Hashing a File: Does it Change It?

G5 Cyber Security

1 week ago

TL;DR

No, hashing a file does not alter the original file itself. Hashing creates a unique ‘fingerprint’ of the file’s contents. The original file remains unchanged.

What is Hashing?

Hashing takes any amount of data (like a file) and turns it into a fixed-size string of characters, called a hash or digest. Think of it like a blender: you can put anything in, but the result will always be the same size smoothie.

Why Use Hashing?

Verification: Check if a file has been changed during download or transfer.
Password Storage: Store passwords securely (never store them in plain text!).
Data Integrity: Ensure data hasn’t been tampered with.

Does Hashing Modify the File?

Absolutely not! The hashing process is one-way. It calculates a value *based on* the file, but doesn’t change the file itself.

How to Hash a File (Examples)

Using Command Line (Linux/macOS):

MD5:
```
md5sum filename.txt
```
SHA-256:
```
sha256sum filename.txt
```

Using PowerShell (Windows):

GetFileHash:

GetFileHash -Algorithm SHA256 filename.txt | Format-List

Online Hash Calculators: Many websites allow you to upload a file and calculate its hash. Be cautious about uploading sensitive files to unknown sites!

Example Scenario

Let’s say you download a software program.

The website provides the SHA-256 hash of the file (e.g., a1b2c3d4...).
You download the file to your computer.
You use sha256sum or GetFileHash to calculate the SHA-256 hash of the downloaded file.
If the calculated hash matches the one on the website, it confirms that the file hasn’t been altered during download. If they don’t match, something went wrong (e.g., corrupted download) and you should not use the file.

Important Considerations

Hash Algorithms: Different algorithms (MD5, SHA-1, SHA-256, etc.) produce different hashes for the same file. Use the algorithm specified by the source you’re verifying against.
Collisions: While rare, it’s theoretically possible for two different files to produce the same hash (a collision). This is why stronger algorithms like SHA-256 are preferred.