在作具体的应用的时候，我们通常都会在config文件中保存一些重要的配置，比如帐号和密码，以及连接串。为了安全，我们需要将其加密后再写入配置文件，以免被居心叵测者非常容易地获取到。.Net Framework提供了几种对称加密算法，比如DES，3DES等，我们可以使用这些算法将敏感的配置项加密。然而，对称加密的天然问题就是密钥的管理——加密算法都是公开的，如果密钥泄漏（比如有人把密钥写在代码中，那完全可以通过反编译代码来得知密钥），那加密就形同虚设了。所以我们需要把密钥保存在相对安全的地方。
可行的做法为：
[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,
int dwFlags,
ref DATA_BLOB pDataOut);
[DllImport("kernel32.dll",
CharSet=System.Runtime.InteropServices.CharSet.Auto)]
private unsafe static extern int FormatMessage(int
dwFlags,
ref IntPtr lpSource,
int dwMessageId,
int dwLanguageId,
ref String lpBuffer,
int nSize,
IntPtr *Arguments);
[StructLayout(LayoutKind.Sequential,
CharSet=CharSet.Unicode)]
internal struct DATA_BLOB
{
public int cbData;
public IntPtr pbData;
}
[StructLayout(LayoutKind.Sequential,
CharSet=CharSet.Unicode)]
internal struct CRYPTPROTECT_PROMPTSTRUCT
{
public int cbSize;
public int dwPromptFlags;
public IntPtr hwndApp;
public String szPrompt;
}
static private IntPtr NullPtr = ((IntPtr)((int)(0)));
private const int CRYPTPROTECT_UI_FORBIDDEN = 0x1;
private const int CRYPTPROTECT_LOCAL_MACHINE = 0x4;
public enum Store {USE_MACHINE_STORE = 1,
USE_USER_STORE};
private Store store;
public void DataProtector(Store tempStore)
{
store = tempStore;
}
public byte[] Encrypt(byte[] plainText, byte[]
optionalEntropy)
{
bool retVal = false;
DATA_BLOB plainTextBlob = new DATA_BLOB();
DATA_BLOB cipherTextBlob = new DATA_BLOB();
DATA_BLOB entropyBlob = new DATA_BLOB();
CRYPTPROTECT_PROMPTSTRUCT prompt = new
CRYPTPROTECT_PROMPTSTRUCT();
InitPromptstruct(ref prompt);
int dwFlags;
try
{
try
{
int bytesSize = plainText.Length;
plainTextBlob.pbData =
Marshal.AllocHGlobal(bytesSize);
if(IntPtr.Zero == plainTextBlob.pbData)
{
throw new Exception("Unable to allocate
plaintext buffer.");
}
plainTextBlob.cbData = bytesSize;
Marshal.Copy(plainText, 0,
plainTextBlob.pbData, bytesSize);
}
catch(Exception ex)
{
throw new Exception("Exception marshalling
data. " + ex.Message);
}
if(Store.USE_MACHINE_STORE == store)
{
dwFlags =
CRYPTPROTECT_LOCAL_MACHINE|CRYPTPROTECT_UI_FORBIDDEN;
if(null == optionalEntropy)
{
optionalEntropy = new byte[0];
}
try
{
int bytesSize = optionalEntropy.Length;
entropyBlob.pbData =
Marshal.AllocHGlobal(optionalEntropy.Length);;
if(IntPtr.Zero == entropyBlob.pbData)
{
throw new Exception("Unable to
allocate entropy data buffer.");
}
Marshal.Copy(optionalEntropy, 0,
entropyBlob.pbData, bytesSize);
entropyBlob.cbData = bytesSize;
}
catch(Exception ex)
{
throw new Exception("Exception entropy
marshalling data. " +
ex.Message);
}
}
else
{
dwFlags = CRYPTPROTECT_UI_FORBIDDEN;
}
retVal = CryptProtectData(ref plainTextBlob, "",
ref entropyBlob,
IntPtr.Zero, ref prompt, dwFlags,
ref cipherTextBlob);
if(false == retVal)
{
throw new Exception("Encryption failed. " +
GetErrorMessage(Marshal.GetLastWin32Error()));
}
if(IntPtr.Zero != plainTextBlob.pbData)
{
Marshal.FreeHGlobal(plainTextBlob.pbData);
