Microsoft use two different file formats to protect users private keys when stored on disk, the original (unnamed) format which was used in older versions of MSIE, IIS, and other software and which is still supported for backwards-compatibility reasons in newer versions, and the newer PFX/PKCS #12 format. Due to a number of design and implementation flaws in Microsofts software, it is possible to break the security of both of these formats and recover users private keys, often in a matter of seconds.
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