Decryption of large file with AES

Question

I'm trying to decrypt 500 mb of data, and im getting a out of memory exception on larger files, so the decryption works for smaller sized files, is there anyway I can ensure that I'm not getting this out of memory exception?

The first part of the key.file is the IV and the second part of the key.file is the key.

My machine got 32 gb of memory, so it's not a local problem.

The code breaks on this line: var so = decrTransform.TransformFinalBlock(file, 0, file.Length);

private void DecryptData() {

 X509Certificate2 cert; var storeLocation = (StoreLocation)Enum.Parse(typeof(StoreLocation), "LocalMachine", true); var storeName = (StoreName)Enum.Parse(typeof(StoreName), "My", true); var findType = (X509FindType)Enum.Parse(typeof(X509FindType), "FindByThumbprint", true); string thumbprint = Thumb; try { X509Store certStore = new X509Store(storeName, storeLocation); certStore.Open(OpenFlags.ReadOnly); X509Certificate2Collection certCollection = certStore.Certificates.Find(findType, thumbprint, false); certStore.Close(); cert = new X509Certificate2(certCollection[0]); } catch (Exception ex) { throw ex; } RijndaelManaged alg = new RijndaelManaged(); try { var asd = ((RSACryptoServiceProvider)cert.PrivateKey).Decrypt("file.key.path"), true); var iv = new byte[16]; Buffer.BlockCopy(asd, 0, iv, 0, 16); var key = new byte[32]; Buffer.BlockCopy(asd, 16, key, 0, 32); alg.Padding = PaddingMode.PKCS7; alg.Mode = CipherMode.CBC; using (ICryptoTransform decrTransform = alg.CreateDecryptor(key, iv)) { byte[] file = ReadFile(@"encrypted.file.path"); var so = decrTransform.TransformFinalBlock(file, 0, file.Length); File.WriteAllBytes(@"SavedData.path", so); decrTransform.Dispose(); } } catch (Exception ex) { throw ex; } }

32bit OS or 64bit OS? Compiling for 32bit .NET or 64bit .NET? — davidbak
– davidbak, Commented Apr 28, 2016 at 17:25
Thing is, a 32bit .NET process can only access 2GB (user region) and may have trouble allocating large blocks of contiguous memory. You've already found one ~500Mb block of contiguous memory for the file you read in. You may not have a second ~500Mb block of contiguous memory in your process address space for the decoded data. If you're running 32-bit. — davidbak
– davidbak, Commented Apr 28, 2016 at 17:28

Maarten Bodewes · Accepted Answer · 2016-04-28 19:53:55Z

Try using streams, especially CryptoStream. The Microsoft example at the bottom of this page actually perform file based encryption with RijndaelManaged so you're in luck. You would first need to extract the IV from the file stream of course, e.g. by reading exactly 16 bytes byte-by-byte. Only wrap the stream after reading the IV.

That way there is no need for memory consumption other than a buffer size, which should range in a few KiB maximum.

Yep you are right, I tried to use stream instead of byte arrays, and now it works as intended - thanks :)

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