Always check and verify!
========================

Published  : 24 March 2025
Reading    : 2 min
Tags       : Security
URL        : https://ctrl-find.nl/posts/checksum/
Plain text : https://ctrl-find.nl/posts/checksum/index.txt

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### What is a checksum
A Checksum is a digital fingerprint of a file or code.
Software creators and/or companies are creating these digital fingerprints for safety reasons.
One of the most common usecases is checking if a downloaded file is changed by a third party.
if the checksum is different from the one that the creators provided, it most likely has been corrupted or tampered with.

We dont need external programs or a website (where we need to upload a file) to give us a checksum of a file.
It is fairly easy and shown down below in two categories: Windows and Linux(MacOS).

#### Windows
By clicking on the windows icon on the server (given it is in graphic mode otherwise your already in a terminal) or desktop.
Search for `Powershell` or `Command Prompt`, we do not need elevated priveleges (admin rights) on the machine.
Then we run the following command;

```powershell
certutil -hashfile <filename> [HashAlgorithm] 
```
The following `HashAgoriths` can be used: `MD2 MD4 MD5 SHA1 SHA256 SHA384 SHA512`

#### Linux
When on desktop, find the launcher and search for `terminal` or use the keyboard shortcut: **ctrl-alt-t**
On a server you are already on the systems terminal.

```bash
[HashAlgorithm]sum <filename>
```
The following `HashAgoriths` can be used: `MD5 SHA1 SHA256 SHA384 SHA512`

#### Example
This example works the same on linux with different commands.
i've created a test.txt file with some info text in it.LEts check the hash
```powershell
certutil -hashfile test.txt sha256
```
#### Output
```powershell
SHA256 hash of test.txt:
9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08
CertUtil: -hashfile command completed successfully.
```
When we change the text inside the test.txt file and run the command again, the hash changes:
```powershell
SHA256 hash of test.txt:
a5b40cf7147479b452c3b51feefc2f7cd0c926121033b2cfd09f1ae1c825ec30
CertUtil: -hashfile command completed successfully.
```
An overview of the changed sha256 checksums.
```powershell
Initial: 9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08
changed: a5b40cf7147479b452c3b51feefc2f7cd0c926121033b2cfd09f1ae1c825ec30
```


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