while(testprime(pr2)==0)
    {
        printf("the number %lu is not prime\nenter a prime no",pr2);
        scanf("%lu",&pr2);
    }
    rsa_algorithm(pr1,pr2,pt);
    return 0;
}