CryptProtectData

[DllImport("CRYPT32.dll", ExactSpelling = true, SetLastError = true)]
[DefaultDllImportSearchPaths(DllImportSearchPath.System32)]
public static extern unsafe BOOL CryptProtectData(
    CRYPT_INTEGER_BLOB* pDataIn,
    PCWSTR szDataDescr,
    [Optional] CRYPT_INTEGER_BLOB* pOptionalEntropy,
    [Optional] void* pvReserved,
    [Optional] CRYPTPROTECT_PROMPTSTRUCT* pPromptStruct,
    uint dwFlags,
    CRYPT_INTEGER_BLOB* pDataOut);

CRYPT_INTEGER_BLOB

Valid dwFlags:

public const uint CRYPTPROTECT_LOCAL_MACHINE = 4U;
public const uint CRYPTPROTECT_UI_FORBIDDEN = 1U;
public const uint CRYPTPROTECT_AUDIT = 16U;