}
if(IntPtr.Zero != entropyBlob.pbData)
{
Marshal.FreeHGlobal(entropyBlob.pbData);
}
}
catch(Exception ex)
{
throw new Exception("Exception encrypting. " +
ex.Message);
}
byte[] cipherText = new byte[cipherTextBlob.cbData];
Marshal.Copy(cipherTextBlob.pbData, cipherText, 0,
cipherTextBlob.cbData);
Marshal.FreeHGlobal(cipherTextBlob.pbData);
return cipherText;
}
private void InitPromptstruct(ref
CRYPTPROTECT_PROMPTSTRUCT ps)
{
ps.cbSize =
Marshal.SizeOf(typeof(CRYPTPROTECT_PROMPTSTRUCT));
ps.dwPromptFlags = 0;
ps.hwndApp = NullPtr;
ps.szPrompt = null;
}
private unsafe static String GetErrorMessage(int
errorCode)
{
int FORMAT_MESSAGE_ALLOCATE_BUFFER = 0x00000100;
int FORMAT_MESSAGE_IGNORE_INSERTS = 0x00000200;
int FORMAT_MESSAGE_FROM_SYSTEM = 0x00001000;
int messageSize = 255;
String lpMsgBuf = "";
int dwFlags = FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS;
IntPtr ptrlpSource = new IntPtr();
IntPtr prtArguments = new IntPtr();
int retVal = FormatMessage(dwFlags, ref ptrlpSource,
errorCode, 0,
ref lpMsgBuf, messageSize,
&prtArguments);
if(0 == retVal)
{
throw new Exception("Failed to format message for
error code " +
errorCode + ". ");
}
return lpMsgBuf;
}
可行的做法为：
1、 使用Aspnet_setreg.exe 工具。此工具使用DPAPI来加密和解密数据，它可以把你需要保护的数据保存在注册表，它受DACL（discretionary access control list）保护，在配置文件中只需要指明注册表键的位置。有一篇How to讲解了如何使用此Aspnet_setreg.exe 工具。在我的随笔系统安全简述中对DPAPI有简单的讲述——DPAPI（Data Protection Application Programming Interface），一个保护密钥的对策。DPAPI使用用户的登录密码加密和解密数据。数据保护过程：（1）生成一个强健的Key，称为主钥，它受用户的密码保护；（2）使用基于口令的密钥生成过程由用户密码生成一个密钥；（3）这个由口令生成的密钥用来加密主钥，它被存储在由计算机上的活动目录生成的用户概述中。安全地存储登录证明，比如用户名和密码，在磁盘上，是安全应用地一个重要方面。DPAPI是一个系统提供的数据保护服务，是一个安全地存储用户证明的解决方案。
2、 直接使用DPAPI和.Net framework提供的加密算法。提供一个key（key越强就越安全），使用DPAPI加密，把它存储在注册表中，然后我们可以使用此key来加密和解密数据了——加密和解密算法我们可以选择.Net framework提供的3DES（DES比较脆弱）。最后，我们把加密过的配置项（如连接串）写入config文件即可。
使用DPAPI时，需要我们提供足够强健的key，否则安全性会降低。另外，如果侵入者获得了系统的管理员权限的话，那这些安全措施都将失效，因为管理员有权限读取我们存入注册表的key。
下面是使用DPAPI加密和解密数据的例子：
[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, int dwFlags, ref DATA_BLOB pDataOut); [DllImport("kernel32.dll", CharSet=System.Runtime.InteropServices.CharSet.Auto)] private unsafe static extern int FormatMessage(int dwFlags, ref IntPtr lpSource, int dwMessageId, int dwLanguageId, ref String lpBuffer, int nSize, IntPtr *Arguments); [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Unicode)] internal struct DATA_BLOB { 
public int cbData; public IntPtr pbData; } [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Unicode)] internal struct CRYPTPROTECT_PROMPTSTRUCT { public int cbSize; public int dwPromptFlags; public IntPtr hwndApp; public String szPrompt; } static private IntPtr NullPtr = ((IntPtr)((int)(0))); private const int CRYPTPROTECT_UI_FORBIDDEN = 0x1; private const int CRYPTPROTECT_LOCAL_MACHINE = 0x4; public enum Store {USE_MACHINE_STORE = 1, USE_USER_STORE}; private Store store; public void DataProtector(Store tempStore) { store = tempStore; } public byte[] Encrypt(byte[] plainText, byte[] optionalEntropy) { bool retVal = false; DATA_BLOB plainTextBlob = new DATA_BLOB(); DATA_BLOB cipherTextBlob = new DATA_BLOB(); DATA_BLOB entropyBlob = new DATA_BLOB(); CRYPTPROTECT_PROMPTSTRUCT prompt = new CRYPTPROTECT_PROMPTSTRUCT(); InitPromptstruct(ref prompt); int dwFlags; try { 
