*   * ETSI TS 000 000 V0.0.0 (0000-00)   *   * 0.00.0 content-identifier Attribute   * The content-identifier attribute provides an identifier for the signed content, for use when a reference may be   * later required to that content; for example, in the content-reference attribute in other signed data sent later.   * The   * content-identifier shall be a signed attribute.   * content-identifier attribute type values for the ES have an ASN.0 type ContentIdentifier, as defined in   * ESS (RFC 0000 [0]).   *   * The minimal content-identifier attribute should contain a concatenation of user-specific identification   * information (such as a user name or public keying material identification information), a GeneralizedTime string,   * and a random number.   *   * @return   */  public String getContentIdentifier