Simple base64-encoder for java

Base64.java

public class Base64
{
    public static String encode(byte[] data)
    {
        char[] tbl = {
            'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P',
            'Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f',
            'g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v',
            'w','x','y','z','0','1','2','3','4','5','6','7','8','9','+','/' };

        StringBuilder buffer = new StringBuilder();
        int pad = 0;
        for (int i = 0; i < data.length; i += 3) {

            int b = ((data[i] & 0xFF) << 16) & 0xFFFFFF;
            if (i + 1 < data.length) {
                b |= (data[i+1] & 0xFF) << 8;
            } else {
                pad++;
            }
            if (i + 2 < data.length) {
                b |= (data[i+2] & 0xFF);
            } else {
                pad++;
            }

            for (int j = 0; j < 4 - pad; j++) {
                int c = (b & 0xFC0000) >> 18;
                buffer.append(tbl[c]);
                b <<= 6;
            }
        }
        for (int j = 0; j < pad; j++) {
            buffer.append("=");
        }

        return buffer.toString();
    }

    public static byte[] decode(String data)
    {
        int[] tbl = {
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63, 52, 53, 54,
            55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2,
            3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
            20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1, -1, 26, 27, 28, 29, 30,
            31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
            48, 49, 50, 51, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 };
        byte[] bytes = data.getBytes();
        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        for (int i = 0; i < bytes.length; ) {
            int b = 0;
            if (tbl[bytes[i]] != -1) {
                b = (tbl[bytes[i]] & 0xFF) << 18;
            }
            // skip unknown characters
            else {
                i++;
                continue;
            }

            int num = 0;
            if (i + 1 < bytes.length && tbl[bytes[i+1]] != -1) {
                b = b | ((tbl[bytes[i+1]] & 0xFF) << 12);
                num++;
            }
            if (i + 2 < bytes.length && tbl[bytes[i+2]] != -1) {
                b = b | ((tbl[bytes[i+2]] & 0xFF) << 6);
                num++;
            }
            if (i + 3 < bytes.length && tbl[bytes[i+3]] != -1) {
                b = b | (tbl[bytes[i+3]] & 0xFF);
                num++;
            }

            while (num > 0) {
                int c = (b & 0xFF0000) >> 16;
                buffer.write((char)c);
                b <<= 8;
                num--;
            }
            i += 4;
        }
        return buffer.toByteArray();
    }
}

Base64Test.java

import java.io.ByteArrayOutputStream;
import java.util.Random;
import java.util.Arrays;
import javax.xml.bind.DatatypeConverter;

public class Base64Test
{
    public static void print(byte[] bytes)
    {
        for (byte b : bytes) {
            System.out.printf("%02X ", b);
        }
        System.out.println();
    }

    public static void main(String[] args)
    {
        int steps = 1000000;
        Random rand = new Random(System.currentTimeMillis());

        System.out.println("Encode, decode");
        for (int count = 0; count < steps; count++) {
            int len = rand.nextInt(100)+1;
            byte[] original = new byte[len];
            for (int i = 0; i < len; i++) {
                original[i] = (byte)rand.nextInt(0xFF);
            }

            String encoded = Base64.encode(original);
            byte[] decoded = Base64.decode(encoded);

            if (!Arrays.equals(original, decoded)) {
                print(original);
                print(decoded);
                System.out.println();
            }
        }

        System.out.println("Encode, decode with other");
        for (int count = 0; count < steps; count++) {
            int len = rand.nextInt(100)+1;
            byte[] original = new byte[len];
            for (int i = 0; i < len; i++) {
                original[i] = (byte)rand.nextInt(0xFF);
            }

            String encoded = Base64.encode(original);
            byte[] decoded = DatatypeConverter.parseBase64Binary(encoded);

            if (!Arrays.equals(original, decoded)) {
                print(original);
                print(decoded);
                System.out.println();
            }
        }

        System.out.println("Encode with other, decode");
        for (int count = 0; count < steps; count++) {
            int len = rand.nextInt(100)+1;
            byte[] original = new byte[len];
            for (int i = 0; i < len; i++) {
                original[i] = (byte)rand.nextInt(0xFF);
            }

            String encoded = DatatypeConverter.printBase64Binary(original);
            byte[] decoded = Base64.decode(encoded);

            if (!Arrays.equals(original, decoded)) {
                print(original);
                print(decoded);
                System.out.println();
            }
        }

        System.out.println("Comparison test");
        for (int count = 0; count < 100000; count++) {
            int len = rand.nextInt(100)+1;
            byte[] original = new byte[len];
            for (int i = 0; i < len; i++) {
                original[i] = (byte)rand.nextInt(0xFF);
            }

            String encoded = DatatypeConverter.printBase64Binary(original);
            String encoded2 = Base64.encode(original);

            if (!encoded.equals(encoded2)) {
                System.out.println("mismatch");
                System.out.println(encoded);
                System.out.println(encoded2);
                System.out.println();
            }
        }
    }
}

gendecodetbl.py

# script to generate the reverse lookup table

s = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"

res = [(ord(c), i) for i,c in enumerate(s)]

lookup = dict(res)

f = []
for i in xrange(0,255):
    if lookup.has_key(i):
        n = str(lookup[i])
        if len(n) == 1:
            n = " " + n
        f.append(" " + n)
    else:
        f.append(" " + str(-1))

print len(f)
print ",".join(f)

There is still a bug in it, try and decode this string:
AAAAAAAAAAABAg==
python says:
base64.b64decode("AAAAAAAAAAABAg==")
'\x00\x00\x00\x00\x00\x00\x00\x00\x01\x02'
this one fails.

@andrewchambers: Confirmed. :/ Seems to be more subtleties to this than I imagined. I've updated the code to work with your example.

I've added a test which generates random bytes and uses the built-in java6 methods for comparison, which hopefully rules out further bugs. This whole thing is obviously of much less utility now that java ships with base64 support, but it might be of some use for legacy code.

There are some bugs in decode method.

1. length of tbl array is 255. add one more.
2. java byte is signed value(-128 ~ 127). When invalid character is feeded, Exception occured. Some text editor add BOM character and it can cause that problem.

for (int i = 0; i < bytes.length;) {
   int b = 0;
   if (bytes[i] >= 0 && tbl[bytes[i]] != -1) {
      b = (tbl[bytes[i]] & 0xFF) << 18;
   }

Kotlin Version https://gist.github.com/hrules6872/e2d4d02a1e8d3c6328ae5aeabc430b96

There are some bugs in decode method.

1. length of tbl array is 255. add one more.
2. java byte is signed value(-128 ~ 127). When invalid character is feeded, Exception occured. Some text editor add BOM character and it can cause that problem.

for (int i = 0; i < bytes.length;) {
   int b = 0;
   if (bytes[i] >= 0 && tbl[bytes[i]] != -1) {
      b = (tbl[bytes[i]] & 0xFF) << 18;
   }

@kimwooglae
May I ask you a question?
Why should the length of the tbl array be 256 instead of 255?