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+---
+title: Changing the cryptography library in your CryptPad server
+date: 2024-10-03
+author: "Fabrice Mouhartem"
+cover: /images/salt-mine-turda.jpg
+summary: A tutorial to explain why and how to change the backend cryptography library on your cryptpad server
+tags:
+- tutorials
+- administration
+- configuration
+- performance
+- security
+---
+
+<figure>
+
+![Cover image: Salt mine of Turda, Romania.](/images/salt-mine-turda.jpg "Cover Image: Salt mine of Turda")
+<figcaption>
+Salt mine of Turda, Romania. From <a href="https://commons.wikimedia.org/wiki/File:Roumanie_Mine_de_sel_de_Turda_2019_5.jpg">Wikimedia Commons</a>
+under CC0 license.
+</figcaption>
+</figure>
+
+⚠️ This tutorial is intended for system administrators. However, the introduction
+can be of interest to anyone who is curious about the inner working of CryptPad.
+
+## 🧑‍🏫 Introduction
+
+When running CryptPad on your server, you may notice that under load, some
+CryptPad processes are consuming a lot of computational resources. So far, there
+is nothing surprising. However, doing a bit of profiling about these
+computations, we noticed that the heavy lifting was done by signature
+verifications. The reason being that the server proceeds with data validation
+before storage to avoid being spammed by false patches. Even if they won’t
+affect the final state of the file as they won’t be displayed with an invalid
+signature, they still need to be processed by the clients, potentially allowing
+to _denial of service_ attacks.
+
+Well, that’s good, but it doesn’t help us with our load issue.
+However, looking at how CryptPad is designed, we remark that both the server and
+the client are written in javascript.
+This allows us to share common libraries between the server and the client·
+Thus it guarantees the same behaviour for both for fundamental building blocks.
+It also stands true for the cryptographic library that is used to perform those
+essential operations for CryptPad.
+[TweetNaCl](https://github.com/dchest/tweetnacl-js) has been picked as a
+lightweight full-javascript library to ensure browser-portability for the
+client.
+However, this portability constraint is not relevant for servers, and the
+lack of low-level optimisation is actually what cause of the bottleneck we
+described previously.
+
+To keep the codebase legible, and as it’s not a problem for small instances, we
+decided to address this problem using CryptPad’s plugin system. Then providing
+sane defaults for reasonable-sized instances, while making possible to easily
+swap the cryptographic library used by the server. The goal of this tutorial is
+to show how to do it in practice.
+
+## 🔩 Installing the plugin
+
+1. To install the plugin, the first step is to download it at the right place.
+From your CryptPad installation directory:
+
+```bash
+cd lib/plugins
+git clone https://github.com/cryptpad/cryptpad-sodium-plugin.git sodium
+```
+
+2. Then, install its dependencies:
+
+```bash
+cd sodium/
+npm ci
+```
+
+3. Restart your CryptPad server.
+
+And that’s it, your server should use its native implementation of _libsodium_
+now.
+
+⚠️ If step 2 is omitted, the server would silently fall back to `TweetNaCl`.
+
+## 🔧 Use another library?
+
+For signatures, CryptPad uses the EdDSA algorithm over the
+[`ed25519`](https://en.wikipedia.org/wiki/EdDSA#Ed25519) curve.
+If you want to run another implementation of it, you can make a copy of our
+[`cryptpad-sodium-plugin`](https://github.com/cryptpad/cryptpad-sodium-plugin/tree/main)
+plugin and edit the
+[`index.js`](https://github.com/cryptpad/cryptpad-sodium-plugin/blob/main/index.js)
+file to provide another implementation for the `open` (combined mode
+verification) and `detachedVerify` (detached mode verification) functions.
+
+Please note that if you do that and run it in production, under the
+[AGPL-3](https://www.gnu.org/licenses/agpl-3.0.en.html) licence, you are to make
+the source code of the plugin available.
+Feel free to contact us at <contact@cryptpad.org> if you want us to advertise
+your back-end crypto plugin as a community contribution 🙂