crates.io token scopes

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+# crates.io token scopes RFC
+
+- Feature Name: `crates_io_token_scopes`
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- crates.io issue: [rust-lang/crates.io#0000](https://github.com/rust-lang/crates.io/issues/0000)
+
+# Summary
+[summary]: #summary
+
+This RFC proposes implementing scopes for crates.io tokens, allowing users to
+choose which endpoints the token is allowed to call and which crates it's
+allowed to affect.
+
+# Motivation
+[motivation]: #motivation
+
+While the current implementation of API tokens for crates.io works fine for
+developers using the `cargo` CLI on their workstations, it's not acceptable for
+CI scenarios, such as publishing crates automatically once a git tag is pushed.
+
+The implementation of scoped tokens would allow users to restrict the actions a
+token can do, decreasing the risk in case of automation bugs or token
+compromise.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+During token creation, the user will be prompted to select the permissions the
+token will have. Two sets of independent scopes will be available: the
+endpoints the token is authorized to call, and the crates the token is allowed
+to act on.
+
+## Endpoint scopes
+
+The user will be able to choose one or more endpoint scopes. This RFC proposes
+adding the following endpoint scopes:
+
+* **publish-new**: allows publishing new crates
+* **publish-update**: allows publishing a new version for existing crates the
+  user owns
+* **yank**: allows yanking and unyanking existing versions of the user's crates
+* **change-owners**: allows inviting new owners or removing existing owners
+* **legacy**: allows accessing all the endpoints on crates.io except for
+  creating new tokens, like tokens created befores the implementation of this
+  RFC.
+
+More endpoint scopes might be added in the future without the need of a
+dedicated RFC.
+
+The crates.io UI will pre-select the scopes needed by the `cargo` CLI, which at
+the time of writing this RFC are `publish-new`, `publish-update`, `yank` and
+`change-owners`. The user will have to explicitly opt into extra scopes or the
+legacy permission model.
+
+Tokens created before the implementation of this RFC will default to the legacy
+scope.
+
+## Crates scope
+
+The user will be able to opt into limiting which crates the token can act on by
+defining a crates scope.
+
+The crates scope can be left empty to allow the token to act on all the crates
+owned by the user, or it can contain the comma-separated list of crate names
+the token can interact with. Crate names can contain `*` to match zero or more
+characters.
+
+For example, a crates scope of `lazy_static,serde*` allows the token to act on
+the `lazy_static` crate or any present or future crates starting with `serde`
+(including `serde` itself), if the user is an owner of those crates.
+
+The crates scope will allow access to all present and future crates matching
+it. When an endpoint that doesn't interact with crates is called by a token
+with a crates scope, the crates scope will be ignored and the call will be
+authorized.
+
+Tokens created before the implementation of this RFC will default to an empty
+crate scope filter (equivalent to no restrictions).
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Endpoint scopes and crates scope are two completly separate systems, and can be
+used independently from one another. Token scopes will be implemented entirely
+on the crates.io side, and there will be no change to `cargo` or alternate
+registries.
+
+## Endpoint scopes
+
+The scopes proposed by this RFC allow access to the following endpoints:
+
+| Endpoint | Required scope |
+| --- | --- |
+| `PUT /crates/new` (new crates) | **publish-new** |
+| `PUT /crates/new` (existing crates) | **publish-update** |
+| `DELETE /crates/:crate_id/:version/yank` | **yank** |
+| `PUT /crates/:crate_id/:version/unyank` | **yank** |
+| `PUT /crates/:crate_id/owners` | **change-owners** |
+| `DELETE /crates/:crate_id/owners` | **change-owners** |
+| everything except `PUT /me/tokens` | **legacy** |
+
+Removing an endpoint from a scope or adding an existing endpoint to an existing
+scope will be considered a breaking change. Adding newly created endpoints to
+an existing scope will be allowed only at the moment of their creation, if the
+crates.io team believes the new endpoint won't grant more privileges than the
+existing set of endpoints in that scope.
+
+## Crates scope
+
+The pattern for the crate scope is desugared into a regular expression,
+following these rules:
+
+* **`^(`** is added at the start of the pattern, and **`)$`** is added at the end of it.
+* **`,`** is desugared into `|`, separating multiple patterns.
+* **`*`** is desugared into `.*`, matching zero or more characters greedily.
