feat(doc): #919 design principles

- Write an argument on how we comply the
  secure design principles

docs/src/security/design-principles.md

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+<!--
+SPDX-FileCopyrightText: 2022 Fluid Attacks and Makes contributors
+
+SPDX-License-Identifier: MIT
+-->
+
+# Design Principles
+
+## Principle of Least Privilege
+
+- The Makes CLI is a python application that runs in user-space.
+  The privileges required are:
+
+  - Write access to the `${HOME}`,
+    which is normally owned by the user,
+    so no extra privileges
+    other than what the user already has are required.
+  - Creating temporary files/directories,
+    which respects the `${TMPDIR}` environment variable,
+    which is a functionality normally available to a user,
+    so no extra privileges are required.
+  - A system with Nix installed.
+  - (optional) privileges to create Kernel namespaces.
+
+- The Makes framework is simply a library
+  that aids the developer in creating build scripts,
+  so no privileges are required,
+  the Makes framework is just source code
+  that the user can opt-in to use.
+
+- When containers are built,
+  they are build by assembling an OCI-compliant image
+  (TAR files per each layer plus a JSON manifest),
+  without resorting to privileged daemons like that of Docker.
+  They are generated as any other build (hermetic, pure, etc)
+  using information from the Nix Store.
+
+## Principle of Fail-Safe Defaults
+
+- By default, builds are run in a sandbox
+  that uses kernel namespaces
+  to prevent the build from accessing the network
+  and the external file system.
+
+  The user is given the option to opt-out from this behavior,
+  but this is enabled by default.
+
+- Generated files are created inside user-owned folders by default,
+  which inherit the security
+  that the user has previously defined for the directory.
+
+  An user may opt-out from this behavior by setting environment variables,
+  but user-owned folders are selected by default.
+
+- In the most common configuration,
+  the contents of the `/nix/store`
+  are never published to the internet.
+
+  A user may want to share artifacts with other users
+  in order to improve performance
+  by writing artifacts to a binary cache,
+  so that other users can download the artifacts
+  if they have already been built by other user,
+  but this behavior
+  requires configuring a read+write binary cache
+  and setting the corresponding access secret.
+
+  A read-only binary cache (<https://cache.nixos.org>)
+  and no write binary cache
+  is the default configuration,
+
+## Principle of Economy of Mechanism
+
+- The Makes CLI is essentially a wrapper over Nix,
+  so the surface is as small as possible (~1000 loc).
+- The Makes Framework defines a common set of utilities
+  a user can opt-in to use,
+  saving the user the work of writing that functionality themselves
+  which would require the same amount of code anyway.
+
+## Principle of Complete Mediation
+
+## Principle of Open Design
+
+- Makes is Free and Open Source Software,
+  anyone can read its internals:
+  https://github.com/fluidattacks/makes
+
+## Principle of Separation of Privilege
+
+## Principle of Least Common Mechanism
+
+- In the most common case
+  each user of Makes has a personal `/nix/store`
+  and a personal installation of Nix.
+  The `/nix/store` contents are not shared between users by default,
+  unless the user configures a read+write binary cache
+  and sets the corresponding binary cache secret.
+
+## Principle of Psychological Acceptability
+
+- The Makes CLI is easy to use.
+  Performing an installation using the default values
+  yields a sufficiently secure version of the system.
+  Users familiar with other build tools would feel at home.