Designing with Capabilities
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The document discusses the importance of capability-based design in API development, emphasizing that good security is inherently linked to good design. It explains how limiting the capabilities of API callers can prevent security risks and encourages practices such as minimizing exposed surface area. The talk integrates principles from both security and design to propose a systematic approach to building secure and flexible interfaces.
![public class CustomerController : ApiController
{
readonly ICustomerDb _db;
public CustomerController(ICustomerDb db)
{
_db = db;
}
[Route("customers/{customerId}")]
[HttpGet]
[GetCustomerProfileAuth]
public IHttpActionResult Get(int customerId)
{
var custId = new CustomerId(customerId);
var cust = _db.GetProfile(custId);
var dto = DtoConverter.CustomerToDto(cust);
return Ok(dto);
}](https://image.slidesharecdn.com/designingwithcapabilities-160116160830/85/Designing-with-Capabilities-78-320.jpg)
Designing with Capabilities
- 1. Designing with capabilities for fun and profit @ScottWlaschin fsharpforfunandprofit.com/cap
- 3. Good security => Complicated I wonât like doing this
- 4. Good security => Complicated X
- 5. Security for free! ...which is good, because Iâm lazy Good security == Good design I like doing this
- 6. The topic of this talk Not about OAuth, JWT etc Good security == Good design
- 8. Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia consequuntur magni dolores eos qui ratione voluptatem sequi nesciunt. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Quis autem vel eum iure reprehenderit qui in ea voluptate velit esse quam nihil molestiae consequatur, Temporibus autem quibus Dacei Megasystems Tech Inc necessitatibust aut officiis debitis auteo 2799 E Dragam Suite 7 quisquam saepe Itaque enieti Los Angeles CA 90002 ut et voluptates repudiandae sint et molestiae non recusandae. Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Please deliver this letter A counterexample
- 9. Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia consequuntur magni dolores eos qui ratione voluptatem sequi nesciunt. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Quis autem vel eum iure reprehenderit qui in ea voluptate velit esse quam nihil molestiae consequatur, Temporibus autem quibus Dacei Megasystems Tech Inc necessitatibust aut officiis debitis auteo 2799 E Dragam Suite 7 quisquam saepe Itaque enieti Los Angeles CA 90002 ut et voluptates repudiandae sint et molestiae non recusandae. Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Please deliver this letter Itâs not just about security... ...hiding irrelevant information is good design!
- 10. EVOLUTION OF AN API
- 11. Evolution of an API (from the security point of view) Say that the UI needs to set a configuration option (e.g. DontShowThisMessageAgain) How can we stop a malicious caller doing bad things?
- 12. Version1 Give the caller the configuration file name interface IConfiguration { string GetConfigFilename(); } var filename = config.GetConfigFilename(); // open file // write new config // close file API Caller ï A malicious caller has the ability to open and write to any file on the filesystem
- 13. Version 2 Give the caller aTextWriter interface IConfiguration { TextWriter GetConfigWriter(); } var writer = config.GetConfigWriter(); // write new config API Caller ï A malicious caller can corrupt the config file We control which file is opened
- 14. Version 3 Give the caller a key/value interface interface IConfiguration { void SetConfig(string key, string value); } config.SetConfig( "DontShowThisMessageAgain", "True"); API Caller ï A malicious caller can set the value to a non-boolean
- 15. Version 4 Give the caller a domain-centric interface enum MessageFlag { ShowThisMessageAgain, DontShowThisMessageAgain } interface IConfiguration { void SetMessageFlag(MessageFlag value); void SetConnectionString(ConnectionString value); void SetBackgroundColor(Color value); } API ï What's to stop a malicious caller changing the connection string when they were only supposed to set the flag?
- 16. Version 5 Give the caller only the interface they need interface IWarningMessageConfiguration { void SetMessageFlag(MessageFlag value); } API ï The caller can *only* do the thing we allow them to do.
- 17. Canât get your work done Too much information passed in Just right Potential for abuse Principle of Least Authority (POLA) Too little information passed in Security spectrum
- 18. Evolution of the same API (from the design point of view)
- 19. DesignVersion1 Give the caller the configuration file name interface IConfiguration { string GetConfigFilename(); } API Bad design: Using a filename means we limit ourselves to file-based config files. Better design: A TextWriter would make the design more mockable.
- 20. DesignVersion 2 Give the caller aTextWriter interface IConfiguration { TextWriter GetConfigWriter(); } API Better design: A generic KeyValue store would make implementation choices more flexible. Bad design: Using a TextWriter means exposing a specific storage format
- 21. DesignVersion 3 Give the caller a key/value interface interface IConfiguration { void SetConfig(string key, string value); } API Bad design: A KeyValue store using strings means possible bugs. So we need to write validation and tests for that ï Better design: A statically typed interface means no corruption checking code. ï
- 22. DesignVersion 4 Give the caller a domain-centric interface enum MessageFlag { ShowThisMessageAgain, DontShowThisMessageAgain } interface IConfiguration { void SetMessageFlag(MessageFlag value); void SetConnectionString(ConnectionString value); void SetBackgroundColor(Color value); } API Bad design: An interface with too many methods violates the ISP. Better design: Reduce the number of available methods to one!
