Is there a way for me to verify if a file uploaded by the user to my server has been altered since their last upload?
My database includes a log table containing User_id and FileName (each User_id is unique). Once I have read the contents of the file, I delete it from the server.
If you want to ensure the integrity of a file before deleting it, you can store a hash of the file and compare it with the previous hash when uploading. The System.Cryptography namespace provides HashAlgorithm classes like SHA1 that can help you achieve this.
"A cryptographic hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string, the (cryptographic) hash value, such that an accidental or intentional change to the data will change the hash value"
Here's some sample code to get you going. Assuming you have a stream variable named stream containing your file data (you could use FileStream to open it):
var sha = new System.Security.Cryptography.SHA1Managed();
byte [] hash = sha.ComputeHash(stream);
Now, the variable hash will contain the fingerprint of the file contents. Even a slight alteration in the file will result in a different hash value, but hashing the same file will always yield the same hash.
Hash functions are commonly used to detect changes in large chunks of data, such as files. One common method is using a cyclic redundancy check (CRC).
In the Linux operating system, there is a built-in utility called cksum that can be used for this purpose.
To generate a checksum for a file, you can simply run the command cksum filename and save the output. This checksum can then be stored in a database and used to verify the integrity of new incoming files.
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