I have some data I need to put into a file, however I want to ensure that the data remains intact. Either from corruption or from a user modifying the file.
Can someone point me in the right direction, of how to add a checksum to a file, read / check the file is correct?
You can use the MD5 hash, that's the best option. Here is how to create a MD5 hash from NSString, NSData or a File:
http://iosdevelopertips.com/core-services/create-md5-hash-from-nsstring-nsdata-or-file.html
Related
I have tried automating this process through SOMA: copying the file from one location to another location using dp:get-file in our local directory and their I've to change the file permissions and then unzip the file and save the folder in another location but didn't achieve.
If anyone tried please help on this scenario.
Thanks in advance !
I don't exactly follow all your steps, can you detail them a bit more indicating what you do on DP and what on remote system?
The key to know is that SOMA dp:get-file returns your file as a base64 encoded XML element value and you have to base64 decode that to have your actual file and then work with it on your remote system.
The other way round to put the file back using SOMA you have to base64 encode it and then sent as SOMA XML element value to DP.
I don't think DP cares of any file permission attributes? Or do you have something specific in mind?
You indicate you unzip your file. Are you playing around with ZIP-format DP config export? That would not be ideal approach. Or what is your usecase?
I'm relatively new to C# and am attempting to adapt a text encryption algorithm I designed in wxMaxima into a Binary encryption program in C# using Visual Studio forms. Because I am new to reading/writing binary files, I am lacking in knowledge regarding what happens when I try to read or write to a filestream.
For example, instead of encrypting a text file as I've done in the past, say I want to encrypt an executable or any other form of binary file.
Here are a few questions I don't understand:
When I open a file stream and use binaryreader will it read in an absolute duplicate of absolutely everything in the file? I want to be able to, for example, read in an entire file, delete the original file, then create a new file with the old name and write the entire binary stream back. Will this reproduce the original file exactly or will there be some sort of corruption that must otherwise be accounted for?
Because it's an encryption program, I was hoping to add in a feature that would low-level "format" the original file before deleting it so it would be theoretically inaccessible by combing the physical data of a harddisk. If I use binarywriter to overwrite parts of the original file with gibberish will it be put on the same spot on the harddisk or will the file become fragmented and actually just redirect via the FAT to some other portion of the harddisk? Obviously there's no point in overwriting the original file with gibberish if it's not over-writing the original cluster on the harddisk.
For your first question: A BinaryReader is not what you want. The name is a bit misleading: it "Reads primitive data types as binary values in a specific encoding." You probably want a FileStream.
Regarding the second question: That will not be easy: please see the "How SDelete Works" section of SDelete for an explanation. Brief extract in case that link breaks in the future:
"Securely deleting a file that has no special attributes is relatively straight-forward: the secure delete program simply overwrites the file with the secure delete pattern. What is more tricky is securely deleting Windows NT/2K compressed, encrypted and sparse files, and securely cleansing disk free spaces.
Compressed, encrypted and sparse are managed by NTFS in 16-cluster blocks. If a program writes to an existing portion of such a file NTFS allocates new space on the disk to store the new data and after the new data has been written, deallocates the clusters previously occupied by the file."
I have generated hash values for files using md5. If there is any change in the file the hash value changes. Does it also change when the file permismsions get modified?
No. The MD5 hash of a file is related to its content, not its permissions.
The MD5 hash will change if there's any change to whatever data you input to the MD5 hashing function. If you fed it the permissions and the permission change, then the MD5 hash will change. If you fed it only the contents, then the MD5 hash will change only if the contents change.
What you get out depends on what you put in. You haven't told us what you put in. So we can't tell you what the output depends on.
Problem:
I have to download some files from the server. In between the connection with the server is lost. And when the file is opened it opened without any problem, except that it was blank.
Question
How to check if the downloaded file from the server is corrupted or not? Is there any way to do that?
If the file is corrupted it must be deleted from the documents folder.
Thank you!
You can create a hash of the file and then use that hash to compare the current hash to the new hash.
Here's an example on creating a hash for iOS:
http://iosdevelopertips.com/core-services/create-md5-hash-from-nsstring-nsdata-or-file.html
It should work pretty well because the hash only changes if the file contents change and is unaffected by creation times, modification times, and the file name.
Edit
You can also sign your files with PGP or GPG and use your public key to verify its contents.
Hope this helps :)
Send a hash of the file with the file, and then compare the hashes.
On some sites, in their download section each file has a md5. md5 of what? i cant understand the purpose
on phpBB.com for example:
Download phpBB 3.0.6 (zip)
Size: 2.30 MiB
MD5: 63e2bde5bd03d8ed504fe181a70ec97a
It is the signature of the file's hash. The idea is that you can run MD5 against the downloaded file, then compare it against that value to make sure you did not end up with a corrupted download.
This is a checksum, for verifying that the file as-downloaded is intact, without transmission errors. If the checksum listing is on a different server than the download, it also may give a little peace of mind that the download server hasn't been hacked (with the presumption that two servers are harder to hack than one).
It's a hash of the file. Used to ensure file integrity once you download said file. You'd use an md5 checksum tool to verify the file state.
Sites will post checksums so that you can make sure the file downloaded is the same as the file they're offering. This lets you ensure that file has not been corrupted or tampered with.
On most unix operating systems you can run md5 or md5sum on a file to get the hash for it. If the hash you get matches the hash from the website, you can be reasonably certain that the file is intact. A quick Google search will get you md5sum utilities for Windows.
You might also see an SHA-1 hash sometimes. It's the same concept, but a different and more secure algorithm.
This is an md5 hash of the entire binary contents of the file. The point is that if two files have different md5 hashes, they are different. This helps you determine whether a local file on your computer is the same as the file on the website, without having to download it again. For instance:
You downloaded your local copy somewhere else and think there might be a virus inside.
Your connection is lossy and you fear the file might be corrupted by the download.
You have changed the local file name and want to know which version you have.