Danny Code V2

#include <iostream>
#include <algorithm>
#include <string>

using namespace std;

string getInput();
int gcd(int a, int b);
void getAB(int & a, int & b);
bool withinLower(char i);
bool withinUpper(char i);
int modInverse(int a);
void encryptText(string & input, int a, int b);
void decryptText(string & input, int a, int b);

int main() {
    string input = getInput();

    int a, b;
    getAB(a, b);

    cout << "\nEncrypted: ";

    encryptText(input, a, b);

    cout << "\n\nDecrypted: ";
    decryptText(input, a, b);
    cout << "\n";
    return 0;
}

string getInput() {
    cout << "\nInput string: ";
    string input;
    getline(cin, input);
    return input;
}

int gcd(int a, int b) {
    return b == 0 ? a : gcd(b, a % b);
}

void getAB(int & a, int & b) {
    do {
        cout << "\na and b must be coprime\na = ";
        cin >> a;

        if (!cin.fail()) {
            cout << "b = ";
            cin >> b;
        }

        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    } while (cin.fail() || gcd(a, b) != 1);
}

bool withinLower(char i) {
    return i >= 'a' && i <= 'z' ? true : false;
}

bool withinUpper(char i) {
    return i >= 'A' && i <= 'Z' ? true : false;
}

// from https://rosettacode.org/wiki/Modular_inverse#C.2B.2B
int modInverse(int a) {
    int b = 26, t, q, x0 = 0, x1 = 1;
    while (a > 1) {
        q = a / b;
        t = b, b = a % b, a = t;
        t = x0, x0 = x1 - q * x0, x1 = t;
    }
    if (x1 < 0) x1 += 26;
    return x1;
}

void encryptText(string & input, int a, int b) {
    for (unsigned int i = 0; i < input.length(); ++i) {
        if (withinLower(input[i])) {
            input[i] = (char)((a * (input[i] - 'a') + b) % 26 + 'a');
        }
        else if (withinUpper(input[i])) {
            input[i] = (char)((a * (input[i] - 'A') + b) % 26 + 'A');
        }
        else {
            input[i] = input[i];
        }
    }
    cout << input;
}

void decryptText(string & input, int a, int b) {
    for (unsigned int i = 0; i < input.length(); ++i) {
        if (withinLower(input[i])) {
            input[i] = (char)(modInverse(a) * (26 + input[i] - 'a' - b) % 26 + 'a');
        }
        else if (withinUpper(input[i])) {
            input[i] = (char)(modInverse(a) * (26 + input[i] - 'A' - b) % 26 + 'A');
        }
        else {
            input[i] = input[i];
        }
    }

    cout << input;
}