C# Decrypting a AES256 encrypted file

Question

I'm trying to decrypt a AES256 coded file but I'm getting a corrupted file output.

I have a 256bit (64 chars) hex AES key and a 128bit (32 chars) hex IV key that I'm converting to byte arrays with the following code.

public byte[] StringToByteArray(String hex) { int NumberChars = hex.Length / 2; byte[] bytes = new byte[NumberChars]; using (var sr = new StringReader(hex)) { for (int i = 0; i < NumberChars; i++) bytes[i] = Convert.ToByte(new string(new char[2] { (char)sr.Read(), (char)sr.Read() }), 16); } return bytes; }

The actual code I'm using for decryption is as follows.

public string DecryptCrypt7(byte[] data, string keyString, string ivString) { byte[] aesHash = StringToByteArray(keyString); byte[] ivHash = StringToByteArray(ivString); try { using (var rijndaelManaged = new RijndaelManaged { Key = aesHash, IV = ivHash, Mode = CipherMode.CBC, Padding = PaddingMode.None, BlockSize = 128, KeySize = 256 }) { using (var memoryStream = new MemoryStream(data)) { using (var cryptoStream = new CryptoStream(memoryStream, rijndaelManaged.CreateDecryptor(aesHash, ivHash), CryptoStreamMode.Read)) { return new StreamReader(cryptoStream).ReadToEnd(); } } } } catch (CryptographicException e) { Console.WriteLine("A Cryptographic error occurred: {0}", e.Message); return null; } }

It does decrypt the file, but it's clearly corrupted as I can't open it with an sqlite db viewer and see the tables, etc.

Decrypting it via CygWin using OpenSSL with the following command decrypts it properly and I can view all the tables in the file using a viewer.

openssl enc -aes-256-cbc -d -nosalt -nopad -bufsize 16384 -in file.crypt7 -K $(cat aes.txt) -iv $(cat iv.txt) > file.db

Is the original data binary data or text? Because you're returning it as plain text... — Jon Skeet
– Jon Skeet, Commented Aug 21, 2014 at 14:13
Then why are you returning it as a string? That may well be the problem. Try just encrypting a plain text file instead first - if you get the same plain text back, then that is indeed the problem, and your method should return a byte[] instead. (Consider using a MemoryStream and cryptoStream.CopyTo(memoryStream) before returning memoryStream.ToArray().) — Jon Skeet
– Jon Skeet, Commented Aug 21, 2014 at 14:52
@JonSkeet That was it. Thank you very much. It's my first time playing with cryptography and I missed the most obvious thing. — Hei
– Hei, Commented Aug 21, 2014 at 14:59

Jon Skeet · Accepted Answer · 2014-08-21 15:04:51Z

The problem is that your original data is binary data, but you're converting it to a string after you've decrypted it. So you just need to change your method to return a byte[], then change the end of your decryption method to:

using (Stream encrypted = new MemoryStream(data), decrypted = new CryptoStream(encrypted, rijndaelManaged.CreateDecryptor(aesHash, ivHash), CryptoStreamMode.Read), copy = new MemoryStream()) { decrypted.CopyTo(copy); return copy.ToArray(); }

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