Crockford Base32 Encoder and Decoder written in Java. It is a shameless adaptation of the Base32 encoder from apache commons-condec. I would have loved to just extend from their BaseNCodec class, but alas it is not designed for extension...

CrockfordBase32.java

import java.nio.charset.Charset;

/**
 * <p>Provides Base32 encoding and decoding as defined by <a href="http://www.ietf.org/rfc/rfc4648.txt">RFC 4648</a>.
 * However it uses a custom alphabet first coined by Douglas Crockford. Only addition to the alphabet is that 'u' and
 * 'U' characters decode as if they were 'V' to improve mistakes by human input.<p/>
 * <p>
 * This class operates directly on byte streams, and not character streams.
 * </p>
 *
 * @version $Id: Base32.java 1382498 2012-09-09 13:41:55Z sebb $
 * @see <a href="http://www.ietf.org/rfc/rfc4648.txt">RFC 4648</a>
 * @see <a href="http://www.crockford.com/wrmg/base32.html">Douglas Crockford's Base32 Encoding</a>
 * @since 1.5
 */
public class CrockfordBase32 {

    /**
     * Mask used to extract 8 bits, used in decoding bytes
     */
    protected static final int MASK_8BITS = 0xff;
    private static final Charset UTF8 = Charset.forName("UTF-8");
    private static final int DEFAULT_BUFFER_RESIZE_FACTOR = 2;
    /**
     * Defines the default buffer size - currently {@value}
     * - must be large enough for at least one encoded block+separator
     */
    private static final int DEFAULT_BUFFER_SIZE = 8192;
    /**
     * Mask used to extract 5 bits, used when encoding Base32 bytes
     */
    private static final int MASK_5BITS = 0x1f;
    /**
     * BASE32 characters are 5 bits in length.
     * They are formed by taking a block of five octets to form a 40-bit string,
     * which is converted into eight BASE32 characters.
     */
    private static final int BITS_PER_ENCODED_BYTE = 5;
    private static final int BYTES_PER_ENCODED_BLOCK = 8;
    private static final int BYTES_PER_UNENCODED_BLOCK = 5;
    private static final byte PAD = '=';
    /**
     * This array is a lookup table that translates 5-bit positive integer index values into their "Base32 Alphabet"
     * equivalents as specified in Table 3 of RFC 2045.
     */
    private static final byte[] ENCODE_TABLE = {
            '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
            'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', 'M',
            'N', 'P', 'Q', 'R', 'S', 'T', 'V', 'W', 'X', 'Y', 'Z'
    };
    /**
     * Convenience variable to help us determine when our buffer is going to run out of room and needs resizing.
     * <code>decodeSize = {@link #BYTES_PER_ENCODED_BLOCK} - 1 + lineSeparator.length;</code>
     */
    private final int decodeSize;
    /**
     * Convenience variable to help us determine when our buffer is going to run out of room and needs resizing.
     * <code>encodeSize = {@link #BYTES_PER_ENCODED_BLOCK} + lineSeparator.length;</code>
     */
    private final int encodeSize;
    /**
     * Wheather this encoder should use a padding character at the end of encoded Strings.
     */
    private final boolean usePaddingCharacter;
    /**
     * Buffer for streaming.
     */
    protected byte[] buffer;
    /**
     * Position where next character should be written in the buffer.
     */
    protected int pos;
    /**
     * Boolean flag to indicate the EOF has been reached. Once EOF has been reached, this object becomes useless,
     * and must be thrown away.
     */
    protected boolean eof;
    /**
     * Writes to the buffer only occur after every 3/5 reads when encoding, and every 4/8 reads when decoding.
     * This variable helps track that.
     */
    protected int modulus;
    /**
     * Place holder for the bytes we're dealing with for our based logic.
     * Bitwise operations store and extract the encoding or decoding from this variable.
     */
    private long bitWorkArea;

    public CrockfordBase32() {
        this(false);
    }

    /**
     * Creates a Base32 codec used for decoding and encoding.
     * <p>
     * When encoding the line length is 0 (no chunking).
     * </p>
     */
    public CrockfordBase32(boolean usePaddingCharacter) {
        this.usePaddingCharacter = usePaddingCharacter;
        this.encodeSize = BYTES_PER_ENCODED_BLOCK;
        this.decodeSize = this.encodeSize - 1;
    }

