Question

I am implementing a method to encrypt with a key and i made a call like this:

Crypto c = new Crypto("mysecretkey");
String enc = c.encrypt("mytext");

But i am getting an exception

“crypto encrypt error: String index out of range: -1”

at this part:

String sKeyChar = getKey().substring((i % getKey().length()) - 1, 1);

And I don’t know what I am doing wrong because I made the same thing in PHP and works good. Maybe this is simple but I am stuck, this is my method:

public String encrypt(String sData) {
        String sEncrypted = null;
        try {
            String sResult = null;
            for (int i = 0; i < sData.length(); i++) {
                String sChar = sData.substring(i, 1);
                String sKeyChar = getKey().substring((i % getKey().length()) - 1, 1);
                char c = (char) (ord(sChar) - ord(sKeyChar));
                String sPart = (new StringBuffer().append(c)).toString();
                sResult += sPart;
            }
            byte[] sResultBuff = sResult.getBytes("UTF-8");
            sEncrypted = Base64.encode(sResultBuff);
        } catch (Exception e) {
            System.out.println("crypto encrypt error: " + e.getMessage());
            sEncrypted = null;
        }
        return sEncrypted;
    }

Other method needed:

public int ord(String sChar) {
    int ascii_code = 0;
    try {
        ascii_code = String.valueOf(sChar.charAt(0)).codePointAt(0);
    } catch (Exception e) {
        System.out.println("crypto ord error: " + e.getMessage());
        ascii_code = 0;
    }
    return ascii_code;
}

PHP equivalent method:

function encrypt($sData, $sKey='mysecretkey'){ 
    $sResult = ''; 
    for($i=0;$i<strlen($sData);$i++){ 
        $sChar    = substr($sData, $i, 1); 
        $sKeyChar = substr($sKey, ($i % strlen($sKey)) - 1, 1); 
        $sChar    = chr(ord($sChar) + ord($sKeyChar)); 
        $sResult .= $sChar; 
    } 
    return encode_base64($sResult); 
} 

Thanks!

Answer 1

You see the difference between PHP and Java because PHP’s substr understands negative numbers, but Java’s substring does not: it throws an exception.

In PHP, passing negative 1 to substr means “get me the last character”, but in Java you need to pass the index of the last character (i.e. str.length()-1) to achieve the same effect.

If this is not a mistake, and this is precisely the effect that you wanted to achieve, you can address this issue with an if condition:

int pos = (i % getKey().length()) - 1;
if (pos == -1) {
    pos = getKey().length() - 1;
}
// EDIT: Second argument needs to be pos+1, not 1. Thomas pointed out this error
String sKeyChar = getKey().substring(pos, pos+1);

EDIT As Thomas correctly pointed out, the other difference between PHP version of substr and Java’s substring is in their treatment of the second argument: PHP thinks it’s length; Java thinks it’s the index of the last character plus one.