In restricted mode, simply trying to start a service brings up a window asking the user for permission. The user can then allow or forbid this. The system records this choice and enters it into the firewall's exceptions list. To achieve this, Apple furnishes unsigned programs with a digital signature in the process. If changes are made to the program subsequently, the permission is withdrawn.

Code signing becomes a problem when an application performs its own self-integrity check and determines that the file on the hard disk has been changed. The firewall's code signature changes the checksum of Skype's binary on the disc:

MD5 (Skype) = 9d7fa7f77b8dc2a3c2ae61737a373c11
MD5 (Skype-org) = 4245cb201a94c76ddcb54b1cc1e58cfa

after which, if the user attempts to start Skype from the command line it displays the following message:

Main starting
Check 1 failed. Can't run Skype

Users who start Skype from the GUI merely see a dancing symbol which then disappears without further comment. Reinstallation is required to restore the application to normal function.

More than $500 billion of potentially worthless paper is padding the balance sheets of the nation's top-10 banks - and could also be contaminating the huge pensions and annuities that support millions of Americans.

What's scaring investors and politicians is the murky nature of many asset-backed investments strewn throughout the global economy, with about $100 billion written off in the past months at funds, banks and other institutions.

The assets, collectively known as level 3 securities, are considered the riskiest breed because their true values are still largely uncalculated.

But tough new standards from the Financial Accounting Standards Board kick in Nov. 15, requiring companies to place values on these assets or write them down.