PDF passwords

What are the two passwords of a PDF?

ISO 32000 defines two, and confusing them is the source of nearly every misunderstanding about PDF security.

User password, also called the open password

This is the one that encrypts. When it exists, the file content is encrypted and the decryption key derives from the password. No reader, no operating system and no tool can read the content without it. There is no shortcut and no remove button: whoever does not know the password stays out. This is the one that protects.

Owner password, also called the permissions password

This is the one that does not encrypt. It flags the file with restrictions (no printing, no copying text, no page extraction) and relies on the PDF reader's goodwill to honour them. The content stays readable in the clear. Any tool that chooses to ignore the restrictions reads and edits the file freely, and plenty do exactly that in seconds without asking for any password. It signals intent, it prevents nothing.

The four ciphers of the PDF format

When a user password exists, the cipher depends on the PDF version and the security handler. These are the combinations that exist:

RC4 40 bit is the worst case and still shows up, because it is the default in several libraries when nothing else is specified. A file encrypted that way is brute forced on an ordinary computer in minutes. If a file comes from an unknown source, check the document properties in your PDF reader, which display the security level.

What BestPDFInTheWorld does exactly

The protect tool sets the same password as both user password and owner password. That means it genuinely encrypts the content rather than merely flagging restrictions. High resolution printing is left allowed.

The cipher is AES-128 (AESV2, V=4, R=4), and that became guaranteed on 31 July 2026. Before that date there was a real problem worth stating: the library does not take the algorithm as an option, it picks it from the version declared in the header of the incoming PDF. A PDF with a 1.3 header, common in older scans, fell into the default branch and came out encrypted with RC4 40 bit. The fix forces the header to 1.7 before encrypting, which removes that branch.

We do not do AES-256. That level requires the header to declare the 1.7ext3 extension, and the library we use can only write versions in major.minor form, so that path is unreachable without changing library. If AES-256 is a requirement for you, use Acrobat or a tool that states it explicitly.

The password matters more than the cipher

AES-256 with the password "1234" is weaker than AES-128 with a long passphrase. The attacker does not attack the algorithm, they attack the password, at millions of guesses per second against known word lists.

• Use a phrase of four or five unrelated words rather than one word with symbols.
• Do not reuse a password from another account of yours.
• Do not send the password in the same email as the file. If the email is intercepted, both are.
• Store it in a password manager. An encrypted PDF whose password is lost is lost, and that is by design.

What PDF encryption does not solve

Encrypting a PDF protects the content at rest and in transit. It does not stop someone with the legitimate password from taking a screenshot, printing it or forwarding the decrypted document. Nothing in the PDF format prevents that, and any product promising otherwise is selling digital rights management, which is a different category with a licence server and online checks.

Sources

• ISO 32000-2, Portable Document Format
• Mapsoft, PDF security: passwords, permissions and encryption
• NIST SP 800-131A, transições de algoritmos criptográficos