C# Signature:

[
DllImport("Crypt32.dll",
SetLastError=true,
CharSet=System.Runtime.InteropServices.CharSet.Auto)
]
private static extern bool CryptProtectData(
    ref DATA_BLOB pDataIn,
    String szDataDescr,
    ref DATA_BLOB pOptionalEntropy,
    IntPtr pvReserved,
    ref CRYPTPROTECT_PROMPTSTRUCT pPromptStruct,
    CryptProtectFlags dwFlags,
    ref DATA_BLOB pDataOut
);

VB .NET Signature:

Declare Function CryptProtectData Lib "crypt32.dll" (TODO) As TODO

See also:

DATA_BLOB

CRYPTPROTECT_PROMPTSTRUCT

CryptProtectFlags

CryptUnprotectData

Notes:

Only a user with logon credentials matching those of the encrypter can decrypt the data. In addition, decryption usually can only be done on the computer where the data was encrypted.

Warning If the logon credentials are lost or forgotten, the data is usually unrecoverable. However, if the dwFlags parameter in the CryptProtectData function is set to CRYPTPROTECT_LOCAL_MACHINE with or without additional entropy specified in the pOptionalEntropy parameter, any user on the same computer where the data was encrypted can recover the data.

The function creates a session key to perform the encryption. The session key is derived again when the data is to be decrypted.

The function also adds a message authentication code (MAC), which is a keyed integrity check, to the encrypted data to guard against data tampering.

Under some circumstances, Microsoft cryptographic service providers (CSPs) may not allow encryption when used in France, in which case this function fails with the error code NTE_PERM.

You can store the encryption or master keys used by CryptProtectData in a file in the Windows folder or in the system registry so that certain persistent registry implementations will retain the encryption keys after losing and regaining power. See Master Key Storage for more information.

Tips & Tricks:

Please add some!

Sample Code:

How To Create a DPAPI Library: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/secmod/html/secmod21.asp

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