我想使用 CryptoStream 将数据附加到已经加密的文件（AES、CBC 模式、填充 PKCS#7）而不读取和写入整个文件。
例子：
旧内容："Hello world..."
新内容："Hello world, with appended text"
当然，我必须读取单个数据块，然后将其附加到已经存在的块中。在上面提到的示例中，我必须读取第一个块中存在的字节数（14 个字节）并将两个字节附加到第一个块，然后写入其余的附加文本
"Hello world, wi"
"th appended text"
我面临的一个问题是我无法读取数据块中的字节数。有没有办法找出存在的字节数（在示例中为 14）？
此外，我被卡住了，因为 CryptoStreamMode 只有读写成员，但没有更新。
有没有办法使用 CryptoStream 完成我想要的功能？
它有点复杂，但不是太多。请注意，这是针对 CBC 模式 + PKCS#7！
三种方法：WriteStringToFile将创建一个新文件，AppendStringToFile将附加到已加密的文件（就像WriteStringToFile文件丢失/空一样），ReadStringFromFile将从文件中读取。
public static void WriteStringToFile(string fileName, string plainText, byte[] key, byte[] iv)
{
using (Rijndael algo = Rijndael.Create())
{
algo.Key = key;
algo.IV = iv;
algo.Mode = CipherMode.CBC;
algo.Padding = PaddingMode.PKCS7;
// Create the streams used for encryption.
using (FileStream file = new FileStream(fileName, FileMode.Create, FileAccess.Write))
// Create an encryptor to perform the stream transform.
using (ICryptoTransform encryptor = algo.CreateEncryptor())
using (CryptoStream cs = new CryptoStream(file, encryptor, CryptoStreamMode.Write))
using (StreamWriter sw = new StreamWriter(cs))
{
// Write all data to the stream.
sw.Write(plainText);
}
}
}
public static void AppendStringToFile(string fileName, string plainText, byte[] key, byte[] iv)
{
using (Rijndael algo = Rijndael.Create())
{
algo.Key = key;
// The IV is set below
algo.Mode = CipherMode.CBC;
algo.Padding = PaddingMode.PKCS7;
// Create the streams used for encryption.
using (FileStream file = new FileStream(fileName, FileMode.OpenOrCreate, FileAccess.ReadWrite))
{
byte[] previous = null;
int previousLength = 0;
long length = file.Length;
// No check is done that the file is correct!
if (length != 0)
{
// The IV length is equal to the block length
byte[] block = new byte[iv.Length];
if (length >= iv.Length * 2)
{
// At least 2 blocks, take the penultimate block
// as the IV
file.Position = length - iv.Length * 2;
file.Read(block, 0, block.Length);
algo.IV = block;
}
else
{
// A single block present, use the IV given
file.Position = length - iv.Length;
algo.IV = iv;
}
// Read the last block
file.Read(block, 0, block.Length);
// And reposition at the beginning of the last block
file.Position = length - iv.Length;
// We use a MemoryStream because the CryptoStream
// will close the Stream at the end
using (var ms = new MemoryStream(block))
// Create a decrytor to perform the stream transform.
using (ICryptoTransform decryptor = algo.CreateDecryptor())
using (CryptoStream cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
{
// Read all data from the stream. The decrypted last
// block can be long up to block length characters
// (so up to iv.Length) (this with AES + CBC)
previous = new byte[iv.Length];
previousLength = cs.Read(previous, 0, previous.Length);
}
}
else
{
// Use the IV given
algo.IV = iv;
}
// Create an encryptor to perform the stream transform.
using (ICryptoTransform encryptor = algo.CreateEncryptor())
using (CryptoStream cs = new CryptoStream(file, encryptor, CryptoStreamMode.Write))
using (StreamWriter sw = new StreamWriter(cs))
{
// Rewrite the last block, if present. We even skip
// the case of block present but empty
if (previousLength != 0)
{
cs.Write(previous, 0, previousLength);
}
// Write all data to the stream.
sw.Write(plainText);
}
}
}
}
public static string ReadStringFromFile(string fileName, byte[] key, byte[] iv)
{
string plainText;
using (Rijndael algo = Rijndael.Create())
{
algo.Key = key;
algo.IV = iv;
algo.Mode = CipherMode.CBC;
algo.Padding = PaddingMode.PKCS7;
// Create the streams used for decryption.
using (FileStream file = new FileStream(fileName, FileMode.Open, FileAccess.Read))
// Create a decrytor to perform the stream transform.
using (ICryptoTransform decryptor = algo.CreateDecryptor())
using (CryptoStream cs = new CryptoStream(file, decryptor, CryptoStreamMode.Read))
using (StreamReader sr = new StreamReader(cs))
{
// Read all data from the stream.
plainText = sr.ReadToEnd();
}
}
return plainText;
}
使用示例：
var key = Encoding.UTF8.GetBytes("Simple key");
var iv = Encoding.UTF8.GetBytes("Simple IV");
Array.Resize(ref key, 128 / 8);
Array.Resize(ref iv, 128 / 8);
if (File.Exists("test.bin"))
{
File.Delete("test.bin");
}
for (int i = 0; i < 100; i++)
{
AppendStringToFile("test.bin", string.Format("{0},", i), key, iv);
}
string plainText = ReadStringFromFile("test.bin", key, iv);
作品如何AppendStringToFile？三种情况：
WriteStringToFileplainTextplainText. 为了重新加密最后一个块，使用的 IV 是倒数第二个块。