How To Store End-to-End Encryption Keys In The Browser

François Best • 13 December 2019 • 4 min read

End-to-end encrypted apps (E2EE) use cryptographic keys that are generated in the client, and never sent to the server in clear-text. This is what makes the strength of this architecture: the server never has the key.

Keys are usually generated from credentials provided by the user, such as a username and a password, which are derived into a strong cryptographic key using key-derivation functions:

Key derivation from master password using PBKDF2

Key derivation from master password using PBKDF2.

Since the key never leaves the client, it needs to be stored there.

We can't reasonably ask the user to enter their credentials every time we need the key to encrypt/decrypt something, it would be a terrible UX and it would lead to users picking weaker passwords.

Let's have a look at what some E2EE apps do, by analyzing the ProtonMail approach.

Key Lifetime#

The first thing to define is the lifetime of the key. For browser-based applications, keys usually last as long as the session.

This rules out localStorage, Indexed DB and cookies, but we could use sessionStorage, or simply keep it in memory only.

Persistence & Page Reloads#

Keeping the key in memory has a serious downside: if your user ever reloads the page, the key is gone, and you would have to show a login screen again. Some E2EE apps like Bitwarden do this for extra security.

If we want our key to survive page reloads, we need to use some form of storage.

One thing to know however, is that most browsers will write the contents of sessionStorage to disk when reloading the page.

This is an issue as we don't want the key to leak, and any write to the filesystem places it outside of our control.

Divide to Conquer#

The approach taken by ProtonMail is to split the key into two parts, store each part using different techniques, and recompose the key on page load.

To split the key, it is XORed with a buffer of random bytes. A copy of the original random data is going to be the other part, so that both of them individually are random, but by XORing them together, the randomness cancels out and reveals the key:

# Split
    a = key ^ random
    b = random

# Recompose
    a ^ b
 => (key ^ random) ^ random
 => key ^ (random ^ random)
 => key ^ 0
 => key

One part is sent to sessionStorage, and the other uses a trick discovered by Thomas Frank named SessionVars.

There is a name property on the global window object in the browser. Its value persists across page reloads, but is not written to disk.

It has been used for cross-domain communications, and because other domains can see its value, we can't send anything there in clear text.

Fortunately, other domains can't access our domain's sessionStorage, so all they would see in is random data.

The Right Amount of Persistence#

The key does not need to be saved in those locations at all times however.

Because is writable by everyone, it would be easy for attackers to erase the key if it was stored there as a single source of truth.

Instead, we can keep the key in memory, and only persist the key to those shared locations when the memory will be destroyed: on page unloads.

If the user reloaded the page, both parts of the key will be preserved and reassembled on page load. If they closed the tab or the window, both parts will be erased by the browser (end of the session).

Cleaning up#

Now our key has been recomposed, both storage locations can be cleared as we don't want our horcruxes to be left around.

The original implementation of this system is available in ProtonMail's shared library.

Introducing session-keystore#

For all to use this key storage technique without depending on ProtonMail's internal library, I built session-keystore, a TypeScript implementation with a few extra features:

François Best

Freelance developer & founder

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