Can I use AES in CTR mode in .NET?

Question

.NET's AES does not implement CTR directly. It only implements CBC, CFB, CTS, ECB and OFB.

Can I use any of these modes and securely implement CTR around them, or do I need to use a different library altogether?

Answer 1

A compact standalone implementation based on the code by @quadfinity.

(Despite naming of the class in the original code) It can work with any key size: 128, 192 and 256. Just provide a key of a correct size. salt must have 128 bits (16 bytes).

The method works both for encryption and decryption.

public static void AesCtrTransform(     byte[] key, byte[] salt, Stream inputStream, Stream outputStream) {     SymmetricAlgorithm aes =         new AesManaged { Mode = CipherMode.ECB, Padding = PaddingMode.None };      int blockSize = aes.BlockSize / 8;      if (salt.Length != blockSize)     {         throw new ArgumentException(             "Salt size must be same as block size " +             $"(actual: {salt.Length}, expected: {blockSize})");     }      byte[] counter = (byte[])salt.Clone();      Queue<byte> xorMask = new Queue<byte>();      var zeroIv = new byte[blockSize];     ICryptoTransform counterEncryptor = aes.CreateEncryptor(key, zeroIv);      int b;     while ((b = inputStream.ReadByte()) != -1)     {         if (xorMask.Count == 0)         {             var counterModeBlock = new byte[blockSize];              counterEncryptor.TransformBlock(                 counter, 0, counter.Length, counterModeBlock, 0);              for (var i2 = counter.Length - 1; i2 >= 0; i2--)             {                 if (++counter[i2] != 0)                 {                     break;                 }             }              foreach (var b2 in counterModeBlock)             {                 xorMask.Enqueue(b2);             }         }          var mask = xorMask.Dequeue();         outputStream.WriteByte((byte)(((byte)b) ^ mask));     } } 

If you want to encrypt or decrypt a file, use File.OpenRead for inputStream and File.Create for the outputStream:

using (Stream inputStream = File.OpenRead("file.in")) using (Stream outputStream = File.Create("file.out")) {     AesCtrTransform(key, salt, inputStream, outputStream); } 

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See also PowerShell version of the code.