Hollie's Hub for Good Supporting each other to make an impact. Write for DigitalOcean You get paid, we donate to tech non-profits. Have you ever wondered what that checksum link was for? That checksum is used to verify the integrity of the file you just downloaded.
On February 20th, , the website for Linux Mint, a popular Linux distribution, was hacked and the ISO used to install the distro was compromised. Before the compromised ISO was discovered, many people downloaded and possibly installed a version of Linux Mint with a backdoor baked in.
This dangerous install could have been avoided at the user level if the individuals who downloaded the altered ISO performed a file verification to see if what they downloaded had the same checksum as the original file.
File verification, also known as hashing , is the process of checking that a file you have on your machine is identical to the source file.. When you hash a file, you are left with a checksum , a random alpha numeric string with a set length.
The process to generate a checksum is often called a one-way cryptographic function. When the file is hashed using the MD5 algorithm, the resulting checksum will be 32 random characters. If you were to hash a character long file, the resulting MD5 checksum is still 32 characters. Even if the source file was only 10 characters long, the MD5 checksum would still be 32 random characters.
But if even one thing is different, like an extra space in the file, the checksum will be completely different. The version is identified as either a number like 1, 2, 3, or by the number of times SHA is run in succession, such as , , or The checksum you use should specify which version of SHA to use.
For the purposes of file verification, both methods are equally valid. Though the algorithm is different, both will return a random string with a set length, although MD5 hashes are shorter than any of the SHA hashes.
Those steps are beyond the scope of this tutorial. Checking the hash on downloaded files provides two different assurances that are both worthwhile. Both of these cases are important since, if either were to happen, the download you have could be harmful to your machine or may not work at all.
Most Linux distributions have command line tools for each hashing algorithm. 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. Connor J's answer gives examples for Windows. Unlike checksums or hashes, a signature involves a secret. This is important, because while the hash for a file can be calculated by anyone, a signature can only be calculated by someone who has the secret.
Signatures use asymmetric cryptography, so there is a public key and a private key. A signature created with the private key can be verified by the public key, but the public key can't be used to create signatures. This way if I sign something with my key, you can know for sure it was me. Of course, now the problem is how to make sure you use the right public key to verify the signature. Key distribution is a difficult problem, and in some cases you're right back where you were with hashes, you still have to get it from a separate trusted source.
But as this answer explains, you may not even need to worry about it. If you're installing software through a package manager or using signed executables, signature verification is probably automatically handled for you using preinstalled public keys i. If you use shasum filename you have to compare the sums yourself which is hard, unreliable and slow.
Solution: Instead, you can create a simple function in your. Please find more details here. Unless you ran that command in a directory that doesn't contain the target of the shasum, in which case you'll get:. Do not use the MD5 algorithm for security related purposes. Instead, use an SHA-2 algorithm, implemented in the programs shasum 1 , shasum 1 , shasum 1 , shasum 1 , or the BLAKE2 algorithm, implemented in b2sum 1.
They all have the same options, with the exception of b2sum which has an extra --length option. If the diff prints out anything at all, those are NOT the droids you're looking for. Otherwise, you're good! Yes No. Never Miss Out Receive updates of our latest tutorials.
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