Homework help - File encrpytion/decryption problem...

The following program is supposed to encrypt and decrpyt a file. The
professor told use that we have to use XOR (exclusive or) when
encrypting and decrypting the file.

//Homework 8, question 2

#include <iostream>
#include <math.h>
#include <string>
#include <fstream>
#include <cstring>
#define MAX 40
#define HASHSIZE 101
using namespace std;

unsigned hash(char *s)
{
         unsigned hashval;
         for (hashval = 0; *s; s++)
         {
             hashval = *s + 31 * hashval;
         }

         return hashval % HASHSIZE;
}

int main(void)
{
    unsigned int value;
    char epassword[MAX];
    char dpassword[MAX];
    char inputFilename[MAX];
    char outputFilename[MAX];
    char c;

    cout << "Enter a password: ";
    cin.getline(epassword, MAX);
    cout << "Enter an input filename: ";
    cin >> inputFilename;
    cout << "Enter an output filename: ";
    cin >> outputFilename;

    srand(hash(epassword));
    value = rand();
    //value = hash(epassword);

    ifstream IS(inputFilename, ios::in);
    ofstream OS(outputFilename, ios::out);

    if (!IS) exit(1);
    if (!OS) exit(1);

    //encrpyt the file
    while (IS.read(&c, 1))
    {
          c = value ^ c;
          OS << c;
    }

    IS.close();
    OS.close();

    cout << "\nThe encrypted file is\n...";
    ifstream IS1(outputFilename, ios::in);
    if (!IS1) exit(1);

    //display the encrypted file
    while (IS1 >> c) cout << c;

    cout << endl;
    IS1.close();

    while (1)
    {
             cout << "Enter the password to decrpyt the file: ";
             cin >> dpassword;
             if( strcmp(dpassword, epassword) == 0) break;
             else cout << "Invalid password\n";
    }
    ifstream IS2(outputFilename, ios::in);
    if (!IS2) exit(1);

    //display the decrypted file
    while (IS2 >> c)
    {
          c = c ^ value;
          cout << c;
    }
    cout << endl;

    IS2.close();
    //system("pause");
    return 0;
}

My input.dat file is..
This is some text

The program appears to works fine for some passwords like "sex" and
"alcohol". However, when I use a password like "underagegirls", the
decrpyted file, output.dat comes out as..

This is se text

Why is the output getting truncated?

Chad