    private static byte decode(byte octet) {
        switch (octet) {
            case '0':
            case 'O':
            case 'o':
                return 0;

            case '1':
            case 'I':
            case 'i':
            case 'L':
            case 'l':
                return 1;

            case '2':
                return 2;
            case '3':
                return 3;
            case '4':
                return 4;
            case '5':
                return 5;
            case '6':
                return 6;
            case '7':
                return 7;
            case '8':
                return 8;
            case '9':
                return 9;

            case 'A':
            case 'a':
                return 10;

            case 'B':
            case 'b':
                return 11;

            case 'C':
            case 'c':
                return 12;

            case 'D':
            case 'd':
                return 13;

            case 'E':
            case 'e':
                return 14;

            case 'F':
            case 'f':
                return 15;

            case 'G':
            case 'g':
                return 16;

            case 'H':
            case 'h':
                return 17;

            case 'J':
            case 'j':
                return 18;

            case 'K':
            case 'k':
                return 19;

            case 'M':
            case 'm':
                return 20;

            case 'N':
            case 'n':
                return 21;

            case 'P':
            case 'p':
                return 22;

            case 'Q':
            case 'q':
                return 23;

            case 'R':
            case 'r':
                return 24;

            case 'S':
            case 's':
                return 25;

            case 'T':
            case 't':
                return 26;

            case 'U':
            case 'u':
            case 'V':
            case 'v':
                return 27;

            case 'W':
            case 'w':
                return 28;

            case 'X':
            case 'x':
                return 29;

            case 'Y':
            case 'y':
                return 30;

            case 'Z':
            case 'z':
                return 31;

            default:
                return -1;
        }
    }

    /**
     * Checks if a byte value is whitespace or not.
     * Whitespace is taken to mean: space, tab, CR, LF
     *
     * @param byteToCheck the byte to check
     * @return true if byte is whitespace, false otherwise
     */
    protected static boolean isWhiteSpace(byte byteToCheck) {
        switch (byteToCheck) {
            case ' ':
            case '\n':
            case '\r':
            case '\t':
                return true;
            default:
                return false;
        }
    }

    /**
     * Tests a given String to see if it contains only valid characters within the alphabet.
     * The method treats whitespace and PAD as valid.
     *
     * @param base32 String to test
     * @return <code>true</code> if all characters in the String are valid characters in the alphabet or if
     *         the String is empty; <code>false</code>, otherwise
     * @see #isInAlphabet(byte[], boolean)
     */
    public static boolean isInAlphabet(String base32) {
        return isInAlphabet(base32.getBytes(UTF8), true);
    }

    /**
     * Tests a given byte array to see if it contains only valid characters within the alphabet.
     * The method optionally treats whitespace and pad as valid.
     *
     * @param arrayOctet byte array to test
     * @param allowWSPad if <code>true</code>, then whitespace and PAD are also allowed
     * @return <code>true</code> if all bytes are valid characters in the alphabet or if the byte array is empty;
     *         <code>false</code>, otherwise
     */
    public static boolean isInAlphabet(byte[] arrayOctet, boolean allowWSPad) {
        for (int i = 0; i < arrayOctet.length; i++) {
            if (!isInAlphabet(arrayOctet[i]) &&
                    (!allowWSPad || (arrayOctet[i] != PAD) && !isWhiteSpace(arrayOctet[i]))) {
                return false;
            }
        }
        return true;
    }

    /**
     * Returns whether or not the <code>octet</code> is in the Base32 alphabet.
     *
     * @param octet The value to test
     * @return <code>true</code> if the value is defined in the the Base32 alphabet <code>false</code> otherwise.
     */
    public static boolean isInAlphabet(byte octet) {
        return decode(octet) != -1;
    }

    /**
     * Returns the amount of buffered data available for reading.
     *
     * @return The amount of buffered data available for reading.
     */
    int available() {  // package protected for access from I/O streams
        return buffer != null ? pos : 0;
    }