- 23. DesignVersion 5 Give the caller only the interface they need interface IWarningMessageConfiguration { void SetMessageFlag(MessageFlag value); } API Good design: The caller has no dependencies on anything else. Bonus: easy to mock! ï
- 24. Canât do anything Too many dependencies Just right Unnecessary coupling Interface Segregation Principle Too few dependencies Design spectrum
- 25. Ak.a. Minimize your surface area (to reduce chance of abuse) OO Design Guidelines Interface Segregation Principle Single Responsibility Principle, etc Security Guidelines Principle of Least Authority (POLA) Ak.a. Minimize your surface area (to reduce coupling, dependencies, etc)
- 26. Good security => Good design Good design => Good security
- 28. Capability based design âą In a cap-based design, the caller can only do exactly one thing -- a "capability". âą In the example, the caller has a capability to set the message flag, and that's all. Prevents both maliciousness and stupidity!
- 29. A one method interface is a capability interface IWarningMessageConfiguration { void SetMessageFlag(MessageFlag value); }
- 30. A one method interface is a function interface IWarningMessageConfiguration { void SetMessageFlag(MessageFlag value); } Action<MessageFlag> messageFlagCapability X
- 31. Capability-based security in the real world
- 34. What does âaccess-controlâ mean? âą Preventing any access at all. âą Limiting access to some things only. âą Revoking access when you are no longer allowed. âą Granting and delegating access to some subset of things. Itâs not always about saying no!
- 35. Using capabilities to control access to a resource Secret Files Alice Bob X
- 36. Alternative for access control
- 37. Using auth/RBAC to control access to a resource Secret Files Alice Bob rejected accepted
- 38. A weakness in capabilities Secret Files Alice Bob ï Anyone with the key can get access.
- 39. A weakness in auth/RBAC Secret Files Alice Bob ï
- 40. A weakness in any security system! Secret Files Alice Bob Do you trust Alice? Then you also trust anyone whom Alice trusts too... (Or whom Alice can be tricked into trusting)
- 41. âDonât prohibit what you canât preventâ
- 43. Supplies Closet Auth/RBAC systems are centralized Alice Bob accepted rejected Bob has only been working there for 6 months ï
- 44. Auth/RBAC systems are centralized Alice âPlease grant Bob access to the supplies closetâ âThen revoke access after 20 minutesâ Authorization System Can your system do this?
- 45. Supplies Closet Auth/RBAC systems are centralized Alice Bob âHere, Bob. You can borrow my swipe cardâ Credential Sharing
- 46. Central systems can revoke access Secret Files Alice rejected
- 47. How to revoke access in a cap-based system? Revoke?
- 48. How to revoke access in a cap-based system?
- 49. Demo: Capabilities as functions
- 50. DESIGNING AN API USING CAPABILITIES
- 51. Client Service Request Response Tic-Tac-Toe as a service Proper name is "Noughts and Crosses" btw
- 52. Tic-Tac-Toe API (obvious version) type TicTacToeRequest = { player: Player row: Row col: Column }
- 53. Tic-Tac-Toe API (obvious version) type TicTacToeResponse = { result : MoveResult display: DisplayInfo } type MoveResult = | KeepPlaying | GameWon of Player | GameTied
- 54. Demo:Tic-Tac-Toe
- 55. What kind of errors can happen? âą A player can play an already played move âą A player can play twice in a row âą A player can forget to check the Result and keep playing
- 56. âMake illegal operations unrepresentableâ Donât let me do a bad thing and then tell me off for doing it... Yes, you could return errors, but...
- 57. Client Service New Game Available Moves Tic-Tac-Toe service with caps Nine available moves
- 58. Client Service 1st move Available Moves Tic-Tac-Toe service with caps Eight available moves
- 59. Client Service Available Moves Tic-Tac-Toe service with caps 2nd move Seven available moves
- 60. Client Service Available Moves Tic-Tac-Toe service with caps 3rd move Six available moves
- 61. Client Service No available Moves Tic-Tac-Toe service with caps Winning move
- 62. Tic-Tac-Toe API (cap-based version) type MoveCapability = unit -> TicTacToeResponse type NextMoveInfo = { playerToPlay : Player posToPlay : CellPosition capability : MoveCapability } These are for client information only. The player and position are baked into the capability
- 63. Tic-Tac-Toe API (cap-based version) type TicTacToeResponse = | KeepPlaying of (DisplayInfo * NextMoveInfo list) | GameWon of (DisplayInfo * Player) | GameTied of DisplayInfo
- 64. Tic-Tac-Toe API (cap-based version) type TicTacToeResponse = | KeepPlaying of (DisplayInfo * NextMoveInfo list) | GameWon of (DisplayInfo * Player) | GameTied of DisplayInfo Where did the "request" type go? Whereâs the authorization?
