Decrypt Text Online

A string arrives, you paste it in, and a red line says decryption failed. You try the password again, more carefully, and it fails again. Most of the time the password was never the problem — so this page checks the data first and tells you what it found.

Why won't my encrypted text decrypt?

Four things go wrong, and only one of them is the password. Knowing which saves you from typing the same eleven characters twenty times.

The string may be truncated, because something between the sender and you had a length limit. Its characters may have been altered, because a mail client wrapped the lines or a phone keyboard capitalised something. It may have come from a different tool entirely. Or the data may be perfect and the password wrong.

The first three are structural, which means we can prove them before you type anything. Paste a string above and the verdict appears straight away, with no password and no waiting. When the data is sound we say so plainly, and then the password really is the only variable left.

How to decrypt a message someone sent you

Get the string in one piece first. That sounds obvious and it is the single biggest cause of failure we see.

Select from the very first character. Our output begins with OWK1, and a paste that starts partway through looks like valid base64 with no header — which we detect and call out, because it is such a common accident. If the sender used a chat app, ask them to send it as a file instead.

Then get the password from a different channel than the message. If both arrived in the same thread, the encryption did very little for either of you, and getting the key to the other end is worth reading before the next one.

What AES decrypt with a password is doing while you wait

The pause before the text appears is not the decryption. Decrypting a paragraph is instant; the wait is your password being turned into a key.

That step runs a hashing function hundreds of thousands of times on purpose. It costs you a fraction of a second once and it costs an attacker the same fraction on every single guess, which is what turns a short password from hopeless into merely risky.

The iteration count is stored inside the string rather than assumed, so files made with our older settings still open with those settings. It also means the pause is longer on newer files than old ones, which is the correct direction for it to move.

The failure we refuse to guess at

When the data is intact and the password is wrong, we say the password is wrong or the data was altered — and that we cannot tell which. That is not hedging, it is the actual limit.

AES-GCM checks the whole message against an authentication tag before releasing any of it. A wrong key produces a failed tag. A single flipped bit produces a failed tag. The two are indistinguishable by design, and a mode that let you tell them apart would leak information to someone probing your data. Why GCM beats CBC goes through what that tag buys you and what it costs.

In practice it is the password nearly every time. We still will not print that as a diagnosis, because the one occasion it is wrong is the occasion someone needed to know their file had been tampered with.

Opening files from the older vault format

Files that begin with STVAULT1.0 came from an earlier version of our own tool. They still open here, and we plan to keep it that way.

Two of them get a warning rather than a clean bill. If a file opens with no password at all, it was saved in a mode that used a fixed key published in the page's own code — it was never protected from anybody, and we say so instead of quietly handing over the text.

If the file carries an old recovery block, it holds the password encrypted under three security answers. That made the whole thing only as strong as a first pet's name. Decrypt it, re-encrypt with the encrypt text online page, and delete the original.

When there is nothing left to try

Sometimes the answer is that the text is gone. We would rather tell you in a minute than let you spend a week on it.

There is no master key, no copy on a server, and no support route that ends in recovery. The deliberate slowness of key derivation also means brute force is not realistic beyond a handful of remembered variations. When the password is simply gone sets out what is genuinely worth attempting and which services are selling hope.

Decrypt text online without guessing at the cause

Paste, read the verdict, then decide. If the string is broken you have learned something in two seconds; if it is intact you know the password is the only thing between you and the text.

For whole files rather than pasted strings, our encrypting a whole file page handles the same job with the original filename preserved. And if what you are protecting is a plain text document on your own machine, a plain .txt file on your own disk covers the operating-system routes and their sharp edges.