Potential Malware Notice / Security Disclaimer:
I cannot vouch for the safety of using the software provided by www.ilovepdf.com. As a general standard, never upload any documents with sensitive information to any site, not just this one. But specific to www.ilovepdf.com, there appears to be suspicion that their desktop application may contain malware. This report may or may not indicate any actual malware. But as with any software, use at your own risk.
Non-standard Encryptionless Password Protection
As @ja1024 pointed out, Adobe may offer some kind of non-standard encryptionless password protection. (That sounds like a really terrible idea to me though.. make it seem like it's safe, when it's totally not??)
Even still, there are a couple things to note here about the standard password-protected PDFs that are actually encrypted.
Use Strong Passwords
Before I go into the specification, please recognize that www.ilovepdf.com tries common passwords (might even use a rainbow table), and may also attempt to crack weak ciphers (e.g. RC4). If it can't figure it out, though, it will ask you to fill in the password.
You can test this yourself by encrypting any random PDF with a weak password, then a strong password, and trying www.ilovepdf.com.
Moral of the story: Use strong passwords!!
The Specification and Restricted PDFs
The PDF specification (ISO 32000-2, accessible here) covers encryption in chapter 7.6, and does clearly indicate that password protection requires encryption, and that encryption encrypts every string and stream in the PDF file, save for a few obvious exceptions (e.g., the strings dictating the type and configuration of the encryption used).
With that said, the encryption algorithm varies depending on PDF version and choice of encryption (The V key in the encryption dictionary, Ch. 7.6.2 Table 20). Most older PDF versions rely on RC4, which is known to be a weaker algorithm. Newer PDF versions, though, allow (and strongly encourage) using AES with either a 128 or 256 bit key.
But another thing I think some people are familiar with is view-only or restricted PDFs, i.e. a form of encrypted PDFs that prevent modifying, printing, etc. This comes from the fact that encrypted PDFs can have two passwords (Ch. 7.6.4.1):
- A User password (optional)
- An Owner password
Both passwords are used to encrypt a single key which is in turn used to encrypt/decrypt the PDF strings and streams. So, knowing either one of these passwords allows decrypting and reading the PDF. Encrypted files always have an owner password. But again, the user password is optional:
If the user password is set, the document cannot be opened or viewed without knowing one of the passwords. Again, the document is encrypted here. So, just opening it anyway is not an option.
But if the user password is not set, it is assigned to an empty value. When the PDF is opened, the reader tries a blank password first. If it succeeds, the PDF will open without asking for a password.
The purpose of the 'owner' password is to allow the creator of the PDF to apply various restrictions to it (Ch. 7.6.4.2 Table 22 lists the bit flags that control these permissions). If a user only knows the user password (or if it's blank), the editor they are using will normally apply these constraints. Whereas, someone opening it with the owner password will not be restricted. With that said, these constraints are solely at the discretion of the PDF viewer. While the specification indicates that all PDF viewers should conform, nothing would prevent a developer from deviating.
So, ultimately, these restrictions on encrypted PDFs are not impossible to bypass, as one would expect. And a tool like www.ilovepdf.com, given the password (or upon guessing it), will simply strip them out.
