CryptProtectData function (dpapi.h)

The CryptProtectData function performs encryption on the data in a DATA_BLOB structure. Typically, only a user with the same logon credential as the user who encrypted the data can decrypt the data. In addition, the encryption and decryption usually must be done on the same computer. For information about exceptions, see Remarks.


DPAPI_IMP BOOL CryptProtectData(
  [in]           DATA_BLOB                 *pDataIn,
  [in, optional] LPCWSTR                   szDataDescr,
  [in, optional] DATA_BLOB                 *pOptionalEntropy,
  [in]           PVOID                     pvReserved,
  [in, optional] CRYPTPROTECT_PROMPTSTRUCT *pPromptStruct,
  [in]           DWORD                     dwFlags,
  [out]          DATA_BLOB                 *pDataOut


[in] pDataIn

A pointer to a DATA_BLOB structure that contains the plaintext to be encrypted.

[in, optional] szDataDescr

A string with a readable description of the data to be encrypted. This description string is included with the encrypted data. This parameter is optional and can be set to NULL.

[in, optional] pOptionalEntropy

A pointer to a DATA_BLOB structure that contains a password or other additional entropy used to encrypt the data. The DATA_BLOB structure used in the encryption phase must also be used in the decryption phase. This parameter can be set to NULL for no additional entropy. For information about protecting passwords, see Handling Passwords.

[in] pvReserved

Reserved for future use and must be set to NULL.

[in, optional] pPromptStruct

A pointer to a CRYPTPROTECT_PROMPTSTRUCT structure that provides information about where and when prompts are to be displayed and what the content of those prompts should be. This parameter can be set to NULL in both the encryption and decryption phases.

[in] dwFlags

This parameter can be one of the following flags.

Value Meaning
When this flag is set, it associates the data encrypted with the current computer instead of with an individual user. Any user on the computer on which CryptProtectData is called can use CryptUnprotectData to decrypt the data.
This flag is used for remote situations where presenting a user interface (UI) is not an option. When this flag is set and a UI is specified for either the protect or unprotect operation, the operation fails and GetLastError returns the ERROR_PASSWORD_RESTRICTION code.
This flag generates an audit on protect and unprotect operations. Audit log entries are recorded only if szDataDescr is not NULL and not empty.

[out] pDataOut

A pointer to a DATA_BLOB structure that receives the encrypted data. When you have finished using the DATA_BLOB structure, free its pbData member by calling the LocalFree function.

Return value

If the function succeeds, the function returns TRUE.

If the function fails, it returns FALSE. For extended error information, call GetLastError.


Typically, only a user with logon credentials that match those of the user who encrypted the data can decrypt the data. In addition, decryption usually can only be done on the computer where the data was encrypted. However, a user with a roaming profile can decrypt the data from another computer on the network.

If the CRYPTPROTECT_LOCAL_MACHINE flag is set when the data is encrypted, any user on the computer where the encryption was done can decrypt 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) (keyed integrity check) to the encrypted data to guard against data tampering.

To encrypt memory for temporary use in the same process or across processes, call the CryptProtectMemory function.


The following example shows encryption of the data in a DATA_BLOB structure. The CryptProtectData function does the encryption by using a session key that the function creates by using the user's logon credentials. For another example that uses this function, see Example C Program: Using CryptProtectData.

// Encrypt data from DATA_BLOB DataIn to DATA_BLOB DataOut.

// Declare and initialize variables.

BYTE *pbDataInput =(BYTE *)"Hello world of data protection.";
DWORD cbDataInput = strlen((char *)pbDataInput)+1;

// Initialize the DataIn structure.

DataIn.pbData = pbDataInput;    
DataIn.cbData = cbDataInput;

//  Begin protect phase. Note that the encryption key is created
//  by the function and is not passed.

     L"This is the description string.", // A description string
                                         // to be included with the
                                         // encrypted data. 
     NULL,                               // Optional entropy not used.
     NULL,                               // Reserved.
     NULL,                               // Pass NULL for the 
                                         // prompt structure.
     printf("The encryption phase worked.\n");
    printf("Encryption error using CryptProtectData.\n");


Requirement Value
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header dpapi.h
Library Crypt32.lib
DLL Crypt32.dll

See also



Data Encryption and Decryption Functions