# aclsystem: encode user and group names into ids to keep acls simple

caveat emptor: this is another fantasy posts where i think how would i design a system based with zero experience in such systems. usually i daydream about being a superhero but today it's about acl systems in a small/mid sized tech company.

suppose you have files in a filesystem, tables in a database, tickets in an issue management software, documents in a content management system, etc. you want to make it configurable which users can access the entities in your system and how. you could have a couple capabilities or access control lists (acls) and for each acls a list of groups or users who have that capability. examples:

suppose you own a file and you want alice to read it. all you need to do is to add alice to the read capability's list. easy peasy. though note that this isn't representible in the standard posix file permissions model. i think that's a very inflexible model and the above is more powerful. these lists don't have to be unbounded. even if you bound them to 4 entries, you already have a quite flexible system.

# ids

how do you represent these acl lists? ideally each user and group in your system has an int64 associated. then each acl is just a list of int64. that's a more compact representation than storing these as list of strings.

how do you map a username to an int64 and vice versa? one approach is to have keep a database around that contains the string<->int64 mappings. but that's overkill! there's a much simpler approach if you accept some limitations.

limit usernames to the form of "[basename]-[suffix]". basename can consist only of at most 10 letters (no digits or underscore allowed). suffix can be one of 8192 hardcoded suffixes.

you can encode one letter out of 26 in 5 bits (2^5 = 32). 10 such letters means you need 50 bits. you can encode one suffix out of 8192 in 13 bits. now we have a 63 bit long number.

there's one bit left: let's use that whether we want group expansion or not. if the id is negative, then username doesn't refer to the user itself, but to a group expansion that is looked up in some other system.

# id mapping example

let's encode 'a' as 00001, ..., 'z' as 11011. and to make the implementation of encoding/decoding simple, store it in reverse. so encode "alice" as "ecila".

that would be the int64 id for those users. the implementation is simple, to decode you would need something like this in go:

  name := ""
  for ; len(name) < 10 && id&31 > 0; id >>= 5 {
    name += string('a' + id&31 - 1)
  }

encoding is similarly simple if the name already meets the limitations.

encoding names like acme-team, login-service, politics-discuss, accesslogs-readers can be done via the suffix logic. you just need a builtin constant map like this: 1-team, 2-service, 3-discuss, 4-readers, 5-group, ...

"politics" translates to 656379523568 and the suffix code for -discuss is 3 so 656379523568 + 3<<50 = 3378356100051440 is the id for politics-discuss. this could be a group that holds all the members subscribed to mailing list called politics-discuss.

to express all members of politics-discuss, use the id of -3378356100051440. note the negative sign. the member expansion would be provided via some external group expansion service.

# acl example

suppose alice has a file that she wants to share with her manager bob and the lawyers-team.

using numbers this translates to this:

checking if a user can read the file consists of two steps: the user's id is in the readers list? it is allowed. if not, then the system needs to group expand each group reference. this is more expensive but with some caching this could be a fast enough operation.

# the suffix map

the list of suffixes would be carefully selected to express common ideas. e.g. many tools and projects want to have a mailing list to discuss it so many teams would like a group with a -discuss ending name. so it makes sense to have that as one of the possible suffixes. this map can grow over time. but each addition must be carefully vetted for usefulness. there are only 8192 possible suffixes, it can run out very quickly if you allow users to register them without oversight.

the suffix map would be embedded into each application as a constant. this means that there's some delay until a new suffix is picked up in all applications. this shouldn't be a problem because most applications only care and communicate via the int64 ids. the map is only needed when the application wants to do a conversion between the id and the human-readable name. but even if the map is not updated, it can just use the raw id as a placeholder.

so decoding 3378356100051440 into politics-3 should be reasonable enough. similarly if an ui wants to encode politics-discuss into an id but doesn't know the id for -discuss then the ui simply returns an error. then the user can enter politics-3 and that should work too.

# namespaces

if it makes sense, you might sacrifice one (or more) bit from that bitmask for namespaces. suppose you are a web company and you have your internal employees and external users. you want to assign ids for both. use this new bit to decide whether an id is for an internal user or an external one.

if it's internal, you will have a selection only from 2¹²=4096 suffixes. if it's external, then the remaining 12 bits could be used differently than suffixes. maybe use it for +2 letter long usernames, 12 letters in total. or have 5 bits (0..31) for suffixes in case your website allows users to form groups (-discuss, -members, -announce) or implement bots (-bot). and then the remaining 7 bits (0..128) for yearstamping with the last two year digits. so if a user registers in year 2024, they get a username like alice24. other users can immediately tell how fresh a user is and prevents account reuse. see @/yseq for other benefits why yearstamping ids in general is good. the internal username decoders can then distinguish between internal and external users solely based on the fact whether the basename part of the username has numbers or not.