+* All other non-alphanumeric characters are quoted to prevent them from having
+  a special meaning.
+
+As an example, the following pattern:
+
+```
+foo,bar-*
+```
+
+... is desugared into the following regex:
+
+```
+^(foo|bar\-.*)$
+```
+
+Any combination of those characters is allowed, but crates.io might define a
+complexity limit for the generated regular expressions.
+
+The pattern will be evaluated during each API call, and if no match is found
+the request will be denied. Because it's evaluated every time, a crates scope
+will allow interacting with matching crates published after token creation.
+
+The check for the crates scope is separate from crate ownership: having a scope
+that technically permits to interact with a crate the user doesn't own will be
+accepted by the backend, but a warning will be displayed if the pattern doesn't
+match any crate owned by the user.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+No drawbacks are known at the time of writing the RFC.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+An alternative implementation for endpoint scopes could be to allow users to
+directly choose every endpoint they want to allow for their token, without
+having to choose whole groups at a time. This would result in more granularity
+and possibly better security, but it would make the UX to create new tokens way
+more complex (requiring more choices and a knowledge of the crates.io API).
+
+An alternative implementation for crate scopes could be to have the user select
+the crates they want to allow in the UI instead of having to write a pattern.
+That would make creating a token harder for people with lots of crates (which,
+in the RFC author's opinion, are more likely to need crate scopes than a person
+with just a few crates), and it wouldn't allow new crates matching the pattern
+but uploaded after the token's creation from being accessed.
+
+Finally an alternative could be to do nothing, and encourage users to create
+"machine accounts" for each set of crates they own. A drawback of this is that
+GitHub's terms of service limit how many accounts a single person could have.
+
+# Prior art
+[prior-art]: #prior-art
+
+The endpoint scopes system is heavily inspired by GitHub, while the rest of the
+proposal is similar to nuget. Here is how popular package registries implements
+scoping:
+
+* [nuget] (package registry for the .NET ecosystem) implements three endpoint
+  scopes (publish new packages, publish new versions, unlist packages), has a
+  mandatory expiration and supports specifying which packages the token applies
+  to, either by checking boxes or defining a single glob pattern.
+  [(documentation)][nuget-docs]
+* [npm] (package registry for JavaScript) implements a binary
+  "read-only"/"read-write" permission model, also allowing to restrict the IP
+  ranges allowed to access the token, but does not allow restricting the
+  packages a token is allowed to change. [(documentation)][npm-docs]
+* [pypi] (package registry for Python) only implements the "upload packages"
+  permission, and allows to scope each token to a *single* package.
+  [(documentation)][pypi-docs]
+* [rubygems] (package registry for Ruby) and [packagist] (package registry for
+  PHP) don't implement any kind of scoping for the API tokens.
+
+[nuget]: https://www.nuget.org/
+[nuget-docs]: https://docs.microsoft.com/en-us/nuget/nuget-org/scoped-api-keys
+[npm]: https://www.npmjs.com
+[npm-docs]: https://docs.npmjs.com/creating-and-viewing-authentication-tokens
+[pypi]: https://pypi.org
+[pypi-docs]: https://pypi.org/help/#apitoken
+[rubygems]: https://rubygems.org/
+[packagist]: https://packagist.org/
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+* Are there more scopes that would be useful to implement from the start?
+* Is the current behavior of crate scopes on endpoints that don't interact with
+  crates the best, or should a token with crate scopes prevent access to
+  endpoints that don't act on crates?
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+A future extension to the crates.io authorization model could be adding an
+optional expiration to tokens, to limit the damage over time if a token ends up
+being leaked.
+
+Another extension could be an API endpoint that programmatically creates
+short-lived tokens (similar to what AWS STS does for AWS Access Keys), allowing
+to develop services that provide tokens with a short expiration time to CI
+builds. Such tokens would need to have the same set or a subset of the parent
+token's scopes: this RFC should consider that use case and avoid the
+implementation of solutions that would make the check hard.
+
+To increase the security of CI environments even more, we could implement an
+option to require a separate confirmation for the actions executed by tokens.
+For example, we could send a confirmation email with a link the owners have to
+click to actually publish the crate uploaded by CI, preventing any mailicious
+action with stolen tokens.
+
+To remove the need for machine accounts, a future RFC could propose adding API
+tokens owned by teams, granting access to all resources owned by that team and
+allowing any team member to revoke them.