How to decrypt an AES-256-CBC encrypted string

c#, encryption

Solution

Looking at your encryption, something like this should do it, passing the resulting string from your encryption in should give the original string back;

// Decrypt a string into a string using a key and an IV 
public static string Decrypt(string cipherData, string keyString, string ivString)
{
    byte[] key = Encoding.UTF8.GetBytes(keyString);
    byte[] iv  = Encoding.UTF8.GetBytes(ivString);

    try
    {
        using (var rijndaelManaged =
               new RijndaelManaged {Key = key, IV = iv, Mode = CipherMode.CBC})
        using (var memoryStream = 
               new MemoryStream(Convert.FromBase64String(cipherData)))
        using (var cryptoStream =
               new CryptoStream(memoryStream,
                   rijndaelManaged.CreateDecryptor(key, iv),
                   CryptoStreamMode.Read))
        {
            return new StreamReader(cryptoStream).ReadToEnd();
        }
    }
    catch (CryptographicException e)
    {
        Console.WriteLine("A Cryptographic error occurred: {0}", e.Message);
        return null;
    }
    // You may want to catch more exceptions here...
}

A small note; you're getting the key using UTF8 encoding from the key string, UTF8 encoding may give you multiple bytes back for international characters, which may give a key or IV of the wrong length for encryption/decryption. Also, using the small range of passwords/keys with 8 characters and printable characters will not give you very secure encryption, you may want to run the string though SHA1 or similar before using it as a key (which will sadly make it incompatible with the current encryption)

Problem

I'm new to C# and I really need help. I need to encrypt/decrypt a string with AES-256-CBC in C#, I found this to encrypt a string: ``` public static string EncryptString(string message, string KeyString, string IVString) { byte[] Key = ASCIIEncoding.UTF8.GetBytes(KeyString); byte[] IV = ASCIIEncoding.UTF8.GetBytes(IVString); string encrypted = null; RijndaelManaged rj = new RijndaelManaged(); rj.Key = Key; rj.IV = IV; rj.Mode = CipherMode.CBC; try { MemoryStream ms = new MemoryStream(); using (CryptoStream cs = new CryptoStream(ms, rj.CreateEncryptor(Key, IV), CryptoStreamMode.Write)) { using (StreamWriter sw = new StreamWriter(cs)) { sw.Write(message); sw.Close(); } cs.Close(); } byte[] encoded = ms.ToArray(); encrypted = Convert.ToBase64String(encoded); ms.Close(); } catch (CryptographicException e) { Console.WriteLine("A Cryptographic error occurred: {0}", e.Message); return null; } catch (UnauthorizedAccessException e) { Console.WriteLine("A file error occurred: {0}", e.Message); return null; } catch (Exception e) { Console.WriteLine("An error occurred: {0}", e.Message); } finally { rj.Clear(); } return encrypted; } ``` I tried to write a decrypt function base on the above code, the following code is what I did: ``` // Decrypt a byte array into a byte array using a key and an IV private byte[] Decrypt(byte[] cipherData, byte[] Key, byte[] IV) { byte[] decryptedData; //string plaintext = null; //MemoryStream ms = new MemoryStream(cipherData); RijndaelManaged alg = new RijndaelManaged(); alg.KeySize = 256; alg.BlockSize = 128; alg.Key = Key; alg.IV = IV; alg.Mode = CipherMode.CBC; alg.Padding = PaddingMode.Zeros; //Array.Copy(Key, 0, IV, 0, IV.Length); ICryptoTransform decryptor = alg.CreateDecryptor(alg.Key, alg.IV); using(MemoryStream ms = new MemoryStream(cipherData)) { using (CryptoStream csDecrypt = new CryptoStream(ms, decryptor, CryptoStreamMode.Read)) { using (StreamReader sw = new StreamReader(csDecrypt)) { sw.ReadToEnd(); sw.Close(); } csDecrypt.Close(); decryptedData = ms.ToArray(); } } //byte[] decryptedData = System.Text.Encoding.Unicode.GetBytes(plaintext); return decryptedData; } ``` But it's nonsense, it can't decrypt anything. I'm really confused and need help. Thank you for any help! P/s: Please don't give me other similar answered questions, I already take a look at them. Their encrypt function doesn't have the same output like the above encrypt function, while I need to decrypt string which MUST be encrypt by the above function. I have two friend who wrote decrypt function in PHP and objective-C, which matched with the above encrypt function, it's bad to have them do it again.

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