A simple RSA implementation for small numbers

rsa.cpp

#include<iostream>
#include <vector>
using namespace std;

using LLong = long long int;

LLong gcd(LLong a, LLong b){
    return (b ? gcd(b,a%b):a);
}

LLong odwr_mod(LLong a, LLong mod){
    for(LLong i = 0; i<mod; i++)
        if(a*i % mod ==1)
            return i;
    return -1;
}

LLong power_mod(LLong value, LLong exp, LLong mod){
    if(exp == 0)
            return 1;
    if(exp % 2 == 0){
        LLong a = (power_mod(value,exp/2, mod))%mod;
        return (a*a)%mod;
    }
    return (power_mod(value, (exp-1), mod)*(value%mod))%mod;
}

int main(){
    const int p=17,q=11;

    LLong n = p*q;
    LLong phi = (p-1)*(q-1);
    LLong e = 3;
    while(gcd(e,phi)!=1)
        e++;
    LLong d = odwr_mod(e,phi);

    cout<<"Public  key = "<<e<<endl<<"Private key = "<<d<<endl<<"Modulo      = "<<n<<endl;
    auto print_tab = [](vector<LLong>&tab){
        for(auto i : tab)
            cout<<i<<" ";
        cout<<endl;
    };
    vector<LLong> tab({9,60,43,65,53,22,1});

    cout<<"Value:      "; print_tab(tab);

    for(auto& i : tab){
        i = power_mod(i,e,n);
    }
    cout<<"Encrypted:  ";print_tab(tab);

    for(auto& i : tab){
        i = power_mod(i,d,n);
    }
    cout<<"Decrypted:  ";print_tab(tab);
}