Xor Encryption Issue

Is this a valid xor encryption function?

Code:

---

#include <iostream>
#include <string>

using namespace std;

void xorEncrypt(string& str, string key)
{
    int strLen = str.length() - 1;

    for(int x=0; x <= strLen; x++)
    {
        str[x] = str[x]^key[x];
    }
}

int main()
{
    string key = "key";
    string text="This my text...";

    xorEncrypt(text,key);
    cout<<text<<endl;

    xorEncrypt(text,key);
    cout<<text<<endl;
}

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I'd do something like this:

Code:

---

void xorEncrypt(string& str, const string& key)
{
    int strLen = str.length();
    int keyLen = key.length();

    for(int x=0; x < strLen; x++)
    {
        str[x] = str[x] ^ key[x % keyLen];
    }
}

---

Quote:

---

str[x] = str[x] ^ key[x % keyLen];

---

I dont understand. How does the modulo operator help?

What happens to your code, when the key is shorter than the text?

Works fine, but when the key string is one or two characters long my CPU beeps.
Why does that happen?

Code:

---

#include <iostream>
#include <string>

using namespace std;

void xorEncrypt(string& str, const string& key)
{
    int strLen = str.length();
    int keyLen = key.length();

    for(int x=0; x < strLen; x++)
    {
        str[x] = str[x] ^ key[x % keyLen];
    }
}


int main()
{
    string key = "ke";
    string text="Hi, my name is John Williams.";

    xorEncrypt(text,key);
    cout<<text<<endl;

    xorEncrypt(text,key);
    cout<<text<<endl;
}

---

edit: I found out that what cause my CPU to beep is not actually the fact that the key string is 1 or 2 characters long.... it's something else i cant seem to figure out,,, could it a problem in my cpu? (btw i'm using codeblock for compiling)


edit2: i'm positive that whats causing my cpu to beep is exactly in this line:

Quote:

---

str[x] = str[x] ^ key[x % keyLen];

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Because when i make it a comment, the CPU doesnt beep

It beeps because the result of the encryption contains non-printable characters, including the bell character.