src/main/java/org/jivesoftware/smack/util/StringUtils.java
public static String encodeBase64(String data) {
        byte[] bytes = null;
        try {
            bytes = data.getBytes("ISO-8859-1");
        } catch (UnsupportedEncodingException uee) {
            throw new IllegalStateException(uee);
        }
        return encodeBase64(bytes);
    }
public static String encodeBase64(String data) {
        byte[] bytes = null;
        try {
            bytes = data.getBytes("ISO-8859-1");
        } catch (UnsupportedEncodingException uee) {
            throw new IllegalStateException(uee);
        }
        return encodeBase64(bytes);
    }
Encodes a String as a base64 String.

@param data
a String to encode.
@return a base64 encoded String.