Password Protect PDF

How the Password Protect PDF works

Payslips, contracts, medical letters and invoices with bank details travel by email all the time, and email is not a private channel. Messages sit in several mailboxes, get forwarded and are synced to phones. A password on the attachment means that anyone who gets hold of the file still cannot read it, and you can send the password separately by text or phone.

How to password protect PDF files

1. Drop the PDF into the box.
2. Type an open password. Anyone opening the file will need it.
3. Optionally set an owner password and choose what readers may do.
4. Run the tool and download the copy ending in -protected.pdf.

Open the downloaded file once to check that the password works before you send it.

What each setting does

The open password is real protection: without it the content is unreadable. Permissions are different. They are a flag that well-behaved readers such as Acrobat, Preview and browser viewers respect, but any tool that has the open password can ignore them. Treat them as a polite request that stops casual copying, not as security.

How it works

The tool opens the file with pdf-lib, an open-source PDF library, and encrypts it using the standard security handler from PDF 2.0 (revision 6), which every current reader supports.

A random 256-bit key is generated with your browser's secure random number generator. Every string and every stream in the document is encrypted with that key using AES in CBC mode, each with its own random starting block. The key itself is then stored twice in the file, wrapped once with a hash of the open password and once with a hash of the owner password. Each hash runs at least 64 rounds of AES and SHA-256, SHA-384 or SHA-512 over the password and a random salt, which makes guessing slow. The permissions are written into an encrypted block too, so they cannot be changed without the key.

Choosing a good password

Length beats complexity. Four or five random words, such as copper lantern yellow tide, are far harder to guess than a short mix of symbols and easier to read out over the phone.

Never reuse an account password. If you send the password alongside the file, assume both may be seen together one day.

Send the password another way. Put the file in the email and the password in a text message or a call. Sending both in the same thread defeats the point.

Tips and pitfalls

There is no recovery. If you forget the open password, nobody can open the file, including this site. Keep the original unprotected copy somewhere safe.

Set an owner password if you restrict permissions. When the owner box is empty a random owner password is used, so the restrictions can only be lifted by unlocking with the open password.

Already-protected files are refused. A file that already has a password or restrictions has to be unlocked first, then protected again with the new settings.

Metadata is encrypted too. The title, author and other document details are hidden until the file is opened. Use the PDF Metadata Remover first if you also want them gone from the opened copy.

Signed PDFs lose their signature. Encrypting rewrites every object, so a digital signature on the original no longer matches. Protect the file before it is signed, not after.

Reading the result

Encryption confirms AES 256-bit. Permissions repeats the option you chose. A note appears if you restricted permissions without setting an owner password, as a reminder to keep the original.

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