Personal key rotation 3: tools for key management

In this article, I present two tools for managing secrets. One is essential. The other is geeky. Both strive for being maximally simple and easy to understand.

This is part #3 in a four-part series on managing my digital life. The series starts with part #1 here. Without further ado: here are pa and psst.

pa: the simplest password safe that I know of

pa is a password safe. It stores passwords in encrypted form on my computer. It lives at https://github.com/biox/pa/.

Interlude: A password safe is an absolutely essential tool. Many accounts still require passwords. The alternative to a password safe is either impossible (remembering all the passwords) or insecure (using the same password everywhere). Moreover, a safe does more than just storing passwords. It also serves as a directory of all my accounts and assets.

A few features make pa nice to use:

• It is integrated with git, so that I can sync my passwords between computers, and back them up easily.
• It has a text interface with tab-completion. pa show is the command to retrieve a password. Tab completion means that to get my Amazon password, instead of typing the full command pa show internet/amazon, I can type pa s in<tab>am<tab>.
• It can encrypt passwords for multiple keys (more on this later).

What makes pa stand out is its simplicity. The entire password manager fits in 300 lines of code (about half of these are comments). The code is carefully written. It is one of the few tools that I depend on, for which I have fully read and understood the source code.

Not included in the 300 lines is pa’s only dependency: age, the tool that performs the actual encryption and decryption of passwords. age is a modern encryption tool that does just that: encrypt and decrypt files. If you know PGP or GPG, think of age as a successor to these tools, built with 30 years of hindsight and experience. age has an easy-to-use interface, supports a single, modern set of encryption standards, and avoids most of the pitfalls that other tools have.

A key feature of the pa and age combination is this: passwords can be encrypted for multiple keys. The encryption needs only the public part of the keys. This means I can add passwords without needing access to any of the secret keys. I keep multiple backups of the password store. This is safe because encryption protects the passwords from prying eyes. That said, I would recommend against making the backups public, since the metadata (the list of all my accounts) is plainly visible.

The secret keys are needed to decrypt and show passwords. In my setup, there are two secret keys. One is stored on my laptop. The other is a backup key with a passphrase, intended for situations where I lose my laptop. I have multiple paper copies of this key, protected using psst.

psst: secret shares made with just pen and paper

psst is a system to make secure paper backups of small secrets. For example, the passphrase for my age key is an excellent use case for psst. psst can split a secret into up to four parts, called “shares”. A single share reveals nothing about the secret. Any two shares can reconstruct it. If I store the shares in separate places, this system provides high security against attackers. It also has high availability, because I can reconstruct my secret even if some of the paper copies are lost.

I made psst to reach new points in the security/availability trade-off. I wanted to make something that is as simple as possible, where I can understand every step.

psst is an implementation of Shamir’s Secret Sharing System. I chose the parameters in a way that leads to a simple, compact implementation. Everything fits on one sheet of paper. At the same time, psst is powerful enough to handle passwords, pass phrases, cryptocurrency seeds and all other types of small secrets.

All that is needed to use psst is a printer, a pen, and a good six-sided dice. A user first print out the psst worksheet. Then, they follow the instructions to encode their secret shares. Finally, the user distributes the shares to separate locations, and destroys the original secret. By seeing the system written step-by-step on paper, users can effortlessly understand the math behind Shamir’s Secret Sharing. It is clear why two shares are necessary to reconstruct the secret, and a single share doesn’t leak any information.

To see this, go print out a copy of the psst worksheet. This is the relevant snippet from the worksheet, showing how a single digit of the secret is split into shares:

• 

• When using `psst`, the secret is first encoded as a set of digits from 0 to 4. Each digit is then split in four shares in a random way, with the help of a dice throw. The snippet shows part of the conversion table: what happens for the digits 0 and 1 for each of the six possible dice faces.

  It is clear that a single share does not reveal the secret: each share can take any value, depending on how the dice falls. Since an attacker does not know what the dice showed, they have no information.

  It is also clear how two shares combined can reconstruct the secret. Each combination of two digits appears exactly once, and uniquely identifies the secret digit.

If you are intrigued, I recommend perusing the psst documentation. It explains in detail how to use psst, and how I made it. There is an in-depth discussion of the security and availability trade-off, and also of potential pitfalls.

Conclusion and outlook

This post presented two tools that can be part of a framework for managing my online stuff. We now have gathered everything needed for a personal key rotation:

1. An overview of the accounts and assets that we want to protect (part #1 of this series)
2. Decisions for how to protect each asset, to gain adequate security and availability (part #2 of this series)
3. The tools we need to achieve that protection (this part)

In the next part, we will finally perform the actual key rotation, and end up with a new set of keys and passwords for a well-protected digital life. Join us on September 1 for part #4 👋🏼