CryptProtectMemory function (dpapi.h)

Article
02/16/2022

The CryptProtectMemory function encrypts memory to prevent others from viewing sensitive information in your process. For example, use the CryptProtectMemory function to encrypt memory that contains a password. Encrypting the password prevents others from viewing it when the process is paged out to the swap file. Otherwise, the password is in plaintext and viewable by others.

Syntax

DPAPI_IMP BOOL CryptProtectMemory(
  [in, out] LPVOID pDataIn,
  [in]      DWORD  cbDataIn,
  [in]      DWORD  dwFlags
);

Parameters

[in, out] pDataIn

A pointer to the block of memory to encrypt. The cbDataIn parameter specifies the number of bytes that will be encrypted. If the data contained in the memory space is smaller than the number of bytes specified, data outside of the intended block will be encrypted. If it is larger than cbDataIn bytes, then only the first cbDataIn bytes will be encrypted.

[in] cbDataIn

Number of bytes of memory pointed to by the pData parameter to encrypt. The number of bytes must be a multiple of the CRYPTPROTECTMEMORY_BLOCK_SIZE constant defined in Wincrypt.h.

[in] dwFlags

This parameter can be one of the following flags. You must specify the same flag when encrypting and decrypting the memory.

Value	Meaning
CRYPTPROTECTMEMORY_SAME_PROCESS	Encrypt and decrypt memory in the same process. An application running in a different process will not be able to decrypt the data.
CRYPTPROTECTMEMORY_CROSS_PROCESS	Encrypt and decrypt memory in different processes. An application running in a different process will be able to decrypt the data.
CRYPTPROTECTMEMORY_SAME_LOGON	Use the same logon credentials to encrypt and decrypt memory in different processes. An application running in a different process will be able to decrypt the data. However, the process must run as the same user that encrypted the data and in the same logon session.

Return value

If the function succeeds, the function returns TRUE.

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

Remarks

Using CryptProtectMemory and CryptUnprotectMemory for password encryption is not secure because the data exists as plaintext in memory before it is encrypted and at any time the caller decrypts it for use.

Typically, you use the CryptProtectMemory function to encrypt sensitive information that you are going to decrypt while your process is running. Do not use this function to save data that you want to decrypt later; you will not be able to decrypt the data if the computer is restarted. To save encrypted data to a file to decrypt later, use the CryptProtectData function.

Call the CryptUnprotectMemory function to decrypt memory encrypted with the CryptProtectMemory function. When you have finished using the sensitive information, clear it from memory by calling the SecureZeroMemory function.

Use the CRYPTPROTECTMEMORY_CROSS_PROCESS or CRYPTPROTECTMEMORY_SAME_LOGON flag if you use RPC or LRPC to pass encrypted data to another process. The receiving process must specify the same flag to decrypt the data. Also, use these flags if you use shared memory.

If the client uses the CRYPTPROTECTMEMORY_SAME_LOGON flag, the server must impersonate the client (RpcImpersonateClient) before decrypting the memory.

Examples

The following example calls the CryptProtectMemory function to encrypt data that is in memory.

#include <windows.h>
#include <stdio.h>
#include <Wincrypt.h>

#define SSN_STR_LEN 12  // includes null

void main()
{
    HRESULT hr = S_OK;
    LPWSTR pSensitiveText = NULL;
    DWORD cbSensitiveText = 0;
    DWORD cbPlainText = SSN_STR_LEN*sizeof(WCHAR);
    DWORD dwMod = 0;

    //  Memory to encrypt must be a multiple of CRYPTPROTECTMEMORY_BLOCK_SIZE.
    if (dwMod = cbPlainText % CRYPTPROTECTMEMORY_BLOCK_SIZE)
        cbSensitiveText = cbPlainText +
		(CRYPTPROTECTMEMORY_BLOCK_SIZE - dwMod);
    else
        cbSensitiveText = cbPlainText;

    pSensitiveText = (LPWSTR)LocalAlloc(LPTR, cbSensitiveText);
    if (NULL == pSensitiveText)
    {
        wprintf(L"Memory allocation failed.\n");
        return E_OUTOFMEMORY;
    }

    //  Place sensitive string to encrypt in pSensitiveText.

    if (!CryptProtectMemory(pSensitiveText, cbSensitiveText,
		CRYPTPROTECTMEMORY_SAME_PROCESS))
    {
        wprintf(L"CryptProtectMemory failed: %d\n", GetLastError());
        SecureZeroMemory(pSensitiveText, cbSensitiveText);
        LocalFree(pSensitiveText);
        pSensitiveText = NULL;
        return E_FAIL;
    }

    //  Call CryptUnprotectMemory to decrypt and use the memory.

    SecureZeroMemory(pSensitiveText, cbSensitiveText);
    LocalFree(pSensitiveText);
    pSensitiveText = NULL;

    return hr;
}

Requirements

Requirement	Value
Minimum supported client	Windows Vista [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

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