Central bank support got the world's banks through the 2008 crisis. But secrecy around the recipients of such largess make it a double-edged sword that can create, as well as reduce, stress in the system.

If a bank seeks aid, it is a scarlet letter and may bring on a full-blown panic in that institution. Yet as the financial crisis showed, secrecy carries its own cost. If investors know an institution is under the gun, but can't tell who it is, that can undermine confidence in all firms.