问如何逆转此方法的输出？

EN

    public String encrypt(String plaintext) {
    MessageDigest md = null;
    try {
        md = MessageDigest.getInstance("SHA");
    } catch (NoSuchAlgorithmException e) {
    }
    try {
        md.update(plaintext.getBytes("UTF-8"));
    } catch (UnsupportedEncodingException e) {
    }
    byte raw[] = md.digest();
    String hash = (new BASE64Encoder()).encode(raw);
    return hash;
}