Encrypting PDF files

Introduction to the PDF encryption

As a kind of media conveying information, the PDF document need applying security to protect its important or privacy contents. Adobe has introduced several ways to ensure files' security, such as using digital signatures to indicate approval of a PDF document or form, certifying documents to disallow subsequent changes, adding passwords and setting security options to restrict opening, editing and printing PDF documents, and so on.  Encrypting PDF files is one of good and easy to use ways to protect the document's contents from unauthorized access so that only a specified set of users have access to it. This security is similar to lock your doors to prevent someone from entering your house without permission. For example, when you receive a restricted PDF document, you may need to enter a password to open it. If a document is encrypted, you may not be able to open it without permission from the person who created it. In addition, restricted or certified documents may prevent you from printing your files or copying information to another application. If you have trouble opening a PDF document, or if you're restricted from using certain features, contact the author of the PDF document.

Encrypting a PDF file can limit access to it by setting passwords and by restricting certain features, such as printing and editing.  A PDF document can have two kinds of passwords: a Document Open password and a Permissions password. When you set a Document Open password (also known as a user password), anyone who tries to open the PDF document must type in the password you specify. If you are restricting printing and editing, you should add a Document Open password to enhance security.

When you set a Permissions password (also known as a master password), only those people who have typed the Permissions password can change security settings. If the PDF document has both types of passwords, it can be opened with either password, but a user can set or change the restricted features only with the Permissions password. If the PDF document has only the Permissions password, or if the user opens the document using the Document Open password, the password prompt appears when the user tries to change security settings. Important: If you forget a password, there is no way to recover it from the document. Keep a backup copy of the document that is not password-protected.

Note: The source PDF must be an unencrypted PDF. Acrobat Reader will show a key in the bottom status bar if the PDF is encrypted.

Encryption applies to all strings and streams in the document's PDF file, but not to other object types such as integers and boolean values, which are used primarily to convey information about the document's structure rather than its content. Leaving these values unencrypted allows random access to the objects within a document, whereas encrypting the strings and streams protects the document's substantive contents.

General Encryption Algorithm

The following algorithms are used when encrypting data in a PDF file:

• A proprietary encryption algorithm known as RC4. RC4 is a symmetric stream cipher: the same algorithm is used for both encryption and decryption, and the algorithm does not change the length of the data.
   
• PDF’s standard encryption methods also make use of the MD5 message-digest algorithm for key generation purposes.

RC4 Algorithm Brief Summary

RC4 encryption algorithm is a shared key stream cipher algorithm, which requires a secure exchange of a shared key that is outside the specification. The algorithm is used identically for encryption and decryption as the data stream is simply XORed with the generated key sequence. The algorithm is serial as it requires successive exchanges of state entries based on the key sequence. Hence implementations can be very computationally intensive. This algorithm has been released to the public and is implemented by many programmers. This encryption algorithm is used by standards such as IEEE 802.11 within WEP (Wireless Encryption Protocol) using a 40 and 128-bit keys. Published procedures exist for cracking the security measures as implemented in WEP.

MD5 Encryption Brief Summary

MD5 is an algorithm that is used to verify data integrity through the creation of a 128-bit message digest from data input (which may be a message of any length) that is claimed to be as unique to that specific data as a fingerprint is to the specific individual.

MD5, which was developed by Professor Ronald L. Rivest of MIT, is intended for use with digital signature applications, which require that large files must be compressed by a secure method before being encrypted with a secret key, under a public key cryptosystem. MD5 is currently a standard, Internet Engineering Task Force (IETF) Request for Comments (RFC) 1321. According to the standard, it is "computationally infeasible" that any two messages that have been input to the MD5 algorithm could have as the output the same message digest, or that a false message could be created through apprehension of the message digest. MD5 is the third message digest algorithm created by Rivest. All three (the others are MD2 and MD4) have similar structures, but MD2 was optimized for 8-bit machines, in comparison with the two later formulas, which are optimized for 32-bit machines. The MD5 algorithm is an extension of MD4, which the critical review found to be fast, but possibly not absolutely secure. In comparison, MD5 is not quite as fast as the MD4 algorithm, but offers much more assurance of data security.

The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. It is conjectured that it is almost impossible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest. The MD5 algorithm is intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA.

How to encrypt your PDF document

We can use Encrypt PDF v2.3 software to realize the encryption function. Encrypt PDF software is a very flexible and powerful program. It allows you to encrypt existing PDFs by using standard 40-bit or 128-bit supported by Acrobat Reader 7.0 and up, set permissions, add user and owner password. For example you can encrypt a PDF without to allow to print it. The button to print the file will be disabled in Acrobat Reader application, you also can encrypt a PDF allowing the user to read it only if he know the correct password.

Double click the Encrypt PDF icon on your desktop or click start-> program-> Encrypt PDF v2.3 to open the software.  Then you can see a file list with several control icons below on the software panel.

File list

The file list records every PDF file's state, including encrypting result, events associated with the result,  source document and target document.

• Result: There are two results can be shown, SUCCEED and ERROR.
• Event: If the encryption action is successful, the event can be shown "File Encrypted", otherwise, if the file has been encrypted, so the word shows "This file has been encrypted before".
• Source document: records the source file name and its path.
• Target document: records the target file name and path.

Encrypting PDF documents

To encrypt a PDF document, you can set the encryption option first by click Setting icon, with the options dialog shows like below,

• Specify Password

To set the user password and master password according your need. With user password, you can only open the document, and master password ( also called owner password) to change permission and passwords.

• Encryption Level

Choose 40-bit or 128-bit according to the version of Acrobat software. 40 and 128 are the length of the encryption key in bits.

• Permission

To set the usage restriction to those who have user password.

• No Printing: You can read the document but not allowed to print it.
• No Changing the Document: You can read the file without permission to edit it.
• No Content Copying or Extraction: Not allowed to select a block of the file to copy to the clipboard.
• No Adding or Changing Form fields: Not allowed to modify forms in the file.

Note: You can set these options before you open more than one PDF documents, letting all the documents be opened with the same password. Or you can set different options to each document before you open every one of them. There is a key shape icon shown on the left bottom of the status bar when the encrypted file is opened, even if you don't choose any option and just open and save as a target file, the icon can still be seen.

Help

Click this button to get help from verypdf.com.

Close

To exit the software.

Register

To register, enter the series number or else you can only try the version in limited times.

The above introduced is GUI version. If you are familiar with the command line, you can also choose Encrypt PDF command line version.