# abnames

the 10 letter, no digit restriction can be quite painful. for human usernames that might be fine, nobody likes long names anyways.

for service roles and product names it might feel more limiting. but judicious use of the @/abnames philosophy can give plenty of short names. these short names don't need to be perfect. the abnames come with a glossary so the user can easily look up the full, human readable name of the product.

in fact most user interfaces should provide a popup window which on popup that explains the details of the role including the full product name. such feature is also useful for human usernames: to see the full name, the profile photo, responsibilities, availability, etc.

# humans vs robots

often there's a desire to distuingish between humans and robots. for example in the above hover-popup-box example a system could look up data differently for humans vs robots. for instance the popup box wouldn't need to try to look at calendar availability for a robot. another example would be enforcing a human-review rule: each commit must be reviewed by a human. in that case the review system would need to be able to tell if an entity is a human or not.

to make this simple, use the following rule: the empty suffix means humans. in other words if a username contains a dash, it's not a human. robots can use a -bot or -service suffix.

i'm not fully sure about the usefulness of this rule because i really like short names. and i can imagine there would be some bots where a short name would be useful. but i think the value of easily recognizing fellow humans in our complex systems is getting more and more valuable so i think it's worth it. this way you can easily tell which one is human between alice and alice-bot.

# groups

i recommend keeping group membership data in version control. you could have the following configuration:

the g/ prefix in "g/acme-team" refers to expanded group. so login-service will contain alice and bob as members.

the group definitions need to be expanded recursively. so accesslog-readers would contain alice, bob, and charlie. this means the group membership lists must be acyclic.

tracking human memberships in a version control for a mailing list like politics-discuss would be overkill. so track groups with high churn (such as memberships for mailing lists) differently, e.g. in a database and have the users join or leave via an ui rather than editing text files.

then create a service that serves these group expansions. make it possible for clients to fetch all members for a group and then watch for updates. this means membership lookup remains local in the client and thus fast.

tip: log every time you look up a member in a group as part of making a decision on access. log it with reason, example:

  func IsMember(group, user int64, reason string) bool
  ...
  acls.IsMember(acls.Id("accesslog-readers"), acls.Id("alice"), "raw access")

log it into a central logging system where users can later look up which memberships users actually used and when was a membership last used. such information will be super useful when trying to lock down privileged groups. eventually you will need such information so it's best if the system is designed with this in mind right away.

# special groups

to make expressing some things easier, create a couple special groups:

the expansion of these groups would be handled in the lookup logic specially: no lookup would be needed.

# management

it makes sense to associate some metadata with users, roles, and groups. e.g. for roles you could configure the full description of the role, the 4 byte linux uid_t, etc. for groups you would configure whether it's a mailing list or not, whether humans can join on their own via an ui, etc.

suppose you have a version control system with per directory access control. then create a directory for every admin team wishing to manage groups and put their roles under them. then all modifications in the files have to be approved via that admin team.

example:

  # plogs-admins/plogs-admins.txtpb
  description: "group admin management team for plogs (Production LOGging System)."
  members: [
    "alice",
    "bob",
  ]

  # plogs-admins/plogs-discuss.txtpb
  description: "mailing list for plogs (Production LOGging System) related topics. anyone can join."
  group_join_mode: "self-service"
  mailinglist {
    moderators: ["alice"]
    readers: ["g/all-special"]
  }

  # plogs-admins/plogs-backend.txtpb
  description: "service for clients wishing to upload production log entries into plogs (Production LOGging System)."
  vm_management {
    linux_uid: 1234
    admins: ["g/plogs-admins"]
  }

  # plogs-admins/plogs-frontend.txtpb
  description: "service for users wishing to browse the production log entries in plogs (Production LOGging System)."
  vm_management {
    linux_uid: 1235
    admins: ["g/plogs-admins"]
  }

then create a service that serves this metadata for other systems. so when the mailserver receives an email to "plogs-discuss@example.com" it can check this service whether it's indeed a mailing list. if so it then asks the group expander service for the members and forwards the email to them.

# disclaimer

i admit, i'm not sure i'd design a real system exactly like this. 10 letters can be quite limiting. this system doesn't scale up to millions of employees creating millions of microservices each with a different username. the names will become very cryptic very fast. but if the company has less than thousand users in its system, this should be a pretty simple way to manage things. i like the simplicity and compactness this design requires so it could be fun to play around with in non-serious environments.

published on 2024-04-01, last modified on 2024-07-01

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