    /**
     * Increases our buffer by the {@link #DEFAULT_BUFFER_RESIZE_FACTOR}.
     */
    private void resizeBuffer() {
        if (buffer == null) {
            buffer = new byte[DEFAULT_BUFFER_SIZE];
            pos = 0;
        } else {
            byte[] b = new byte[buffer.length * DEFAULT_BUFFER_RESIZE_FACTOR];
            System.arraycopy(buffer, 0, b, 0, buffer.length);
            buffer = b;
        }
    }

    /**
     * Ensure that the buffer has room for <code>size</code> bytes
     *
     * @param size minimum spare space required
     */
    protected void ensureBufferSize(int size) {
        if ((buffer == null) || (buffer.length < pos + size)) {
            resizeBuffer();
        }
    }

    /**
     * Extracts buffered data into the provided byte[] array, starting at position bPos,
     * up to a maximum of bAvail bytes. Returns how many bytes were actually extracted.
     *
     * @param b byte[] array to extract the buffered data into.
     * @return The number of bytes successfully extracted into the provided byte[] array.
     */
    int readResults(byte[] b) {  // package protected for access from I/O streams
        if (buffer != null) {
            int len = available();
            System.arraycopy(buffer, 0, b, 0, len);
            buffer = null; // so hasData() will return false, and this method can return -1
            return len;
        }
        return eof ? -1 : 0;
    }

    /**
     * Resets this object to its initial newly constructed state.
     */
    private void reset() {
        buffer = null;
        pos = 0;
        modulus = 0;
        eof = false;
    }

    /**
     * Encodes a String containing characters in the Base32 alphabet.
     *
     * @param pArray A String containing Base32 character data
     * @return A String containing only Base32 character data
     */
    public String encodeToString(String pArray) {
        return encodeToString(pArray.getBytes(UTF8));
    }

    /**
     * Encodes a byte[] containing binary data, into a String containing characters in the Base-N alphabet.
     *
     * @param pArray a byte array containing binary data
     * @return A String containing only Base32 character data
     */
    public String encodeToString(byte[] pArray) {
        return new String(encode(pArray), UTF8);
    }

    /**
     * Encodes a String containing characters in the Base32 alphabet.
     *
     * @param pArray A String containing Base32 character data
     * @return A UTF-8 decoded String
     */
    public String decodeToString(String pArray) {
        return decodeToString(pArray.getBytes(UTF8));
    }

    /**
     * Decodes a byte[] containing binary data, into a String containing UTF-8 decoded String.
     *
     * @param pArray a byte array containing binary data
     * @return A UTF-8 decoded String
     */
    public String decodeToString(byte[] pArray) {
        return new String(decode(pArray), UTF8);
    }

    /**
     * Decodes a String containing characters in the Base-N alphabet.
     *
     * @param pArray A String containing Base-N character data
     * @return a byte array containing binary data
     */
    public byte[] decode(String pArray) {
        return decode(pArray.getBytes(UTF8));
    }

    /**
     * Encodes a String containing characters in the Base32 alphabet.
     *
     * @param pArray A String containing Base-N character data
     * @return a byte array containing binary data
     */
    public byte[] encode(String pArray) {
        return encode(pArray.getBytes(UTF8));
    }

    /**
     * Decodes a byte[] containing characters in the Base-N alphabet.
     *
     * @param pArray A byte array containing Base-N character data
     * @return a byte array containing binary data
     */
    public byte[] decode(byte[] pArray) {
        reset();
        if (pArray == null || pArray.length == 0) {
            return pArray;
        }
        decode(pArray, 0, pArray.length);
        decode(pArray, 0, -1); // Notify decoder of EOF.
        byte[] result = new byte[pos];
        readResults(result);
        return result;
    }

    // The static final fields above are used for the original static byte[] methods on Base32.
    // The private member fields below are used with the new streaming approach, which requires
    // some state be preserved between calls of encode() and decode().

