I'm actually trying to make a secure file transfer program and I would like to encrypt the sent file with the c# Aes.Create() method but I wanted a AES-256 encryption and I'm not sure that the method does a 256 bits key so I searched on Microsoft docs and many sketchy websites but I did find nothing.

So, how many bits generate Aes.Create()?

There is my code:

using System.Security.Cryptography;

namespace ConsoleApp1
{
internal class Program
{
    public static void Main()
    {
        string original = File.ReadAllText(@"C:\SomePath");

        // Create a new instance of the Aes
        // class.  This generates a new key and initialization
        // vector (IV).
        using (Aes myAes = Aes.Create())
        {

            // Encrypt the string to an array of bytes.
            byte[] encrypted = EncryptStringToBytes_Aes(original, myAes.Key, myAes.IV);

            // Decrypt the bytes to a string.
            string roundtrip = DecryptStringFromBytes_Aes(encrypted, myAes.Key, myAes.IV);

            //Display the original data and the decrypted data.
            Console.WriteLine("Original:   {0}", original);
            Console.WriteLine("Encrypted:  {0}", System.Text.Encoding.Default.GetString(encrypted));
            Console.WriteLine("Round Trip: {0}", roundtrip);
        }
    }
    static byte[] EncryptStringToBytes_Aes(string plainText, byte[] Key, byte[] IV)
    {
        // Check arguments.
        if (plainText == null || plainText.Length <= 0)
            throw new ArgumentNullException("plainText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        if (IV == null || IV.Length <= 0)
            throw new ArgumentNullException("IV");
        byte[] encrypted;

        // Create an Aes object
        // with the specified key and IV.
        using (Aes aesAlg = Aes.Create())
        {
            aesAlg.Key = Key;
            aesAlg.IV = IV;

            // Create an encryptor to perform the stream transform.
            ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);

            // Create the streams used for encryption.
            using (MemoryStream msEncrypt = new MemoryStream())
            {
                using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                    {
                        //Write all data to the stream.
                        swEncrypt.Write(plainText);
                    }
                    encrypted = msEncrypt.ToArray();
                }
            }
        }

        // Return the encrypted bytes from the memory stream.
        return encrypted;
    }

    static string DecryptStringFromBytes_Aes(byte[] cipherText, byte[] Key, byte[] IV)
    {
        // Check arguments.
        if (cipherText == null || cipherText.Length <= 0)
            throw new ArgumentNullException("cipherText");
        if (Key == null || Key.Length <= 0)
            throw new ArgumentNullException("Key");
        if (IV == null || IV.Length <= 0)
            throw new ArgumentNullException("IV");

        // Declare the string used to hold
        // the decrypted text.
        string? plaintext = null;

        // Create an Aes object
        // with the specified key and IV.
        using (Aes aesAlg = Aes.Create())
        {
            aesAlg.Key = Key;
            aesAlg.IV = IV;

            // Create a decryptor to perform the stream transform.
            ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);

            // Create the streams used for decryption.
            using (MemoryStream msDecrypt = new MemoryStream(cipherText))
            {
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                    {

                        // Read the decrypted bytes from the decrypting stream
                        // and place them in a string.
                        plaintext = srDecrypt.ReadToEnd();
                    }
                }
            }
        }

        return plaintext;
    }
}
}

Yes, it's a modified version of Microsoft docs on Aes class: https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.aes

CodePudding user response：

AES is a block cipher. That means it encrypts a fixed-sized block of clear text bytes into a same-sized block of cipher text bytes (hence the term block cipher). AES uses 128-bit blocks, i.e. 16 bytes long. This is irrespective of key size.

To be able to encrypt data of an arbitrary length, block ciphers use different modes of operation. Depending on the mode, padding is applied, an initialization vector may be used, and dependencies between blocks are employed.