🔐 Portable Secret

Better privacy, without special software.

TL;DR;

Portable Secret is a little hack that allows you to:

Send encrypted messages/images/files over insecure channels (email, messaging, ...)
Store sensitive information (passwords, documents) in insecure locations (web, Cloud drives, USB drives)

It's portable because: decrypting these secrets does not require special software! All you need is a browser.

I created Portable Secret to securely exchange documents via email with my mother, who can't be expected to learn PGP, age, or similar.

I also use Portable Secret to store some my most sensitive secrets (private keys, 2FA recovery codes, etc.)

Finally, I use it to store copies of important documents (like a picture of passport). These documents are accessible to me from anywhere, even if all my trusted devices have been stolen or lost.

Sounds too good to be true? Keep reading. This is for you.

Portable Secret is not a product and it is barely a project. It is just a neat trick, a hack.

The source code and creator tool are provided as a demonstration.

Update January 2023: Rocky W. took this idea and ran with it, creating a beautiful full-fledged product: PrivacyProtect.dev

How it works

A 'Portable Secret' is simply an HTML file that also contains:

An encrypted payload
Some Javascript that calls into the browser's Web Cryptography APIs

Any (reasonably modern) web browser can open the file, even without an internet connection! If you know the password, you can recover the secret within.

Here's an example (the password is banana)

Notice that the file is self-contained and has no external dependencies! It can be carried on a USB drive and decrypted without an internet connection, on any device that has a web browser.

To understand how it works, go ahead and create yourself a secret. Download and inspect the generated Portable Secret.

The embedded code is straightforward:

Take the password and generate a key
Use the key to decrypt the payload
Display the decrypted secret

How I use Portable Secret

Private communication

Do you want to communicate privately with people, but you can't expect them to learn how to use PGP?

Send a Portable Secret. For example as email attachment.

Hey ___, attached to this email is the PDF and data you requested, but it's encrypted. Never double-click on attachments, it's dangerous! Give me a call when you get this, and I'll show you how to read it.

Whenever they call me, I tell them:

Right-click on the attachment and 'Open with...' any browser. The password is 'banana_split'. Now you can save the decrypted PDF.

Store top-level secrets

Some secrets don't belong in your password manager. Things like backup private keys, 2FS recovery keys, wallet keys, safe combinations, treasure maps, etc.

Using Portable Secret, I can keep copies of these critical keys all over the place (Cloud drives, USB drives, all my devices, etc).

Even if some of these copies end up stolen (e.g. I lose a USB stick), I am not concerned anyone will be able to recover the secrets within.

The passwords are long sequence of words that are trivial for me to remember (thanks to the hints provided), but impossible for anyone else to guess or crack.

Emergency documents on the go

Have you ever gotten stranded in a foreign country without any of your devices or documents? It's not fun.

I keep a copy of my passport encrypted on the internet. It's just an HTML file, it's easy to host. If I find myself stranded again, I can use any computer/device to retrieve it.

Here's an example of ID document safely encrypted in plain sight.

Cryptography

The following is a schematic representation of the encryption scheme implemented by the PortableSecret creator, as well as the decryption happening in the PortableSecret itself.

This scheme and its parameters follow best practices and guidelines recommended by NIST and OWASP.

Encryption

AES-GCM is used for Symmetric Authenticated Encryption.

Symmetric because the same key is used to encrypt and decrypt
Authenticated because it can verify the message integrity (if the ciphertext is tampered with, then it will fail to decrypt. As opposed to producing an invalid plaintext)

Key derivation

PBKDF2 is used to turn a text password into an AES key. The purpose of this module is slowing down dictionary-based brute-force attacks.

Unfortunately, the Web Crypto APIs do not support stronger KDFs such as scrypt, bcrypt, or Argon2.

Bounty: Crack me if you can

Do you think this cannot possibly be secure? Great, prove it.

This secret contains the recovery key for a Bitcoin wallet. Crack it and take my money!

Miscellaneous

Choosing a good password

Choosing a strong-enough password is key (pun intended).

Eventually I'll fill in this paragraph. For now all you get is the obligatory XKCD: correct-horse-battery-staple

On tools

Portable Secret is a tool. As such, it can be used wrong (e.g. weak password), or used to do bad things (e.g., exfiltrate intellectual property).

I cannot take responsibility for such misuse any more than a hammer manufacturer can take responsibility for me hammering my thumb, or using the hammer to attack someone.