try { int bytesSize = plainText.Length; plainTextBlob.pbData = Marshal.AllocHGlobal(bytesSize); if(IntPtr.Zero == plainTextBlob.pbData) { throw new Exception("Unable to allocate plaintext buffer."); } plainTextBlob.cbData = bytesSize; Marshal.Copy(plainText, 0, plainTextBlob.pbData, bytesSize); } catch(Exception ex) { throw new Exception("Exception marshalling data. " + ex.Message); } if(Store.USE_MACHINE_STORE == store) { dwFlags = CRYPTPROTECT_LOCAL_MACHINE|CRYPTPROTECT_UI_FORBIDDEN; if(null == optionalEntropy) { optionalEntropy = new byte[0]; } try { int bytesSize = optionalEntropy.Length; entropyBlob.pbData = Marshal.AllocHGlobal(optionalEntropy.Length);; if(IntPtr.Zero == entropyBlob.pbData) { throw new Exception("Unable to allocate entropy data buffer."); } Marshal.Copy(optionalEntropy, 0, entropyBlob.pbData, bytesSize); entropyBlob.cbData = bytesSize; } catch(Exception ex) { throw new Exception("Exception entropy marshalling data. " + ex.Message); } } else { dwFlags = CRYPTPROTECT_UI_FORBIDDEN; } retVal = CryptProtectData(ref plainTextBlob, "", ref entropyBlob, IntPtr.Zero, ref prompt, dwFlags, ref cipherTextBlob); if(false == retVal) { throw new Exception("Encryption failed. " + GetErrorMessage(Marshal.GetLastWin32Error())); } if(IntPtr.Zero != plainTextBlob.pbData) { Marshal.FreeHGlobal(plainTextBlob.pbData); } if(IntPtr.Zero != entropyBlob.pbData) { Marshal.FreeHGlobal(entropyBlob.pbData); } } catch(Exception ex) { throw new Exception("Exception encrypting. " + ex.Message); } byte[] cipherText = new byte[cipherTextBlob.cbData]; Marshal.Copy(cipherTextBlob.pbData, cipherText, 0, cipherTextBlob.cbData); Marshal.FreeHGlobal(cipherTextBlob.pbData); return cipherText; } private void InitPromptstruct(ref CRYPTPROTECT_PROMPTSTRUCT ps) { ps.cbSize = Marshal.SizeOf(typeof(CRYPTPROTECT_PROMPTSTRUCT)); ps.dwPromptFlags = 0; ps.hwndApp = NullPtr; ps.szPrompt = null; } private unsafe static String GetErrorMessage(int errorCode) { int FORMAT_MESSAGE_ALLOCATE_BUFFER = 0x00000100; int FORMAT_MESSAGE_IGNORE_INSERTS = 0x00000200; int FORMAT_MESSAGE_FROM_SYSTEM = 0x00001000; int messageSize = 255; String lpMsgBuf = ""; int dwFlags = FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS; IntPtr ptrlpSource = new IntPtr(); IntPtr prtArguments = new IntPtr(); int retVal = FormatMessage(dwFlags, ref ptrlpSource, errorCode, 0, ref lpMsgBuf, messageSize, &prtArguments); if(0 == retVal) { throw new Exception("Failed to format message for error code " + errorCode + ". "); } return lpMsgBuf; } 
