code: func (s *SecureCookie) BlockFunc(f func([]byte) (cipher.Block, error)) *SecureCookie {
  	if s.blockKey == nil {
  		s.err = errBlockKeyNotSet
  	} else if block, err := f(s.blockKey); err == nil {
  		s.block = block
  	} else {
  		s.err = cookieError{cause: err, typ: usageError}
  	}
  	return s
  }
docs: // BlockFunc sets the encryption function used to create a cipher.Block.
  //
  // Default is crypto/aes.New.