Add some documentation about security (#858)

Co-authored-by: Baptiste Jonglez <git@bitsofnetworks.org>

docs/index.rst

@@ -19,6 +19,7 @@ Table of contents
    configuration
    upgrade
    api
+   security
    contributing
 
 Indices and tables

docs/security.rst

@@ -0,0 +1,97 @@
+Security
+########
+
+Ihatemoney does not have user accounts. Instead, authorization is based around
+shared projects: this is a bit unusual and deserves some explanation about
+the security model.
+
+First of all, Ihatemoney fundamentally assumes that all members of a project trust
+each other. Otherwise, you would probably not share expenses in the first place!
+
+That being said, there are a few mechanisms to limit the impact of a malicious
+member and to manage changes in membership (e.g. ensuring that a previous member
+can no longer access the project). But these mechanisms don't prevent a malicious member
+from breaking things in your project!
+
+Security model
+==============
+
+A project has three main parameters when it comes to security:
+
+- **project identifier** (equivalent to a "login")
+- **private code** (equivalent to a "password")
+- **token** (cryptographically derived from the private code)
+
+Somebody with the private code can:
+
+- access the project through the web interface or the API
+- add, modify or remove bills
+- view project history
+- change basic settings of the project
+- change the email address associated to the project
+- change the private code of the project
+
+Somebody with the token can manipulate the project through the API to
+do essentially the same thing:
+
+- access the project
+- add, modify or remove bills
+- change basic settings of the project
+- change the email address associated to the project
+- change the private code of the project
+
+The token can also be used to build "invitation links". These links allow
+to login on the web interface without knowing the private code, see below.
+
+Giving access to a project
+==========================
+
+There are two main ways to give access to a project to a new person:
+
+- share the project identifier and private code using any out-of-band
+  communication method
+
+- share an invitation link that allows to login on the web interface
+  without knowing the private code
+
+The second method is interesting because it does not reveal the private code.
+In particular, somebody that is logged-in through the invitation link will not be able
+to change the private code, because the web interface requires a confirmation
+of the existing private code to change it.
+However, a motivated person could extract the token from the invitation link,
+use it to access the project through the API, and change the private code through
+the API.
+
+Removing access to a project
+============================
+
+If a person should no longer be able to access a project, the only way is to change
+the private code.
+
+This will also automatically change the token: old invitation links won't
+work anymore, and anybody with the old token will no longer be able to access
+the project through the API.
+
+Recovering access to a project
+==============================
+
+If the private code is no longer known, the creator of the project can still recover
+access. He/she must have provided an email address when creating the project,
+and Ihatemoney can send a reset link to this email address (classical "forgot
+your password" functionality).
+
+Note, however, that somebody with the private code could have changed the email
+address in the settings at any time.
+
+Recovering lost data
+====================
+
+A member can delete or change bills. There is no way to revert such actions for now.
+However, each project has an history page that lists all actions done on the project.
+This history can be used to manually correct previous changes.
+
+Note, however, that the history feature is primarily meant to protect against mistakes:
+a malicious member can easily remove all entries from the history!
+
+The best defense against this kind of issues is... backups! All data for a project can be
+exported through the settings page or through the API.