Convert hex string to byte[] array have error " java.lang.StringIndexOutOfBoundsException"

Question

For my previous question, I want to encrypt string on Java and decrypt on Android, I still do not have any solution.

But it is very important to me so I try to think differently. I do not use Base64 encode/decode, instead I use convert bytesToHex and hexStringToByteArray (see this reference).

It looks better, but I got an error I did not understand.

Here is my code:

//encrypt
try {
        Cipher c = Cipher.getInstance("ECIES",BouncyCastleProvider.PROVIDER_NAME);
        c.init(Cipher.ENCRYPT_MODE,publicKey);
        encodeBytes = c.doFinal(origin.getBytes());           
        //encrypt = Base64.getEncoder().encodeToString(encodeBytes);
        encrypt = bytesToHex(encodeBytes);

        System.out.println(encrypt);


    } catch (Exception e) {
        e.printStackTrace();
    }
    //decrypt
    try
    {
        //abc = Base64.getDecoder().decode(encrypt);
        abc = hexStringToByteArray(encrypt);
        Cipher c = Cipher.getInstance("ECIES","BC");
        c.init(Cipher.DECRYPT_MODE,privateKey);
        decodeBytes = c.doFinal(abc);
        String deCrypt = new String(decodeBytes,"UTF-8");

        System.out.println("Decrypt:"+ deCrypt +"\n");
    }
    catch (Exception ex)
    {
        ex.printStackTrace();
    }
public static String bytesToHex(byte[] bytes) 
{
    char[] hexChars = new char[bytes.length * 2];
    for ( int j = 0; j < bytes.length; j++ ) 
    {
        int v = bytes[j] & 0xFF;
        hexChars[j * 2] = hexArray[v >>> 4];
        hexChars[j * 2 + 1] = hexArray[v & 0x0F];
    }
    return new String(hexChars);
}
public static byte[] hexStringToByteArray(String s) {
    int len = s.length();
    byte[] data = new byte[len / 2];
    for (int i = 0; i < len; i += 2) {
        data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                             + Character.digit(s.charAt(i+1), 16));
    }
    return data;
}

The encrypt result is:

0446CB4A315EED9BE6F5698EAEF87E900A0A0868D0F7C7B1D30A17FBE8AB7D22DEC9E6DB15D70D01C8DA1DD69727D6DA9341844BE84673865F99EDD648FEA5F278FB88956E12D4154C8F7386D61E7118BA6C1AEC72A0EB7CAF187E1DE88D860A9A8A5A2B0526672958C2ABD6360E75649FD9D6457BF410EDC4563B1B10D19590E2C3

This code work very good in local (java - java) or (android - android). It means if I encrypt/ decrypt local on Java program or Android app, it doesn't have any error.

But I want encrypt string on Java and decrypt on Android. It means, I copy the encrypted string above to android app and use this code to decrypt this string:

Cipher c = Cipher.getInstance("ECIES","SC");
c.init(Cipher.DECRYPT_MODE,privateKeyFromFile);
encodeBytes = hexStringToByteArray(my_encrypted_string);
decodeBytes = c.doFinal(encodeBytes);
String deCrypt = new String(decodeBytes,"UTF-8");

//////////////////And the function//////////////

public static byte[] hexStringToByteArray(String s) {
    int len = s.length();
    byte[] data = new byte[len / 2];
    for (int i = 0; i < len; i += 2) {
        data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                + Character.digit(s.charAt(i+1), 16));
    }
    return data;
}

When I do that, it has this error:

10-03 22:54:07.078 19949-19949/com.example.napoleon.luanvana W/System.err: java.lang.StringIndexOutOfBoundsException: length=261; index=261
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at java.lang.String.charAt(Native Method)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at com.example.napoleon.luanvana.MessageFragment.hexStringToByteArray(MessageFragment.java:194)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at com.example.napoleon.luanvana.MessageFragment$3.onClick(MessageFragment.java:156)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at android.view.View.performClick(View.java:6213)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at android.widget.TextView.performClick(TextView.java:11074)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at android.view.View$PerformClick.run(View.java:23645)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at android.os.Handler.handleCallback(Handler.java:751)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:95)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at android.os.Looper.loop(Looper.java:154)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:6692)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
10-03 22:54:07.079 19949-19949/com.example.napoleon.luanvana W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
10-03 22:54:07.080 19949-19949/com.example.napoleon.luanvana W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)

I do not really understand why it cause "java.lang.StringIndexOutOfBoundsException", because when I use exactly code block on local Java - Java/ Android - Android it not have that error

Tyler · Accepted Answer · 2017-10-03 16:56:30Z

3

It looks like this is your issue

for (int i = 0; i < len; i += 2) {
    data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
            + Character.digit(s.charAt(i+1), 16)); //i+1 goes out of bounds

i maxes out at len-1 if len is odd. i+1 would = len, and because arrays (Essentially what a string is, a char array), the bounds are as you have in your for loop, (0,len-1)

It is very common to remember the proper for loop bounds for arrays, but forget them when doing any array access that isn't equal to i (i.e. i+1)

Edit:

One possible fix could be this for loop

for(int i = 0; i < len-1; i+=2)

edited Oct 3, 2017 at 16:56

answered Oct 3, 2017 at 16:26

Tyler

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3 Comments

ThanhLam112358 Over a year ago

How can I fix it?

Tyler Over a year ago

Modify the for loop so that i never goes below 0 or above len.

ThanhLam112358 Over a year ago

I have to try like your edit, but I have error "org.spongycastle.jcajce.provider.util.BadBlockException: unable to process block". It mean the encrypted string is not true. Like my previous question stackoverflow.com/questions/46536535/…

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