Question

I use AES encryption. It’s ok when I encrypt and then decrypt whole file. I want to add multiple files to one encrypted. That’s where the problem is. Encryption is fine, but decryption causes CryptographicException – bad data length. Is it even possible to decrypt part of file or is it encrypted as whole ? I used one cryptostream and passed there all files I want to encrypt to single file. I am trying to do opposite:

AesManaged aes = AES.InitAes(key, salt);
            ICryptoTransform transform = aes.CreateDecryptor(aes.Key, aes.IV);
            int defChunkSize = 1024 * 1024 * 50;

            using (FileStream source = new FileStream(header.data.filename, FileMode.Open))
            {
                foreach (CryptHeader.fileStruct file in header.data.files)
                {
                    preparePath(file.filename);

                    using (FileStream target = new FileStream(file.filename, FileMode.Create))
                    {
                        using (CryptoStream cryptoStream = new CryptoStream(target, transform, CryptoStreamMode.Write))
                        {
                            long padding = source.Length - header.data.files.Sum(x => x.length);//Just test
                            int chunkSize = (defChunkSize > (int)file.length) ? (int)file.length : defChunkSize;
                            byte[] chunkData = new byte[chunkSize];
                            int bytesRead = 0;
                            int totalRead = 0;
                            while (totalRead < file.length)
                            {
                                bytesRead = source.Read(chunkData, 0, chunkSize);
                                if (bytesRead <= 0) break;
                                totalRead += bytesRead;
                                cryptoStream.Write(chunkData, 0, bytesRead);
                            }      
                            chunkData = null;
                        }
                    }
                }
            }    

Answer 1

I’ve done the same few years ago without any problem. The logic I used is the following:

Encryption

1. define number of files

2. define array for keeping encrypted sizes

3. open output stream

4. seek (forced) to (number of files * 4) + 4 (assuming lengths are integers)

5. loop for encryption (encrypt- write encrypted data -assigned encrypted size)

6. seek to 0 (begin)

7. write number of files

8. write encrypted size array

9. close output stream

Decryption

1. open input stream

2. read number of files

3. define-read-fill array with encrypted sizes

4. loop for decryption (read using known sizes)

5. close output stream

I hope that this helps.