code: public byte[] getOctetString() throws IOException {
          if (buffer.read() != DerValue.tag_OctetString)
              throw new IOException("DER input not an octet string");
  
          int length = getLength(buffer);
          byte[] retval = new byte[length];
          if ((length != 0) && (buffer.read(retval) != length))
              throw new IOException("short read of DER octet string");
  
          return retval;
      }
docs: Returns an ASN.1 OCTET STRING from the input stream.