CODEWITHSUNDEEP
c#encryptionaes
alexhak
alexhak

Reputation: 131

AES-ECB block-decryption in C#

I wrote code that decrypts the input string completely. But since this is an ECB mode, I wanted to somehow decrypt not the entire input text, but only a separate block of it.

As far as I understand, ECB AES encrypts in blocks of 8 bytes. How can I add the AES_Decrypt function to it so that it decrypts only the last 8 bytes of the input string, for example.

    byte[] bytesToBeDecrypted = new byte[32];

    byte[] 8_bytesToBeDecrypted = new byte[8];  // Only 8 bytes of bytesToBeDecrypted

    public static byte[] AES_Decrypt(byte[] bytesToBeDecrypted, byte[] passwordBytes)
        {
            byte[] decryptedBytes = null;

            string salt = "12345678";
            Encoding unicode = Encoding.Unicode;
            byte[] saltBytes = unicode.GetBytes(salt);

            using (MemoryStream ms = new MemoryStream())
            {
                using (RijndaelManaged AES = new RijndaelManaged())
                {
                    AES.KeySize = 256;
                    AES.BlockSize = 128;
                    AES.Padding = PaddingMode.Zeros;

                    var key = new Rfc2898DeriveBytes(passwordBytes, saltBytes, 65535);
                    AES.Key = key.GetBytes(AES.KeySize / 8);
                    AES.IV = key.GetBytes(AES.BlockSize / 8);

                    AES.Mode = CipherMode.ECB;

                    using (var cs = new CryptoStream(ms, AES.CreateDecryptor(), CryptoStreamMode.Write))
                    {
                        cs.Write(bytesToBeDecrypted, 0, bytesToBeDecrypted.Length);
                        cs.Close();
                    }
                    decryptedBytes = ms.ToArray();
                }
            }

            return decryptedBytes;
        }

Upvotes: 1

Views: 1787

Answers (1)

Maarten Bodewes
Maarten Bodewes

Reputation: 93998

You can likely just Seek the MemoryStream with a multiple of 16 bytes from the beginning (or apparently even 16 bytes from the last part of the stream) and then connect the CryptoStream to decrypt whatever is left. With CBC that would be a bit more tricky as you would have to set the IV to the previous ciphertext block, but with ECB this should be a breeze.

Notes:

  • Of course, you don't need to set the IV for the ECB mode, that should not be required.
  • Rijndael with a block size of 128 is actually AES. C#/.NET only supports a subset of Rijndael by the way: only 64 increments of the block size and key size are supported, if I remember correctly.
  • I'd still use aes = Aes.Create() rather than aes = new RijndaelManaged() as that leaves the choice of the AES implementation to the system, and it will probably choose the hardware accelerated one over the managed one.

Upvotes: 1

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