RSA Decryption

///////////////////////////////////////////////////////////////////////////////
// This program takes a RSA encrypted message in a text file, 
// by given parameters, and decrypts it. After which it outputs to console.
///////////////////////////////////////////////////////////////////////////////
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <math.h>
///////////////////////////////////////////////////////////////////////////////
void fileRead(std::ifstream &inFile, std::vector &fileBuff);
void decrypt(std::vector<int> cipherText, std::vector<char> &plaintText,
		int pq, int d); // decrypts passed cipher text
char toChar(int i); // change to char, a-z = 0 - 26
int residue(int number, int power, int modulo);
int residualPowTwo(int number, int power, int modulo);
int BaseTwoConv(int a, int b[]);
///////////////////////////////////////////////////////////////////////////////
int main(void)
{
	std::string filePath;
	std::ifstream inFile;
	int pq = 323, d = 107, e = 35; // provided decryption parameters
	std::vector<int> cipherText; // vector to hold encrypted message
	std::vector<char> plainText; // vector to hold decrypted message

	std::cout << "Enter filepath: "; std::cin >> filePath;
	inFile.open(filePath.c_str());

	while(!(inFile.is_open()))
	{
		std::cout << "\nFile not found\nEnter filepath: "; std::cin >> filePath;
		inFile.open(filePath.c_str()); // cast to c string to open
	}

	fileRead(inFile, cipherText); // read from file into vector
	decrypt(cipherText, plainText, pq, d); // decrypt the ciphertext

	std::cout << "Decrypted Message: ";
	
	for(auto it : plainText) // iterate through and print decrypted message
	{
		std::cout << it;
	}

	std::cout << std::endl; // endline at end looks better

	return 0;
}
///////////////////////////////////////////////////////////////////////////////
void fileRead(std::ifstream &inFile, std::vector<int> &fileBuff)
{
	std::string ln; // string to buffer input
	while(std::getline(inFile, ln)) // while reading by line
	{
		std::istringstream iss(ln); // use stringstream to parse
		int buff; // declare int for easy type casting w/ iss 

		while(iss >> buff) // while stringstreaming and casting to int
		{
			fileBuff.push_back(buff); // pushback to vector
		}
	}		

	return;
}
///////////////////////////////////////////////////////////////////////////////
void decrypt(std::vector<int> cipherText, std::vector<char> &plainText,
	int pq, int d)
{
	int ctBuff; // ciphertext buff

	for(auto it : cipherText) // iterate through ciphertext ints
	{
		ctBuff = residue(it, d, pq); // calculate residue
		plainText.push_back(toChar(ctBuff)); // convert to char, push
	}
	return;
}
///////////////////////////////////////////////////////////////////////////////
char toChar(int i)
{
	if(i > 0 && i < 27) // if it is an uppercase letter
	{
		return (char)(i + 64);
	}
	
	else if(i == 27) // if it is a space
	{
		return ' ';
	}

	else
	{
		std::cout << "\nErroneous output detected";
		exit(1);
	}
}
///////////////////////////////////////////////////////////////////////////////
int residue(int number, int power, int modulo)
{
	int b[100], digits, i;

	long long result;

	result = 1;

	digits = BaseTwoConv(power, b); 

	for(i=0; i < digits; i++) 
	{
		if(b[i] == 1)
		{
			result = result * residualPowTwo(number, pow(2,i), 
				modulo);
		}
	}
	
	return(result % modulo);
}
///////////////////////////////////////////////////////////////////////////////
int residualPowTwo(int number, int power, int modulo)
{
	int result = number % modulo;

	for (int i = 2; i <= power; i = (2*i)) { result = (result * result) % modulo; } return(result); } /////////////////////////////////////////////////////////////////////////////// int BaseTwoConv(int a, int b[]) { int i = 0; while (a>0)
	{
		b[i] = (a%2);
		a /= 2;
		i++;
	}
	
	return i;
}
///////////////////////////////////////////////////////////////////////////////