What this covers
- There is no recovery of any kind - write it down first
- Length beats character variety, by a large margin
- Four unrelated words beat eight mixed characters
- Send the password by a different route from the file
Encrypting a PDF is the easy part. The decision that matters is the password, and it is made under a constraint people consistently underestimate: if it is lost, the document is gone permanently.
Write it down before you encrypt, not after
Not afterwards, when you have moved on. Before. The number of permanently unreadable documents created by someone intending to remember a password is not small, and every one of them was preventable by thirty seconds of writing.
Why there is no recovery
AES-256 derives a key from the password. Without the password there is no key, and without the key the content streams are indistinguishable from random data. There is nothing to attack except guessing the password, and guessing a reasonable one is not feasible.
This is not a policy decision by any tool. It is a property of the mathematics, and any service offering to recover a password from a properly encrypted PDF is either operating on files that were only permission-flagged or is not telling the truth.
Length beats complexity
The strength of a password is roughly the number of possibilities an attacker has to work through, and that grows exponentially with length and only linearly with the size of the character set.
Eight characters drawn from 94 printable ASCII characters gives about 52 bits if chosen truly at random - and people do not choose at random, so the real figure is much lower. Four unrelated common words drawn from a 7,000-word vocabulary gives about 50 bits with no cleverness required, and you can actually remember it.
Roughly how strong
Assuming each is genuinely chosen at random, which is the part people get wrong.
| Password | Search space | Memorable? |
|---|---|---|
| 8 random printable characters | About 52 bits | No - it will be written on a note or forgotten |
| A word with digits substituted | Far less than it looks | Yes, and it is the first thing guessed |
| 3 unrelated words | About 38 bits | Yes |
| 4 unrelated words | About 50 bits | Yes |
| 5 unrelated words | About 63 bits | Yes, with a little effort |
Substituting 3 for e and 1 for i adds almost nothing. Every guessing tool applies those substitutions first.
Transmitting it
A document encrypted and then emailed with its password in the same message is not protected against anything, because whatever could read the message could read both parts.
Send the file one way and the password another: the file by email, the password by text message or spoken aloud. This is not paranoia. It is the entire mechanism by which encryption provides any benefit in transit, and skipping it makes the encryption decorative.
Doing this properly
Three minutes.
- Choose four unrelated words
By a method that is not your own preference. Write them down now.
- Store the note somewhere you will find it
A password manager, or a physical note somewhere you already keep important things. Not in the same folder as the document.
- Encrypt
AES-256, which is what Protect PDF uses. Set permissions separately and do not mistake them for protection.
- Verify you can open it
Close everything, reopen the encrypted file, type the password. Do this before you delete the unencrypted original.
- Send the password separately
A different channel from the file. Always.
Encrypting and decrypting
When not to encrypt at all
If a document is going to one person who will open it once, and the risk is casual rather than targeted, encryption often creates more problems than it solves - a password to transmit, a password to store, and a document that becomes unreadable if either goes wrong.
Consider whether the actual requirement is confidentiality or just not-in-public. Sending a link that expires, or simply not attaching the document to a thread that will be forwarded, solves some problems better than encryption does.
Before you encrypt anything
- Password chosen as four or more unrelated words
- Chosen by a method other than your own judgement
- Written down somewhere you will find it, before encrypting
- Stored separately from the document
- Encrypted file opened once, successfully, before deleting the original
- Password sent to any recipient by a different route from the file
Questions and answers
What happens if I forget the password?
The document is unreadable, permanently. There is no recovery by this site or any other. AES-256 with a reasonable password is not opened by guessing.
Is a longer password really better than a complicated one?
Yes, by a large margin. Search space grows exponentially with length and only linearly with character variety, and the substitutions people make to look complicated are the first things a guessing tool tries.
Can I email the password with the file?
You can, and it defeats the purpose entirely - anything that could read the message could read both parts. Send them by different channels.
Should I encrypt my revision notes?
Almost certainly not. The risk is low and the cost of a lost password is losing your notes. Encrypt things whose exposure would actually harm someone.
