Encrypting and decrypting part of the ASCII table - c++

Question

I have made small encryption program that uses to keys rot7 and rot13. Everything works fine except two 6 letters uvwxyz.
If I type ABCDEFGHIJKLMNOPQRSTUVWXYZ it encrypts and decrypts no problem. However, if I enter the same in lower case letters then uvwxyz do not work.
Having said this, I have allowed all the writable characters within the ascii table as the valid range as follows:
// allow all writable characters from 32 to 255
if ((str[i] >= 32 ) && (str[i] <=255))
{
str[i] -= key;
}
Here is the process of encryption:
cout << endl;
cout << "Encrypting process started " << endl << endl;
cout << "--------------------------- " << endl;
//get the string length
int i = 0;
int length = str.length();
int key = rot13 ;
int k = 5;
int multiple = 0;
int count = 0;
cout << "the text to encrypt is: " << str << endl;
cout << "text length is: " << length << endl;
cout << "using rot13"<<endl;
cout <<"---------------------------" << endl;
cout << "using rot13" << endl;
//traverse the string
for(i = 0; i < length; i++)
{
count ++;
cout << left;
//if it is a multiple of 5 not the first character change the key
if((multiple = (( i % 5 ) == 0)) && (count != 1) && (key == rot13)){
key = rot7;
}
//if it is a multiple of 5 not the first character change the key
else if((multiple = (( i % 5 ) == 0)) && (count != 1) && (key == rot7) ) {
key = rot13;
}
// Capital letters are 65 to 90 (a - z)
if ((str[i] >= 32) && (str[i] <= 255))
{
str[i] += key;
}
}
return str;
How is it possible that the capital letters work and not the lower case if I have allowed this range? Can it be because of something else? I have added these captures with what happens step by step... hopefully this helps

Answer 1

In your code :
if ((str[i] >= 32) && (str[i] <= 255))
{
if (str[i] + key > 255)
str[i] = ((str[i] + key) % 255 )+ 32;
else
str[i] += key;
}
if key has a value of 13 and str[i] is 'u' or greater, str[i] has a value higher than 255.
You should use modulo % operator in this case, this is rotation, not only a shift