Sign-In with Ethereum Capabilities, ReCaps | Ethereum Improvement Proposals

Abstract

ERC-4361, or Sign-In with Ethereum (SIWE), describes how Ethereum accounts authenticate with off-chain services. This proposal, known as ReCaps, describes a mechanism on top of SIWE to give informed consent to authorize a Relying Party to exercise certain scoped capabilities. How a Relying Party authenticates against the target resource is out of scope for this specification and depends on the implementation of the target resource.

Motivation

SIWE ReCaps unlock integration of protocols and/or APIs for developers by reducing user friction, onchain state and increasing security by introducing informed consent and deterministic capability objects on top of Sign-In With Ethereum (ERC-4361).

While SIWE focuses on authenticating the Ethereum account against the service (relying party or SIWE client) initiating the SIWE flow, there is no canonical way for the authenticated Ethereum account to authorize a relying party to interact with a third-party service (resource service) on behalf of the Ethereum account. A relying party may want to interact with another service on behalf of the Ethereum account, for example a service that provides data storage for the Ethereum account. This specification introduces a mechanism that allows the service (or more generally a Relying Party) to combine authentication and authorization of such while preserving security and optimizing UX.

Note, this approach is a similar mechanism to combining OpenID Connect (SIWE auth) and OAuth2 (SIWE ReCap) where SIWE ReCap implements capabilities-based authorization on top of the authentication provided by SIWE.

Specification

This specification has three different audiences:

Web3 application developers that want to integrate ReCaps to authenticate with any protocols and APIs that support object capabilities.
Protocol or API developers that want to learn how to define their own ReCaps.
Wallet implementers that want to improve the UI for ReCaps.

Terms and Definitions

ReCap - A SIWE Message complying with this specification, i.e. containing at least one ReCap URI in the Resources section and the corresponding human-readable ReCap Statement appended to the SIWE statement.
ReCap URI - A type of URI that resolves to a ReCap Details Object.
ReCap Details Object - A JSON object describing the actions and optionally the resources associated with a ReCap Capability.
Resource Service (RS) - The entity that is providing third-party services for the Ethereum account.
SIWE Client (SC) - The entity initiating the authorization (SIWE authentication and ReCap flow).
Relying Party (RP) - same as SC in the context of authorization.

Overview

This specification defines the following:

ReCap SIWE Extension
ReCap Capability
- ReCap URI Scheme
- ReCap Details Object Schema
ReCap Translation Algorithm
ReCap Verification

ReCap SIWE Extension

A ReCap is an ERC-4361 message following a specific format that allows an Ethereum account to delegate a set of ReCap Capabilities to a Relying Party through informed consent. ReCap Capabilities MUST be represented by the final entry in the Resources array of the SIWE message that MUST deterministically translate the ReCap Capability in human-readable form to the statement field in the SIWE message using the ReCap Translation Algorithm.

The following SIWE message fields are used to further define (or limit) the scope of all ReCap Capabilities:

The URI field MUST specify the intended Relying Party, e.g., https://example.com, did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK. It is expected that the RS authenticates the Relying Party before invoking an action for the ReCap Capability.
The Issued At field MUST be used to specify the issuance date of the ReCap Capabilities.
If present, the Expiration Time field MUST be used as the expiration time of the ReCap Capabilities, i.e. the time at which the RS will no longer accept an invocation of the capabilities expressed in this form.
If present, the Not Before field MUST be used as the time that has to expire before the RS starts accepting invocations of the capabilities expressed in the message.

The following is a non-normative example of a SIWE message with the SIWE ReCap Extension:

example.com wants you to sign in with your Ethereum account:
0x0000000000000000000000000000000000000000

I further authorize the stated URI to perform the following actions on my behalf: (1) 'example': 'append', 'read' for 'https://example.com'. (2) 'other': 'action' for 'https://example.com'. (3) 'example': 'append', 'delete' for 'my:resource:uri.1'. (4) 'example': 'append' for 'my:resource:uri.2'. (5) 'example': 'append' for 'my:resource:uri.3'.

