Image packet decoding on Android

Question

I am trying to request a captcha code via the Clickatell api and show the image on my activity. I manage to get the xml response containing the image, but I can't decode it properly in order to show it (the bitmap created is always null).

The xml response:

<ClICKATELLSDK>
<Action>get_captcha</Action>
<Result>Success</Result>
<Values>
<Value>
<captcha_id>c202a3fc98c50a5f8dccc8540077614e</captcha_id>
<captcha_image>%89PNG%0D%0A%1A%0A%00%00%00%0DIHDR%00%00%00%AF%00%00%00-%08%02%00%00%00W%88%AC%F1%00%00%12%1AIDATx%9C%ED%5CyxTU%B2%AFs%97%5E%93%EE%A4%B3%27%84l%24%21%40X%24%A8%04%81%A8++%22H%40% B8l%2C%E7%60%3D%14%27%831%D2%C5Z%E2Fu%08%3Cm3%A9%DB%EB%A2%2C% D2FsZ%83H%2B%7B%13s%0Du%87%D4%A6%F3DE%0E%5Db%7B.a%FFZ%D7T%1E% %06%94%96%89%F4%91%00%D0%9D2%85%1E%1B%FA%01%8A%AB+%E0%FF%00%CDSh%D94%F1%00%D8%00%00%00%00IEND%AEB%60%82
</captcha_image>
<Value>
</Values>
<Timestamp>1316511558</Timestamp>
</ClICKATELLSDK>

I parse this xml and I save the captcha_image to a String that I use in this code to create the bitmap:

captchaImage = <captcha_image from xml>
byte[] encodeByte = Base64.decode(captchaImage, Base64.DEFAULT);
Bitmap bitmap = BitmapFactory.decodeByteArray(encodeByte, 0, encodeByte.length);

Answer 1

First of all, I think Url encoded value pasted above is corrupted. In url encoding, hexedecimal representation must lead after % character. But there is part like this: %1E% %06

You can use this TextUtil class to encode/decode from/to byte array!

String xmlValue = <your xml captcha value>
byte[] decoded = TextUtil.urlDecode(xmlValue.getBytes("UTF-8"));

public final class TextUtil {
static final String HEX_DIGITS = "0123456789ABCDEF";

protected static String urlEncode(byte[] rs) {
    StringBuffer result = new StringBuffer(rs.length * 2);

    for (int i = 0; i < rs.length; i++) {
        char c = (char) rs[i];

        switch (c) {
        case '_':
        case '.':
        case '*':
        case '-':
        case '/':
            result.append(c);
            break;

        case ' ':
            result.append('+');
            break;

        default:
            if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9')) {
                result.append(c);
            } else {
                result.append('%');
                result.append(HEX_DIGITS.charAt((c & 0xF0) >> 4));
                result.append(HEX_DIGITS.charAt(c & 0x0F));
            }
        }

    } 

    return result.toString();
}

protected static byte[] urlDecode(byte[] bytes) throws UnsupportedEncodingException,
        IllegalArgumentException {
    if (bytes == null) {
        return null;
    }

    byte[] decodeBytes = new byte[bytes.length];
    int decodedByteCount = 0;

    try {
        for (int count = 0; count < bytes.length; count++) {
            switch (bytes[count]) {
            case '+':
                decodeBytes[decodedByteCount++] = (byte) ' ';
                break;

            case '%':
                decodeBytes[decodedByteCount++] = (byte) ((HEX_DIGITS.indexOf(bytes[++count]) << 4) + (HEX_DIGITS
                        .indexOf(bytes[++count])));

                break;

            default:
                decodeBytes[decodedByteCount++] = bytes[count];
            }
        }

    } catch (IndexOutOfBoundsException ae) {
        throw new IllegalArgumentException("Malformed encoding");
    }

    return decodeBytes;
}
}

Answer 2

A lot of % indicate that your image is Url encoded. Base-64 uses just letters & numbers.

One of the url-decode methods in Android is Uri.decode(String) which results in a String. A String can be turned into a byte[] via String#getBytes().

So the following could work

String captchaImage = <captcha_image from xml>;
String decodedString = Uri.decode(captchaImage);
byte[] decodedBytes = decodedString.getBytes("UTF-8");
Bitmap bitmap = BitmapFactory.decodeByteArray(decodedBytes, 0, decodedBytes.length);

There is also java.net.URLDecoder which also let's you specify the charset.