Unable to create CryptoStream to read from - System.NotSupportedException

Question

There is really simple code to decrypt file (triple des encryption):

        FileStream fin = new FileStream(FilePath, FileMode.Open, FileAccess.Read);
        TripleDES tdes = new TripleDESCryptoServiceProvider();
        CryptoStream cs = new CryptoStream(fin, tdes.CreateDecryptor(key, iv),CryptoStreamMode.Read); //<---- Exceptions

And it does not work. 'cs' is invalid and it's impossible to read from it. There are some exceptions while creating CryptoStream:

Length = 'cs.Length' threw an exception of type 'System.NotSupportedException'
base {System.SystemException} = {"Stream does not support seeking."}

Why I can't create crypto stream and read from it and how to fix this issue?

[added]

Thanks for responses, now it's more clear for me. But - still, this is impossible to read from 'cs'.

Encryption:

FileStream fout = new FileStream(FilePath, FileMode.OpenOrCreate, FileAccess.Write);
TripleDES tdes = new TripleDESCryptoServiceProvider();
CryptoStream cs = new CryptoStream(fout, tdes.CreateEncryptor(key, iv), CryptoStreamMode.Write);

byte[] d = Encoding.ASCII.GetBytes(Data);
cs.Write(d, 0, d.Length);
cs.WriteByte(0);

cs.Close();
fout.Close();

There is iv and key defined somewhere else. And, decryption - entire method:

    FileStream fin = new FileStream(FilePath, FileMode.Open, FileAccess.Read);
    TripleDES tdes = new TripleDESCryptoServiceProvider();
    CryptoStream cs = new CryptoStream(fin, tdes.CreateDecryptor(key, iv),CryptoStreamMode.Read);

    StringBuilder SB = new StringBuilder();
    int ch;
    for (int i = 0; i < fin.Length; i++)
    {
     ch = cs.ReadByte(); //Exception - CryptographicException: Bad data
     if (ch == 0)
     break;
     SB.Append(Convert.ToChar(ch));
    }
  cs.Close();
  fin.Close();

As you can see, there is the same key and iv like in encryption code. But it' still impossible to read from 'cs' stream -exception is thrown. How do you think - what's wrong here?

This is my key and iv used:

public static byte[] key = { 21, 10, 64, 10, 100, 40, 200, 4,
                    21, 54, 65, 246, 5, 62, 1, 54,
                    54, 6, 8, 9, 65, 4, 65, 9};

    private static byte[] iv = { 0, 0, 0, 0, 0, 0, 0, 0 };

Answer 1

It seams to me that you are watching sc.Length property in visual studios tool dedicated to inspect variables and you get exceptions there. If so just ignore them they would be relevant if you use Length in your code. It is quite normal for streams to not support features that require knowledge about all data that is inside.

Edit

First of all you assume that length of encrypted file is equal to length of decrypted data. I suppose this might be true but I doubt it.

try:

var textReader = new StreamReader(cs);// you might need to specify encoding 
var text = textReader.ReadToEnd();

Note that this will read whole file into memory and this will be a problem for large files.

If I would write this code I would use StreamWritter to write to CryptoStream and StreamReader to read from it code is strait forward.

Answer 2

Your code gives no errors on my machine (VS2010, .NET 4, Windows). What Client Profile/Platform are you running this on? The error indicates your FileStream is of a type that does not support seeking, is the the normal .NET FileStream?