What is an MD5 hash?

What MD5 produces

Run a file through MD5 and you get a string like d41d8cd98f00b204e9800998ecf8427e. That is 128 bits written as 32 hex characters. The same file always produces the same string, and changing a single byte changes the whole thing. The length is fixed no matter how large the file is.

What it is good at

MD5 is fast, so it is a practical choice when you need to hash a lot of files quickly:

• Finding duplicates. Group files by MD5 and identical files land together, whatever they are named.
• Quick integrity checks. Confirm a file copied without corruption by comparing before and after.
• Deduplicating backups. Skip storing a file you already have a copy of.

For all of these, the concern is accidental difference, not deliberate tampering, and MD5 handles that well.

Why it is not secure

MD5 is broken for security. Researchers have shown it is practical to create two different files that produce the same MD5 on purpose, which is called a collision. That means MD5 cannot be trusted to prove a file has not been swapped by someone acting in bad faith. For downloads that must resist tampering, or for anything cryptographic, SHA-256 is the right tool.

The practical takeaway

Use MD5 when you are checking your own files against accidental duplication or corruption, where speed matters and nobody is trying to fool you. Switch to SHA-256 when the check has to hold up against someone deliberately faking a match. Many file tools compute both, so you can choose based on the job.