- 66. What kind of errors can happen? âą A player can play an already played move âą A player can play twice in a row âą A player can forget to check the Result and keep playing Is this good security or good design? All fixed now! ï
- 67. HATEOAS Hypermedia As The Engine Of Application State âA REST client needs no prior knowledge about how to interact with any particular application or server beyond a generic understanding of hypermedia.â RESTful done right
- 68. How NOT to do HATEOAS POST /customers/ GET /customer/42 If you can guess the API youâre doing it wrong Security problem! Also, a design problem â too much coupling.
- 69. How to do HATEOAS POST /81f2300b618137d21d / GET /da3f93e69b98 You can only know what URIs to use by parsing the page Each of these URIs is a capability
- 70. Demo: HATEOAS
- 71. Some Benefits of HATEOAS âą Client decoupled from server â The server owns the API model and can change it without breaking any clients â E.g. Change links to point to CDN â E.g. Versioning âą Simpler clients in many cases â No need for client-side checking of moves âą Explorable API â Choose your own adventure!
- 72. Client Service Login Available Capabilities Service API with caps Many available actions initially
- 73. Client Service Use a capability Available Capabilities Service API with caps Fewer available actions in a given state
- 74. CAPABILITY DRIVEN DESIGN Using these design techniques throughout your domain
- 75. Controller/ API Business Logic Database Client Where shall we put the authorization logic? But then any other path has complete access to the database ï
- 76. Controller/ API Business Logic Database Client Where shall we put the authorization logic? But then it doesnât have enough context ï
- 77. Controller/ API Business Logic Database Client Where shall we put the authorization logic? Global Authorizer Are you doing this already?
- 78. public class CustomerController : ApiController { readonly ICustomerDb _db; public CustomerController(ICustomerDb db) { _db = db; } [Route("customers/{customerId}")] [HttpGet] [GetCustomerProfileAuth] public IHttpActionResult Get(int customerId) { var custId = new CustomerId(customerId); var cust = _db.GetProfile(custId); var dto = DtoConverter.CustomerToDto(cust); return Ok(dto); }
- 79. public interface ICustomerDb { CustomerProfile GetProfile(CustomerId id); void UpdateProfile(CustomerId id, CustomerProfile cust); void CreateAccount(CustomerId id, CustomerProfile cust); void DeleteAccount(CustomerId id); void UpdateLoginEmail(CustomerId id, string email); void UpdatePassword(CustomerId id, string password); } How much authority do you really need?
- 80. Controller /API Business Logic Database Use Case Controller /API Business Logic Database Use Case Controller /API Business Logic Database Use Case Global Authorizer Every controller has one method
- 81. Controller /API Business Logic Database Use Case Controller /API Business Logic Database Use Case Controller /API Business Logic Database Use Case Global Authorizer Vertical slices Donât mention microservices
- 85. type MessageFlag = ShowThisMessageAgain | DontShowThisMessageAgain type ConfigurationCapabilities = { GetMessageFlag : unit -> MessageFlag SetMessageFlag : MessageFlag -> unit GetBackgroundColor : unit -> Color SetBackgroundColor : Color -> unit GetConnectionString : unit -> ConnectionString SetConnectionString : ConnectionString -> unit } â
- 86. let dontShowMessageAgainDialogBox capabilities = let getFlag,setFlag = capabilities let ctrl= new CheckBox( Text="Don't show this message again") ctrl.Checked <- getFlag() // etc
- 87. Demo: Vertical slices and delegation
- 89. Controller /API Business Logic Database Global Authorizer Passing tokens Overkill? Too complicated to implement?
- 90. Controller /API Business Logic Database Inject a value of specific type Global Authorizer Passing tokens as types Represent tokens by types!
- 91. Demo: Using types for access tokens
- 92. What have we covered? âą Donât use complicated security patterns â This will ensure that they are never used, or used wrong. â Donât rely on other people doing the right thing. â Donât rely on other people reading the documentation! âą Do use techniques where you get security for free. â You can be lazy! â You donât have to remember to do anything.
- 93. What have we covered? âą Donât develop first, add security later â Right after you implement the âqualityâ module âą Do use security-driven design â Bonus: get a modular architecture!
- 94. What have we covered? âą Donât only do security at the outermost layer âą Do use POLA everywhere, which ensures that you have minimal dependencies.
- 95. Common questions âą How do you pass these capabilities around? â Dependency injection or equivalent âą Are you saying that all external IO should be passed around as capabilities? â Yes!You should never access any ambient authority. â You should be doing this anyway for mocking.
- 96. Common questions âą Wonât there be too many parameters? â Less than you think! â Encourages vertical slices (per use-case, scenario) â âFunctional core, imperative shellâ âą Canât this be bypassed by reflection or other backdoors? â Yes. This is really all about design not about total security.
- 97. Resources for capability-based thinking âą LMGTFY âCapability based securityâ âą âLazy developers guide to secure computingâ talk by Marc Stiegler âą erights.org âą Google's Caja built over JavaScript âą Emily, a capability based language (via Ocaml)
- 98. Thanks! @ScottWlaschin fsharpforfunandprofit.com/cap Contact me Slides and video here Let us know if you need help with F#