Checksum downloaded files

Checksums have the advantage that they detect strictly more problems. In theory, the network would deliver every single segment properly and they'd be assembled properly on the disk and nothing would go wrong. In reality, computers are machines and software, both of which are designed and built by fallible humans. In the event that a download does somehow not come down right for one reason or another, such as the download being through some intermediary device whether innocuous or nefarious which mangles the data, it's nice to have a way to check that the file almost certainly was downloaded as an accurate replica of the file on the provider's side.

When we add another checksum to the train we multiply the probability of detecting an error. The server has an internal error while serving the download hence the download is terminated. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why compute checksums of downloaded files? Ask Question. Asked 6 years, 4 months ago.

Active 6 years, 4 months ago. Viewed 3k times. Improve this question. The TCP checksum is actually pretty lousy: it's only 16 bits. If you're serving up large files to thousands of people think: installation DVD images , it's virtually certain that some of those downloads will be undetectably corrupted.

Mihai Of course, it probably does decrease the risk a bit, though. For example, if your server is infected by a virus that automatically modifies all binary responses or just replaces all executables you download.

It's not perfect, but it can help in some cases. Add a comment. Active Oldest Votes. Not always. Improve this answer. PSkocik PSkocik It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. Maybe I have been negligent towards the verification of software I download over the Internet, but I or anybody I ever met have never tried to verify the checksum of the contents I download.

And because of this, I have no idea about how to verify the integrity of the downloaded item. Usually this would start on the owners side displaying the checksum for the file that you wish to download. Which would look something like the following:. Now depending on what operating system you are using, once you have downloaded the required file you can compute a hash of it.

First navigate to the directory of the file you downloaded, than:. The issue that comes with checking a hash from a website is that it doesn't determine that the file is safe to download, just that what you have downloaded is the correct file, byte for byte. If the website has been compromised then you could be shown the hash for a different file, which in turn could be malicious. A checksum simply verifies with a high degree of confidence that there was no corruption causing a copied file to differ from the original for varying definitions of "high".

In general a checksum provides no guarantee that intentional modifications weren't made, and in many cases it is trivial to change the file while still having the same checksum. Cryptographic hashes provide additional properties over simple checksums all cryptographic hashes can be used as checksums, but not all checksums are cryptographic hashes. Cryptographic hashes that aren't broken or weak provide collision and preimage resistance.

Collision resistance means that it isn't feasible to create two files that have the same hash, and preimage resistance means that it isn't feasible to create a file with the same hash as a specific target file.

MD5 and SHA1 are both broken in regard to collisions, but are safe against preimage attacks due to the birthday paradox collisions are much easier to generate. SHA is commonly used today, and is safe against both.

If you plan to use a hash to verify a file, you must obtain the hash from a separate trusted source. Retrieving the hash from the same site you're downloading the files from doesn't guarantee anything. If an attacker is able to modify files on that site or intercept and modify your connection, they can simply substitute the files for malicious versions and change the hashes to match. Using a hash that isn't collision resistant may be problematic if your adversary can modify the legitimate file for example, contributing a seemingly innocent bug fix.

They may be able to create an innocent change in the original that causes it to have the same hash as a malicious file, which they could then send you. The best example of where it makes sense to verify a hash is when retrieving the hash from the software's trusted website using HTTPS of course , and using it to verify files downloaded from an untrusted mirror.

On Linux you can use the md5sum , sha1sum , shasum , etc utilities. How to check the checksum of a file in Windows. How to view the checksum of a file in Linux. Tip If you copy the fciv. Additional information How to protect yourself while on the Internet. See our checksum page for further information and related links. Microsoft Windows help and support. Drag the downloaded file from the Finder window into the Terminal window.

Press Enter and wait a few moments. The MD5 hash of the file is displayed in the Terminal. Open the checksum file provided on the Web page where you downloaded your file from. The file usually has a. For example: md5sum: 25dcc23b44c3bbd7a66c76d52af46 Compare the MD5 hash in the checksum file to the one displayed in the Terminal. If they are exactly the same, your file was downloaded successfully.