evaluate secret management schemes

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+# Evaluation of secret management schemes
+
+2024-12-03 Robert, Nicolas, Valentin
+
+## Requirements
+
+- Store and manage secrets in a central place
+- Must be able to rotate keys (some state management)
+- Minimal state on contributors' end, ideally exactly one per-user credential or even SSO
+
+## Non-requirements
+
+- Don't need (or need only very basic) RBAC, all contributors are equal (maybe infra admins have special access)
+- Components which require secrets don't have to be a secret (this would be a requirement for personal setups, where we don't want to leak e.g. which accounts exist)
+- No need to retrieve secrets for very old versions
+- No need for forward secrecy (thoroughly destroying keys as required by e.g. secure messaging protocols)
+
+## Design considerations
+
+- Storing secrets
+
+  Some secrets need to be persisted, and there are multiple formats and technologies to do that.
+  
+- Managing secrets
+
+  Secrets need to be shared with contributors, and changed or rotated.
+  Different systems have different degrees of comfort for these operations.
+
+- Deploying secrets
+
+  Secrets need to be made available to programs and services.
+  
+- Versioning
+
+  For key rotation we need at least two versions: old to access the machine, new for rotating in
+
+- Setup complexity
+
+  Different systems have different requirements to get going, and may require more or less manual intervention for new contributors. This distinguishes:
+
+  - complexity to set up for experts
+  - complexity to contribute as a beginner
+
+- Scalability, sustainability
+
+  Questions to consider:
+  - What if a contributor works on 100 such projects?
+  - What if a project has 100 contributors?
+  - What if a project runs over 10 years, how much effort does secret handling incur?
+  - What if someone messes up the central server?
+  - How fast can we set up a working system?
+  - How hard is it to migrate from one scheme to another?
+
+## Overview
+
+|Name|management|deployment|storage|versioning|setup|scalability|
+|-|-|-|-|-|-|-|
+|[agenix]|yes (CLI)|yes (tempfiles)|repo ([age])|Git|[partially manual](#agenix-setup)|[details](#agenix-scalability)|
+|[sops-nix]|yes (CLI)|yes (tempfiles)|repo ([SOPS])|Git|[partially manual](#sops-nix-setup)|[more moving parts than agenix](#sops-nix-scalability)|
+|[Vaultwarden]|yes (web GUI)|no|database|yes, on demand|[details](#Vaultwarden-setup)|[more up-front effort](#Vaultwarden-scalability)|
+|ssh/scp|yes (manual) |yes (manual)|per-user|manually|[details](#sshscp-setup)|[details](#sshscp-scalability)|
+
+[agenix]: https://github.com/ryantm/agenix
+[age]: https://github.com/FiloSottile/age
+[sops-nix]: https://github.com/Mic92/sops-nix
+[SOPS]: https://github.com/getsops/sops
+[LoadCredential]: https://systemd.io/CREDENTIALS/
+[Vaultwarden]: https://github.com/dani-garcia/vaultwarden
+
+## Details on setup complexity
+
+### agenix setup
+
+- include module into configuration
+- manage per-user ssh public keys
+- each user needs to manage their public keys manually 
+
+### sops-nix setup
+
+- include module into configuration
+- manage per-user ssh public keys
+- each user needs to manage their public keys manually 
+
+### Vaultwarden setup
+
+- deploy Vaultwarden, set up backups
+- manage per-user authentication with Vaultwarden
+
+### ssh/scp setup
+
+- each contributor has to manage private keys and ssh config manually 
+- have to take care of distribution of secrets and deployment separately
+
+## Details on scalability
+
+### agenix scalability
+
+- allows reusing ssh key workflows
+
+### sops-nix scalability
+
+- some extra complexity due to multiple encryption schemes
+- allows reusing ssh key workflows
+- some additional local setup for contributors
+
+### Vaultwarden scalability
+
+- allows reusing password handling workflows (typically better automation than for ssh keys)
+- more up-front work for initial deployment
+- disaster recovery needs special care, doesn't implicitly distribute copies to contributors
+- less interaction for managing contributor access
+- separate source of truth (workflows, audit log, etc.) as opposed to everything in the Git repo
+- adds an extra security boundary; encrypted secrets are not world-readable
+
+### ssh/scp scalability
+
+- requires taking care of distributing keys
+- per-user key management typically not automated, requires taking care of that separately
+
+## Additional notes
+
+- Managing the interface between public confiuration and secrets is a concern of the code
+    - For a scalable setup you want something like modules that take secrets as settings
+- It is possible to split the git-stored secret schemes into private repositories
+    - Then you have to handle synchronisation, e.g. by importing the public part from the secret part
+    - This would incur extra overhead for managing access, but that would be the same workflow as managing access to the rest of the Git server
+- With secrets stored in Git there's a potential for running into merge conflicts, which can be avoided but requires extra care
+    - Probably you want a monorepo for the entire organisation
+        - Separating public and private parts through git subtrees is possible but requires even more care and automation/tooling when managing outside contributions
+        - The upfront effort may be similar (but different in nature) to deploying and maintaining a Vaultwarden server
+- There's an experience and skill issue involved in maintaining a sophisticated Git repo or a live server, and what is more appropriate will depend on who will be responsible for the setup long-term