Data Protection API: Difference between revisions

Revision as of 20:15, 16 August 2016

Revision as of 20:21, 16 August 2016

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For nearly all cryptosystems, one of the most difficult challenges is "key management" - in part, how to securely store the decryption key. If the key is stored in ''plain text'', then any user that can access the key can access the encrypted data. If the key is to be encrypted, another key is needed, and so on. DPAPI allows developers to encrypt keys using a symmetric key derived from the user's logon secrets, or in the case of system encryption, using the system's ___domain authentication secrets.

The DPAPI keys used for encrypting the user's RSA keys are stored under "<code>%APPDATA%\Microsoft\Protect\{[[Security Identifier|SID]]}"</code> directory, where {[[Security Identifier|SID]]} is the security[[Security identifierIdentifier]] of that user. The DPAPI key is stored in the same file as the master key that protects the users private keys. It usually is 64 bytes of random data.

In 2010 [[Elie Bursztein]] and [[Jean-Michel Picod]] presented an analysis of the protocol titled ''Reversing DPAPI and Stealing Windows Secrets Offline'' at [https://www.blackhat.com/html/bh-dc-10/bh-dc-10-briefings.html Black Hat DC 2010]. In addition to their briefing, Bursztein and Picod released [http://www.dpapick.com DPAPIck] which allows offline decryption of data encrypted with DPAPI. In 2012 Passcape Software published in their blog more detailed article on [http://passcape.com/index.php?section=blog&cmd=details&id=20 DPAPI internal logic] and presented [http://passcape.com/windows_password_recovery_dpapi_decoder a tool] for fully DPAPI offline decryption and analysis. Unlike previous one, the tool utilizes some old Windows bugs (for example, you can decrypt Windows 2000 DPAPI blobs without knowing the owner logon password) and is fully compatible with Windows 8 DPAPI data structure. In Windows 8 Microsoft changed the way the DPAPI logic works. Now multiple user keys can be used to derive an encryption key to decrypt the user masterkey which is used then to decode a single DPAPI blob.