![]() To decrypt symmetrically encrypted data, you need that same secret key. ![]() Symmetric encryption works by using a secret key (password) to encrypt some data. ![]() Symmetric encryption is the type of encryption most people are familiar with. Symmetric vs Asymmetric EncryptionĪs I mentioned before, there are two primary forms of encryption: symmetric and asymmetric. The hardest thing about encrypting data isn’t encryption, it’s key management. While you can easily Google “symmetric encryption best practices” and figure out the best algorithms and developer libraries to use (more on this later) to encrypt and decrypt data, one thing isn’t so easy: figuring out how to properly store and manage your data encryption keys. There are two primary forms of data encryption: symmetric and asymmetric. The original implementation of this system is available in ProtonMail's shared library.Encrypting data is all about making sure that only the right people can view the data you’ve encrypted. Now our key has been recomposed, both storage locations can be cleared as we don't want our horcruxes to be left around. If they closed the tab or the window, both parts will be erased by the browser (end of the session). If the user reloaded the page, both parts of the key will be preserved and reassembled on page load. 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. The key does not need to be saved in those locations at all times however.īecause window.name 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. 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.įortunately, other domains can't access our domain's sessionStorage, so all they would see in window.name is random data. Its value persists across page reloads, but is not written to disk. There is a name property on the global window object in the browser. One part is sent to sessionStorage, and the other uses a trick discovered by Thomas Frank named SessionVars. 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 To split the key, it is XORed with a buffer of random bytes. 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. 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. One thing to know however, is that most browsers will write the contents of sessionStorage to disk when reloading the page. If we want our key to survive page reloads, we need to use some form of storage. Some E2EE apps like Bitwarden do this for extra security. 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. This rules out localStorage, Indexed DB and cookies, but we could use sessionStorage, or simply keep it in memory only. For browser-based applications, keys usually last as long as the session. The first thing to define is the lifetime of the key. Let's have a look at what some E2EE apps do, by analyzing the Key to encrypt/decrypt something, it would be a terrible UX and it would lead to We can't reasonably ask the user to enter their credentials every time we need the Since the key never leaves the client, it needs to be stored there. Key derivation from master password using PBKDF2.
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