code: protected void engineInitSign(PrivateKey privateKey,
                                    SecureRandom random)
          throws InvalidKeyException {
              this.appRandom = random;
              engineInitSign(privateKey);
      }
docs: Initializes this signature object with the specified
  private key and source of randomness for signing operations.
  
  <p>This concrete method has been added to this previously-defined
  abstract class. (For backwards compatibility, it cannot be abstract.)
  
  @param privateKey the private key of the identity whose signature
  will be generated.
  @param random the source of randomness
  
  @exception InvalidKeyException if the key is improperly
  encoded, parameters are missing, and so on.