public class base64EncryAndDecry {
	public static final String CODES = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

	public static void main(String args[]) {
		String input = "中國";
		String encode = base64Encode( input.getBytes() );
		System.out.println( "encode: " + encode );

		input = "1tC5+g==";
		String decode = new String( base64Decode(input) );
		System.out.println( "decode: " + decode );
	}
	
	// base64加密
	public static String base64Encode( byte[] in ) {
		StringBuilder out = new StringBuilder( (in.length * 4) / 3 );
		int b = 0;
		for ( int i = 0; i < in.length; i += 3 ) {
			b = (in[i] & 0xFC) >> 2;
			out.append( CODES.charAt(b) );
			b = (in[i] & 0x03) << 4;
			if (i + 1 < in.length) {
				b |= (in[i + 1] & 0xF0) >> 4;
				out.append(CODES.charAt(b));
				b = (in[i + 1] & 0x0F) << 2;
				if (i + 2 < in.length) {
					b |= (in[i + 2] & 0xC0) >> 6;
					out.append(CODES.charAt(b));
					b = in[i + 2] & 0x3F;
					out.append(CODES.charAt(b));
				} else {
					out.append(CODES.charAt(b));
					out.append('=');
				}
			} else {
				out.append(CODES.charAt(b));
				out.append("==");
			}
		}
		return out.toString();
	}

	// base64解密
	public static byte[] base64Decode( String input ) {
		if ( input.length() % 4 != 0 ) {
			throw new IllegalArgumentException( "Invalid base64 input" );
		}
		byte decoded[] = new byte[((input.length() * 3) / 4)
				- (input.indexOf('=') > 0 ? (input.length() - input.indexOf('=')) : 0)];
		char[] inChars = input.toCharArray();
		int j = 0;
		int b[] = new int[4];
		for (int i = 0; i < inChars.length; i += 4) {
			b[0] = CODES.indexOf(inChars[i]);
			b[1] = CODES.indexOf(inChars[i + 1]);
			b[2] = CODES.indexOf(inChars[i + 2]);
			b[3] = CODES.indexOf(inChars[i + 3]);
			decoded[j++] = (byte) ((b[0] << 2) | (b[1] >> 4));
			if (b[2] < 64) {
				decoded[j++] = (byte) ((b[1] << 4) | (b[2] >> 2));
				if (b[3] < 64) {
					decoded[j++] = (byte) ((b[2] << 6) | b[3]);
				}
			}
		}
		return decoded;
	}
}