File Checksum Verifier

Drop a file in, get SHA-1, SHA-256, SHA-384 and SHA-512 back — and if you've got a published checksum to check against, paste it in and this tells you straight away whether it matches. Nothing gets uploaded; the file never leaves your browser, hashing happens locally through the Web Crypto API.

Why you'd actually bother checking a checksum

A checksum tells you two different things depending on context, and it's worth being clear on which one you're actually checking for. Comparing against a checksum published on the same site as the download mainly catches a corrupted or incomplete transfer — a download that got interrupted, a bad mirror, a filesystem issue. Comparing against a checksum published somewhere independent of the download source — a security advisory, a different trusted channel — is closer to actually verifying you got the real file and not a tampered one, though even that only holds if the independent source itself is trustworthy and wasn't compromised too.

What a mismatch is actually telling you

Any mismatch, even a single bit different, produces a completely different hash — that's the avalanche effect, and it's deliberate. A mismatch doesn't tell you what changed or how much, just that the file you have isn't byte-for-byte identical to whatever the published checksum was computed from. Don't assume it's benign (a bad download) or malicious (tampering) without more context — but don't run the file either way until you know which.

Why MD5 isn't offered here

MD5 has known, practical collision attacks — it's genuinely possible to construct two different files that hash to the same MD5 value, which defeats the entire point of using a checksum to verify integrity. Browsers' native Web Crypto API deliberately doesn't implement MD5 for exactly this reason. If you're downloading something that only publishes an MD5 checksum, that's worth noticing as a signal about how seriously integrity verification is being taken on that particular download — it's not necessarily a red flag on its own, but it's a datapoint.

SHA-256 is the one to reach for by default

SHA-1 has its own known weaknesses at this point and shouldn't be your first choice for anything new, even though it still shows up on older download pages. SHA-256 is the practical default across the industry — it's what most modern software distributors, package managers and container registries publish, and it's the one worth checking first if a page offers a choice.