    /**
     * Encodes a byte[] containing binary data, into a byte[] containing characters in the alphabet.
     *
     * @param pArray a byte array containing binary data
     * @return A byte array containing only the basen alphabetic character data
     */
    public byte[] encode(byte[] pArray) {
        reset();
        if (pArray == null || pArray.length == 0) {
            return pArray;
        }
        encode(pArray, 0, pArray.length);
        encode(pArray, 0, -1); // Notify encoder of EOF.
        byte[] buf = new byte[pos];
        readResults(buf);
        return buf;
    }

    /**
     * Calculates the amount of space needed to encode the supplied array.
     *
     * @param pArray byte[] array which will later be encoded
     * @return amount of space needed to encoded the supplied array.
     *         Returns a long since a max-len array will require > Integer.MAX_VALUE
     */
    public long getEncodedLength(byte[] pArray) {
        // Calculate non-chunked size - rounded up to allow for padding
        // cast to long is needed to avoid possibility of overflow
        long len = ((pArray.length + BYTES_PER_UNENCODED_BLOCK - 1) / BYTES_PER_UNENCODED_BLOCK) * (long) BYTES_PER_ENCODED_BLOCK;
        return len;
    }

    /**
     * <p>
     * Decodes all of the provided data, starting at inPos, for inAvail bytes. Should be called at least twice: once
     * with the data to decode, and once with inAvail set to "-1" to alert decoder that EOF has been reached. The "-1"
     * call is not necessary when decoding, but it doesn't hurt, either.
     * </p>
     * <p>
     * Ignores all non-Base32 characters. This is how chunked (e.g. 76 character) data is handled, since CR and LF are
     * silently ignored, but has implications for other bytes, too. This method subscribes to the garbage-in,
     * garbage-out philosophy: it will not check the provided data for validity.
     * </p>
     *
     * @param in      byte[] array of ascii data to Base32 decode.
     * @param inPos   Position to start reading data from.
     * @param inAvail Amount of bytes available from input for encoding.
     *                <p/>
     *                Output is written to {@link #buffer} as 8-bit octets, using {@link #pos} as the buffer position
     */
    void decode(byte[] in, int inPos, int inAvail) { // package protected for access from I/O streams
        if (eof) {
            return;
        }
        if (inAvail < 0) {
            eof = true;
        }
        for (int i = 0; i < inAvail; i++) {
            byte b = in[inPos++];
            if (b == PAD) {
                // We're done.
                eof = true;
                break;
            } else {
                ensureBufferSize(decodeSize);
                if (isInAlphabet(b)) {
                    int result = decode(b);
                    modulus = (modulus + 1) % BYTES_PER_ENCODED_BLOCK;
                    bitWorkArea = (bitWorkArea << BITS_PER_ENCODED_BYTE) + result; // collect decoded bytes
                    if (modulus == 0) { // we can output the 5 bytes
                        buffer[pos++] = (byte) ((bitWorkArea >> 32) & MASK_8BITS);
                        buffer[pos++] = (byte) ((bitWorkArea >> 24) & MASK_8BITS);
                        buffer[pos++] = (byte) ((bitWorkArea >> 16) & MASK_8BITS);
                        buffer[pos++] = (byte) ((bitWorkArea >> 8) & MASK_8BITS);
                        buffer[pos++] = (byte) (bitWorkArea & MASK_8BITS);
                    }
                }
            }
        }

        // Two forms of EOF as far as Base32 decoder is concerned: actual
        // EOF (-1) and first time '=' character is encountered in stream.
        // This approach makes the '=' padding characters completely optional.
        if (eof && modulus >= 2) { // if modulus < 2, nothing to do
            ensureBufferSize(decodeSize);

            //  we ignore partial bytes, i.e. only multiples of 8 count
            switch (modulus) {
                case 2: // 10 bits, drop 2 and output one byte
                    buffer[pos++] = (byte) ((bitWorkArea >> 2) & MASK_8BITS);
                    break;
                case 3: // 15 bits, drop 7 and output 1 byte
                    buffer[pos++] = (byte) ((bitWorkArea >> 7) & MASK_8BITS);
                    break;
                case 4: // 20 bits = 2*8 + 4
                    bitWorkArea = bitWorkArea >> 4; // drop 4 bits
                    buffer[pos++] = (byte) ((bitWorkArea >> 8) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea) & MASK_8BITS);
                    break;
                case 5: // 25bits = 3*8 + 1
                    bitWorkArea = bitWorkArea >> 1;
                    buffer[pos++] = (byte) ((bitWorkArea >> 16) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea >> 8) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea) & MASK_8BITS);
                    break;
                case 6: // 30bits = 3*8 + 6
                    bitWorkArea = bitWorkArea >> 6;
                    buffer[pos++] = (byte) ((bitWorkArea >> 16) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea >> 8) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea) & MASK_8BITS);
                    break;
                case 7: // 35 = 4*8 +3
                    bitWorkArea = bitWorkArea >> 3;
                    buffer[pos++] = (byte) ((bitWorkArea >> 24) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea >> 16) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea >> 8) & MASK_8BITS);
                    buffer[pos++] = (byte) ((bitWorkArea) & MASK_8BITS);
                    break;
            }
        }
    }

    /**
     * <p>
     * Encodes all of the provided data, starting at inPos, for inAvail bytes. Must be called at least twice: once with
     * the data to encode, and once with inAvail set to "-1" to alert encoder that EOF has been reached, so flush last
     * remaining bytes (if not multiple of 5).
     * </p>
     *
     * @param in      byte[] array of binary data to Base32 encode.
     * @param inPos   Position to start reading data from.
     * @param inAvail Amount of bytes available from input for encoding.
     */
    void encode(byte[] in, int inPos, int inAvail) { // package protected for access from I/O streams
        if (eof) {
            return;
        }
        // inAvail < 0 is how we're informed of EOF in the underlying data we're
        // encoding.
        if (inAvail < 0) {
            eof = true;
            if (0 == modulus) {
                return; // no leftovers to process
            }
            ensureBufferSize(encodeSize);
            int savedPos = pos;
            switch (modulus) { // % 5
                case 1: // Only 1 octet; take top 5 bits then remainder
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 3) & MASK_5BITS]; // 8-1*5 = 3
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea << 2) & MASK_5BITS]; // 5-3=2
                    if (usePaddingCharacter) {
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                    }
                    break;

                case 2: // 2 octets = 16 bits to use
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 11) & MASK_5BITS]; // 16-1*5 = 11
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 6) & MASK_5BITS]; // 16-2*5 = 6
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 1) & MASK_5BITS]; // 16-3*5 = 1
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea << 4) & MASK_5BITS]; // 5-1 = 4
                    if (usePaddingCharacter) {
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                    }
                    break;
                case 3: // 3 octets = 24 bits to use
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 19) & MASK_5BITS]; // 24-1*5 = 19
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 14) & MASK_5BITS]; // 24-2*5 = 14
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 9) & MASK_5BITS]; // 24-3*5 = 9
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 4) & MASK_5BITS]; // 24-4*5 = 4
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea << 1) & MASK_5BITS]; // 5-4 = 1
                    if (usePaddingCharacter) {
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                        buffer[pos++] = PAD;
                    }
                    break;
                case 4: // 4 octets = 32 bits to use
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 27) & MASK_5BITS]; // 32-1*5 = 27
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 22) & MASK_5BITS]; // 32-2*5 = 22
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 17) & MASK_5BITS]; // 32-3*5 = 17
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 12) & MASK_5BITS]; // 32-4*5 = 12
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 7) & MASK_5BITS]; // 32-5*5 =  7
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 2) & MASK_5BITS]; // 32-6*5 =  2
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea << 3) & MASK_5BITS]; // 5-2 = 3
                    if (usePaddingCharacter) {
                        buffer[pos++] = PAD;
                    }
                    break;
            }
        } else {
            for (int i = 0; i < inAvail; i++) {
                ensureBufferSize(encodeSize);
                modulus = (modulus + 1) % BYTES_PER_UNENCODED_BLOCK;
                int b = in[inPos++];
                if (b < 0) {
                    b += 256;
                }
                bitWorkArea = (bitWorkArea << 8) + b; // BITS_PER_BYTE
                if (0 == modulus) { // we have enough bytes to create our output
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 35) & MASK_5BITS];
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 30) & MASK_5BITS];
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 25) & MASK_5BITS];
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 20) & MASK_5BITS];
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 15) & MASK_5BITS];
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 10) & MASK_5BITS];
                    buffer[pos++] = ENCODE_TABLE[(int) (bitWorkArea >> 5) & MASK_5BITS];
                    buffer[pos++] = ENCODE_TABLE[(int) bitWorkArea & MASK_5BITS];
                }
            }
        }
    }

}

Why don't you send a pull request to org.apache.commons.codec.binary ?