feat: introduce cryptographic documentation

Documents all our current key generation algorithms.

explanation/cryptography/index.md

@@ -0,0 +1,16 @@
+(cryptography)=
+# Cryptography
+
+This section describes the cryptographic algorithms used by the Charmed HPC project.
+
+- {ref}`munge`
+- {ref}`jwt`
+
+```{toctree}
+:titlesonly:
+:maxdepth: 1
+:hidden:
+
+MUNGE <munge>
+JSON Web Tokens <jwt>
+```

explanation/cryptography/jwt.md

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+(jwt)=
+# JSON Web Tokens (JWT)
+
+Some Slurm charms support [JSON Web Tokens](https://jwt.io/) as an alternative authentication method for a Slurm cluster.
+
+This service is used by the Slurm charms:
+
+- [`slurmctld`](https://charmhub.io/slurmctld)
+- [`slurmrestd`](https://charmhub.io/slurmrestd)
+
+A shared private encryption key is required to verify the signature of client tokens. The current method uses RSA with a length of 2048 bits, which is generated using the [`cryptography`](https://pypi.org/project/cryptography/) package for Python.
+
+The [Slurm documentation](https://slurm.schedmd.com/jwt.html) contains more information about the topic.
+
+## Libraries used
+
+- [`cryptography`](https://pypi.org/project/cryptography/), from PyPi.

explanation/cryptography/munge.md

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+(munge)=
+# MUNGE
+
+[MUNGE (MUNGE Uid 'N' Gid Emporium)](https://dun.github.io/munge/) is an authentication service for creating and validating credentials.
+
+This service is used by all our Slurm charms, including:
+
+- [`slurmctld`](https://charmhub.io/slurmctld)
+- [`slurmd`](https://charmhub.io/slurmd)
+- [`slurmdbd`](https://charmhub.io/slurmdbd)
+- [`slurmrestd`](https://charmhub.io/slurmrestd)
+
+MUNGE requires sharing a cryptographically secure key between all the Slurm nodes in a cluster. To generate this key, the charms
+use the [mungectl](https://github.com/charmed-hpc/mungectl) utility, which uses Go's [`crypto/rand`](https://pkg.go.dev/crypto/rand) library to generate a cryptographically secure key of 1024 bytes of length, using either [`getrandom(2)`](https://man7.org/linux/man-pages/man2/getrandom.2.html) if available, and [`/dev/urandom`](https://en.wikipedia.org/wiki//dev/random) otherwise.
+
+You can find more information about MUNGE on its [official wiki](https://github.com/dun/munge/wiki).
+
+## Packages used
+
+- [`crypto/rand`](https://pkg.go.dev/crypto/rand), from the [Go standard library](https://pkg.go.dev/std).

explanation/index.md

@@ -1,4 +1,14 @@
 (explanation)=
 # Explanation
 
+- {ref}`cryptography`
+
 🚧 Under construction 🚧
+
+```{toctree}
+:titlesonly:
+:maxdepth: 1
+:hidden:
+
+cryptography/index
+```