Simple Encryption/Decryption in C++

/*
    Program for simple encryption and decryption.

    c0d3d by sm@rtw3ap0n
*/

#include<iostream>
#include<conio.h>
#include<cctype>
#include<cstring>
#include<Windows.h>

using namespace std;

// read key from user

int readkey()
{
    int key=0;
    cout<<"\nEnter key: ";
    // input key through a loop to prevent invalid input
    char ch;
    do
    {
        ch = getche();
        if (isdigit(ch))
        {
            key = key*10 + atoi(&ch);
        }
        else if (ch != 13)
        {
            cout<<"\b";
            Beep(800, 250);
        }
    } while (ch != 13);

    return key;
}

// encrypt the message

void encrypt(int key, char msg[])
{
    int length = strlen(msg);
    int k = key;
    for (int i = length - 1; i >= 0; --i)
    {
        msg[i] += k%10;
        if (msg[i] > 126)
        {
            msg[i] -= 94;
        }
        else if (msg[i] < 32)
        {
            msg[i] += 95;
        }
        if (k%10 == k)
        {
            k = key;
        }
        else
        {
            k /= 10;
        }
    }
}

// decrypt the message

void decrypt(int key, char msg[])
{
    int length = strlen(msg);
    int k = key;
    for (int i = length - 1; i >= 0; --i)
    {
        msg[i] -= k%10;
        if (msg[i] > 126)
        {
            msg[i] -= 94;
        }
        else if (msg[i] < 32)
        {
            msg[i] += 95;
        }
        if (k%10 == k)
        {
            k = key;
        }
        else
        {
            k /= 10;
        }
    }
}

int main()
{
    char msg[100];
    int key;
    cout<<"\nEnter message: ";
    cin.getline(msg, 99);
    key = readkey();

    // encrypt it!
    encrypt(key, msg);
    cout<<"\nEncrypted Message: ";
    puts(msg);

    // decrypt it!
    decrypt(key, msg);
    cout<<"\n\nDecrypted Message: ";
    puts(msg);
    _getch();
    return 0;
}