Decoding text using a random generated substitution cipher

I am almost done with a program for a random cipher which encodes and decodes. However, I can't get my program to also decode messages. It confuses me as I don't have a key to the cipher, since the letters are replaced in a random manner each time, so I have no idea how a decryption code should look like for this program. What approach should I take?

Here is the code:

void randomizer()       //cipher

{ srand(time(0)); //seed for rand() static char alphabet[]="ABCDEFGHIJKLMNOPQRSTUVWXYZ"; //static array to hold uppercase Latin letters static char alphabetlow[]="abcdefghijklmnopqrstuvwxyz"; //lower-case Latin letters const int LENGTH=sizeof(alphabet)-1; //length of the array

int r;
char temp;
char tempb;
for (unsigned int i=0; i<LENGTH; i++)   //loop which shuffles the array
{
    r=rand() % LENGTH;  //generate a sequnce of pseudo-random numbers

    temp=alphabet[i];          //temp gets the int values of the array elements
    alphabet[i] = alphabet[r];  //randomized letters are copied into the ordered alphabet (now scrambled)
    alphabet[r]=temp;   //ordered letters are copied into the scrambled alphabet

    tempb=alphabetlow[i];   //shuffle second array
    alphabetlow[i]=alphabetlow[r];
    alphabetlow[r]=tempb;
}
string text, textb;
getline(cin,text);

for (unsigned int i=0; i<text.length(); i++)     //loop to encrypt
{
    if (isalpha(text[i]))   //encrypt only input text which is Latin letters
    {
        if (islower(text[i]))   //checks if input text is lowercase letters
        {
            text[i]=alphabetlow[text[i] - 'a']; //scrambles lowercase letters
        }
        else
        {
            text[i]=alphabet[text[i] - 'A'];    //scrambles uppercase letters
        }
    }
}
cout<<"Encrypted: "<<text<<endl;

getline (cin, textc);
for (unsigned i=0; i<textc.length(); i++)
{
    //supposedly code to decode should be here
}
cout<<"Decrypted: "<<textc;