Lock PDF
Lock and password protect PDF files securely in your browser. Add 128-bit encryption, restrict printing, and batch lock multiple PDFs. 100% private & offline.
Lock PDF Files with Password
Choose PDF files or drag & drop anywhere to add 128-bit encryption and reader restrictions. 100% client-side, your files never leave your device.
or drag & drop PDFs anywhere
Security Deep Dive: Encryption Techniques & Protection Guarantees
This tool applies the international ISO 32000-1 (PDF Standard Security Handler Revision 3) standard. The encryption key is derived using a 50-round iterated MD5 hashing protocol over the user password, permissions bitmask, and unique document ID. All internal binary streams and text objects are encrypted with 128-bit symmetric stream ciphers using distinct per-object keys.
Document Open Passwords are mathematically unbreakable without the key. However, short or common passwords (e.g. 123456) are vulnerable to dictionary brute-force attacks. Using a strong password with 8+ mixed characters prevents brute-forcing. Permissions restrictions (e.g. print blocking) are honored by standard PDF viewers, but can be bypassed by third-party software if the user already has the Open password.
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Frequently Asked Questions
This tool uses the industry-standard PDF Standard Security Handler (Revision 3 per ISO 32000-1 specification). The encryption key is derived using 50-round iterated MD5 hashing over the user password, permissions bitmask, and document ID. The document's internal binary streams and text objects are encrypted with 128-bit symmetric stream ciphers using unique per-object keys.
Standard 128-bit PDF encryption is cryptographically secure and cannot be mathematically reversed without the password. However, short or predictable passwords (like '123456' or dictionary words) can be discovered by offline brute-force attacks. Using a strong password with 8+ mixed characters (uppercase, lowercase, numbers, and symbols) makes the lock practically impossible to break. Note that while Open Passwords strictly protect document contents, Permissions restrictions (like print blocking) are honored by standard PDF readers but can be bypassed if the user already possesses the Open password.
Yes! You can select or drag and drop up to 20 PDF files at once. The tool will lock all files in a single batch using your chosen password and permissions. Once finished, you can download all locked PDFs together as a convenient ZIP archive or download each protected file individually.
No. All encryption and file locking occur 100% locally in your web browser using client-side Web Workers. Your files, passwords, and sensitive information never leave your computer or phone.
An Open (User) Password prevents unauthorized users from opening or reading the document by encrypting the underlying data streams. Permissions (Owner restrictions) allow you to specify whether authorized readers are allowed to print, copy text, or modify annotations in standard PDF readers.
Because encryption happens locally on your device and we do not store your passwords on any server, forgotten passwords cannot be recovered by anyone. Make sure to remember or store your password safely in a password manager.
Yes! Once this web page is loaded, the service worker caches all required scripts so you can lock, encrypt, and batch process PDF files completely offline.
No. Encryption only adds cryptographic protection to the document structure and streams. All vector fonts, high-resolution images, links, and layout formatting remain 100% lossless and identical to the original.
Yes, Lock PDF is completely free with no usage limits, no forced account signups, and zero added watermarks.