Add base32 encoder/decoder.

bramble-api/src/main/java/org/briarproject/bramble/util/Base32.java

+package org.briarproject.bramble.util;
+
+import java.io.ByteArrayOutputStream;
+
+public class Base32 {
+
+	private static final char[] DIGITS = {
+			'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', '2', '3', '4', '5', '6', '7'
+	};
+
+	public static String encode(byte[] b) {
+		StringBuilder s = new StringBuilder();
+		int byteIndex = 0, currentCode = 0x00;
+		int byteMask = 0x80, codeMask = 0x10;
+		while (byteIndex < b.length) {
+			if ((b[byteIndex] & byteMask) != 0) currentCode |= codeMask;
+			// After every 8 bits, move on to the next byte
+			if (byteMask == 0x01) {
+				byteMask = 0x80;
+				byteIndex++;
+			} else {
+				byteMask >>>= 1;
+			}
+			// After every 5 bits, move on to the next digit
+			if (codeMask == 0x01) {
+				s.append(DIGITS[currentCode]);
+				codeMask = 0x10;
+				currentCode = 0x00;
+			} else {
+				codeMask >>>= 1;
+			}
+		}
+		// If we're part-way through a digit, output it
+		if (codeMask != 0x10) s.append(DIGITS[currentCode]);
+		return s.toString();
+	}
+
+	public static byte[] decode(String s) {
+		ByteArrayOutputStream b = new ByteArrayOutputStream();
+		int digitIndex = 0, digitCount = s.length(), currentByte = 0x00;
+		int byteMask = 0x80, codeMask = 0x10;
+		while (digitIndex < digitCount) {
+			int code = decodeDigit(s.charAt(digitIndex));
+			if ((code & codeMask) != 0) currentByte |= byteMask;
+			// After every 8 bits, move on to the next byte
+			if (byteMask == 0x01) {
+				b.write(currentByte);
+				byteMask = 0x80;
+				currentByte = 0x00;
+			} else {
+				byteMask >>>= 1;
+			}
+			// After every 5 bits, move on to the next digit
+			if (codeMask == 0x01) {
+				codeMask = 0x10;
+				digitIndex++;
+			} else {
+				codeMask >>>= 1;
+			}
+		}
+		// If any extra bits were used for encoding, they should all be zero
+		if (byteMask != 0x80 && currentByte != 0x00)
+			throw new IllegalArgumentException();
+		return b.toByteArray();
+	}
+
+	private static int decodeDigit(char c) {
+		if (c >= 'A' && c <= 'Z') return c - 'A';
+		if (c >= 'a' && c <= 'z') return c - 'a';
+		if (c >= '2' && c <= '7') return c - '2' + 26;
+		throw new IllegalArgumentException("Not a base32 digit: " + c);
+	}
+}