How do I check a PDF's encryption algorithm?

With a tool like qpdf --show-encryption, pdftk, or a technical library, inspect the /Encrypt dictionary. The /V and /R entries give the encryption version, /Length the key size, and /CF/StdCF/CFM shows /AESV2 for AES-128 or /AESV3 for AES-256.

Why do some tools advertise AES-256 on PDF 1.7?

Most often as a marketing simplification: an implementation that follows the document version produces AES-128 (revision 4) on a standard PDF 1.7. Real AES-256 requires forcing revision 5 (1.7 Extension Level 3 profile) — which is what Document Flare does, verifiable in the result's /Encrypt dictionary.

Does password encryption prevent copying the text?

Not automatically. In Document Flare's current implementation, the password only controls opening the file; copy and print permissions are not restricted. To make an area unrecoverable, you must redact it before encrypting.

Privacy

PDF Passwords: What Encryption Really Does (AES, RC4 and the Honest Limits)

AES-256, AES-128 or RC4? How to check a protected PDF's real algorithm, why password encryption doesn't do everything, and why Document Flare always produces AES-256.

Published on September 23, 20265 min readby Hamza El Hajjaji

This page contains affiliate links: we may earn a commission if you use them, at no extra cost to you. This does not influence our recommendations. As an Amazon Associate, we earn from qualifying purchases.

Table of contents

A PDF password is not a single security level

Encrypting a PDF with a password locks access to the file — it doesn’t necessarily harden the content. Yet many tools display “AES-256” as if it were a universal label. Reality is more nuanced: with default implementations, the source document’s version can determine the resulting algorithm — RC4 40-bit, RC4 128-bit or AES-128. We document that gradation — because a misidentified encryption gives false assurance — and explain why Document Flare systematically forces the strongest revision.

YubiKey 5 NFC — FIDO2 security keyHardware two-factor authentication: phishing-proof sign-in for sensitive accounts.affiliate

Why the AES-256 advertised everywhere is often wrong

The PDF standard does not define one encryption. It defines several revisions:

revision 2: RC4 with a 40-bit key (PDF 1.3 and earlier);
revision 3: RC4 with a 128-bit key (PDF 1.4 / 1.5);
revision 4: AES with a 128-bit key in CBC mode (PDF 1.6 / 1.7);
revision 5: AES with a 256-bit key, reserved for PDF 1.7 Extension Level 3.

A classic PDF 1.7 file — the most common case today — will therefore be encrypted as AES-128 by an implementation that follows the document’s declared version, not AES-256. When a tool advertises “AES-256” without explicitly forcing revision 5, it is often simplifying its marketing.

How to verify what a tool really does

PDF stores its encryption parameters in an /Encrypt dictionary. You can inspect it with technical tools:

qpdf --show-encryption file.pdf displays the algorithm, key size and permissions;
pdftk file.pdf dump_data gives indications about the protection;
advanced PDF readers or Python libraries (pikepdf, PyPDF2) expose the /V, /R, /Length and /CF entries.

What to look for:

/V = 1 and /R = 2 → RC4 40-bit;
/V = 2 and /R = 3 → RC4 128-bit;
/V = 4 and /R = 4, with /CF/StdCF/CFM = /AESV2 and /Length = 128 → AES-128;
/V = 5 and /R = 5, with /AESV3 and /Length = 256 → AES-256.

If a tool promises AES-256 without forcing revision 5, check the result’s /Encrypt dictionary — the label may be marketing only.

The real level in Document Flare: forced AES-256

Document Flare uses the @cantoo/pdf-lib library to encrypt and decrypt in the browser. By default, this library picks the algorithm according to the source PDF’s declared version — with the gradation described above. Document Flare does not stick to that default: when writing, the tool declares the PDF 1.7 Extension Level 3 profile, which systematically selects revision 5 — AES-256 (AESV3).

Concretely: even if your source document is an old PDF 1.4, the protected file that comes out is AES-256 encrypted. You can verify it yourself with qpdf --show-encryption: you will read /V = 5, /R = 5, /CFM = /AESV3, /Length = 256.

What the password protects — and doesn’t

Password encryption protects opening the file. Without the password, the content stays unreadable — in theory. In practice, resistance depends heavily on the password itself: “123456” or the company name fall in seconds, even with AES-128.

What the tool does not do, as it stands:

it does not restrict copy / print permissions; the entered password serves as both user password and owner password;
it does not encrypt the document metadata separately;
it is not an electronic signature: it only proves that whoever holds the password can open the file.

If you need to make information unrecoverable, password encryption is not enough: you must redact the sensitive area before protecting the file.

Limits and pitfalls

Weak password = weak protection. Prefer a long passphrase, even one that is easy to remember.
RC4 40-bit should raise alarm. If you receive a protected PDF dating from before PDF 1.4, its protection is now considered obsolete; re-encrypt it as AES-256 before relying on it.
An already-encrypted PDF cannot be re-encrypted. The tool rejects protected PDFs; unlock them first with the password in the Unlock PDF tool.
Size and pagination are capped. The tool accepts up to 50 MB and 1,000 pages.
Short passwords are flagged. The interface shows a warning if the password is under 6 characters.
Forgetting the password means losing the document. No backdoor is kept; processing is entirely local.

Honesty as differentiation

Many tools ride on the AES-256 label. We prefer to state precisely which algorithm is applied and under what conditions. An informed user chooses better: they know when to add a redaction step, when to avoid RC4, and when to send the password through a separate channel.

If you want to protect a PDF without sending it to a server: