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1

It depends on what whether you really mean the "most trustworthy". As with many aspects of digital security, there is a sliding scale of strength / reliability vs usability / feasibility. I think some important questions to consider are: Who are your users? To what extent do they care about proof of file integrity? Even if the users don't care, how ...


1

You already have your answer on methods to demonstrate integrity. Any of those techniques are as "trustable" as the entity implementing them is. Perhaps what you meant to say was "most appropriate"? Assuming this is your homework question: A user transfers several files to a destination, and wants to prove that the files have not been altered in ...


3

Question 1:- assuming the polling interval is acceptable, is this a strong enough defense? Could the malicious code fool the polling code? No, its not a strong enough defense. Yes, the malicious code can identify the polling server/service by its IP, User Agent etc. and serve the clean file to your polling agent while continue to serve malicious files ...


0

As far as Question 1 goes, it would be easy for malicious code to fool the polling code. As am example, a lot of PHP malware looks at User Agent strings, and gives back 404 codes to Google, Bing, Yahoo and others. It would be almost as easy to keep track of IP addresses that make repetitive requests, and spoof them. Given a dedicated opponent, the whole ...


4

There are a number of local processes that will watch files and directories for any changes, writes, deletions, and accesses. When these events occur, the process creates an event log through syslog. This can happen in a second. If the syslog entries are sent to a remote server (as they should be) you will have nearly instant alerting to file changes, ...


4

You can append both the intended recipient, possible a timestamp or counter (see Ángel's comment), and the hash to the original data, and then encrypt. Upon decryption, you can compare the origin, and the original intended recipient, as well as the hash. This will not prevent other parties from tampering with the encrypted blob before it reaches your ...


1

Since you are already using a signature verification method on the file itself, just modify that system a little bit: Sign individual chunks of the file, and store the signatures inline. If your signature takes 32 bytes, and you use a 4,096 byte chunk size, you can have 4,064 bytes data + 32 bytes signature in each chunk. Sign the list of signatures, and ...


2

Because it's not needed. All protocols which are commonly used to download files either have a buildin mechanism to ensure integrity of the data stream or rely on a lower-level protocol (like TCP) to provide this. This is a reliable precation against accidental corruption of files in transfer. When it comes to intentional corruption of files by a ...


0

I haven't tried this, but I know there are APIs for file change notification: ReadDirectoryChangesW for Windows (Archived here.) FSEvents for Mac OS X



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