Secure Checked before download

Password-protect a PDF, and see it proved

The file is really encrypted — AES-128, the standard security handler at revision 4, which every PDF reader made since 2005 can open. Then it is re-opened from its own bytes: refused without your password, opened with it, and searched for page content left in the clear.

On this device

Choose a password

The password never leaves this device, and there is no way to recover the file without it — not by us, not by anyone.

    Opened on this device · never transmitted

    What this tool does, exactly

    What it is
    AES-128 in CBC mode, the standard security handler at revision 4 — the “Acrobat 7 and later” encryption every PDF reader made since 2005 can open. The file is genuinely encrypted: its text does not appear in the bytes, and a reader refuses it until the password is supplied.
    The one thing to understand
    The strength of the encryption is the strength of the password. Revision 4 derives its key with 50 rounds of MD5, which a laptop runs millions of times a second, so a short or obvious password can be found by running a guess list against the file. That is why an empty password, anything under six characters and short runs of digits are refused outright.
    What it is not
    AES-256. The library used here can also write revision 5, but revision 5 is Adobe's withdrawn Extension Level 3, whose password check is a single SHA-256 — faster to attack than what ships here. The revision 6 that PDF 2.0 standardised is not implemented by it at all. If you need AES-256, use a desktop tool; this one does not have it and will not pretend to.
    What it proves
    Before you are offered anything, the finished file is re-opened from its own bytes: it must declare encryption, it must be refused with no password, it must open with yours, it must still have every page, and a run of the original page content must be absent from the file. If any of the five fails, there is no download.
    What it cannot do
    Recover the password. Nothing about the file or the password leaves your device, so there is no reset, no recovery and nobody to ask. Keep the password somewhere before you close this tab.

    About this tool

    Is the file really encrypted, or just marked read-only?

    Really encrypted. The page content, the strings and the metadata are all encrypted with AES-128 under the standard security handler at revision 4, and the file cannot be opened without the password by any reader. You can watch the tool prove it: it re-opens the finished bytes with no password, which must fail, and then with your password, which must succeed.

    Is this AES-256?

    No, and we will not imply otherwise. It is AES-128 at revision 4. The library used here can also write revision 5, but revision 5 is Adobe's withdrawn Extension Level 3 whose password check is a single SHA-256 — weaker in practice than what ships here. The AES-256 that PDF 2.0 actually standardised, revision 6, is not implemented by it. If AES-256 is your requirement, this tool is not the right one.

    Why does it refuse my password?

    Because the encryption is only as strong as the password. Revision 4's key derivation is 50 rounds of MD5, which is cheap to run over a guess list, so a short password is the whole attack. Empty passwords, anything under six characters and short runs of digits are refused; a passphrase of several words is far stronger than a clever short one.

    What happens if I forget the password?

    The file is gone. There is no recovery, no back door and no copy of the password anywhere — it exists only in this tab while you type it. That is what makes the protection real, and it is also why you should write the password down before you close the page.

    What do the permission settings do?

    They record what a reader is allowed to do once the file is open: copying text, printing, changing it. They are enforced by the reader rather than by mathematics, so treat them as an instruction to well-behaved software, not as a guarantee. The password is the part that cannot be argued with.

    Can I protect a file that already has a password?

    No. An encrypted PDF cannot be read without its password, so there is nothing here to re-encrypt. Open it in a reader, save an unlocked copy, and protect that.

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