Help with simple Arry encoder

I am triying to make a program which encodes a user password and writes it to a file this is what I have:

Code:

---

/*ENCODER*/
void Encoder(char *Code)
{
        for (int i = 0; i < strlen(Code); i++)
                Code[i] -= 30;             
               
        Code[i] = '\0';
}

---

Code:

---

/*DECODER*/
void Decoder(char *Code)
{
        for (int i = 0; i < strlen(Code); i++)
                Code[i] += 30;

        Code[i] = '\0';
}

---

it works fine, but I want to do something better like

Code:

---

/*ENCODER*/
void Encoder(char *Code)
{
        for (int i = 0; i < strlen(Code); i++)
                Code[i] = (Code[i] * 100) % 256;             
               
        Code[i] = '\0';
}

---

The only problem with that is that I would have no idea how to decode it, could someone describe how the modulus works so I can find a way to decode the arry please?

Ok thanks, but is there any way to encode something like that other than doing what I did?

Bitwise exclusive-or seems to be a popular encyption method. It's reversable. You just exclusive-or with your "key".