URI: did:key:example
Version: 1
Chain ID: 1
Nonce: mynonce1
Issued At: 2022-06-21T12:00:00.000Z
Resources:
- urn:recap:eyJhdHQiOnsiaHR0cHM6Ly9leGFtcGxlLmNvbSI6eyJleGFtcGxlL2FwcGVuZCI6W10sImV4YW1wbGUvcmVhZCI6W10sIm90aGVyL2FjdGlvbiI6W119LCJteTpyZXNvdXJjZTp1cmkuMSI6eyJleGFtcGxlL2FwcGVuZCI6W10sImV4YW1wbGUvZGVsZXRlIjpbXX0sIm15OnJlc291cmNlOnVyaS4yIjp7ImV4YW1wbGUvYXBwZW5kIjpbXX0sIm15OnJlc291cmNlOnVyaS4zIjp7ImV4YW1wbGUvYXBwZW5kIjpbXX19LCJwcmYiOltdfQ

ReCap Capability

A ReCap Capability is identified by their ReCap URI that resolves to a ReCap Details Object which defines the associated actions and optional target resources. The scope of each ReCap Capability is attenuated by common fields in the SIWE message as described in the previous chapter, e.g., URI, Issued At, Expiration Time, Not Before.

ReCap URI Scheme

A ReCap URI starts with urn:recap: followed by the unpadded base64url-encoded payload of the ReCap Details Object. Note, the term base64url is defined in RFC4648 - Base 64 Encoding with URL and Filename Safe Alphabet. If present, a Recap URI MUST occupy the final entry of the SIWE resource list.

The following is a non-normative example of a ReCap Capability:

urn:recap:eyJhdHQiOnsiaHR0cHM6Ly9leGFtcGxlLmNvbS9waWN0dXJlcy8iOnsiY3J1ZC9kZWxldGUiOlt7fV0sImNydWQvdXBkYXRlIjpbe31dLCJvdGhlci9hY3Rpb24iOlt7fV19LCJtYWlsdG86dXNlcm5hbWVAZXhhbXBsZS5jb20iOnsibXNnL3JlY2VpdmUiOlt7Im1heF9jb3VudCI6NSwidGVtcGxhdGVzIjpbIm5ld3NsZXR0ZXIiLCJtYXJrZXRpbmciXX1dLCJtc2cvc2VuZCI6W3sidG8iOiJzb21lb25lQGVtYWlsLmNvbSJ9LHsidG8iOiJqb2VAZW1haWwuY29tIn1dfX0sInByZiI6WyJ6ZGo3V2o2Rk5TNHJVVWJzaUp2amp4Y3NOcVpkRENTaVlSOHNLUVhmb1BmcFNadUF3Il19

Ability Strings

Ability Strings identify an action or Ability within a Namespace. They are serialized as <namespace>/<ability>. Namespaces and Abilities MUST contain only alphanumeric characters as well as the characters ., *, _, +, -, conforming to the regex ^[a-zA-Z0-9.*_+-]$. The ability string as a whole MUST conform to ^[a-zA-Z0-9.*_+-]+\/[a-zA-z0-9.*_+-]+$. For example, crud/update has an ability-namespace of crud and an ability-name of update.

ReCap Details Object Schema

The ReCap Details Object denotes which actions on which resources the Relying Party is authorized to invoke on behalf of the Ethereum account for the validity period defined in the SIWE message. It can also contain additional information that the RS may require to verify a capability invocation. A ReCap Details Object MUST follow the following JSON Schema:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "att": {
      "type": "object",
      "propertyNames": {
        "format": "uri"
      },
      "patternProperties": {
        "^.+:.*$": {
          "type": "object",
          "patternProperties": {
            "^[a-zA-Z0-9.*_+-]+\/[a-zA-z0-9.*_+-]+$": {
              "type": "array",
              "items": {
                "type": "object"
              }
            }
          },
          "additionalProperties": false,
          "minProperties": 1
        }
      },
      "additionalProperties": false,
      "minProperties": 1
    },
    "prf": {
      "type": "array",
      "items": {
        "type": "string",
        "format": "CID"
      },
      "minItems": 1
    }
  }
}

A ReCap Details Object defines the following properties:

att: (CONDITIONAL) If present, att MUST be a JSON object where each key is a URI and each value is an object containing Ability Strings as keys and a corresponding value which is an array of qualifications to the action (i.e. a restriction or requirement). The keys of the object MUST be ordered lexicographically.
prf: (CONDITIONAL) If present, prf MUST be a JSON array of string values with at least one entry where each value is a valid Base58-encoded CID which identifies a parent capability, authorizing the Ethereum account for one or more of the entries in att if the SIWE address does not identify the controller of the att entries.

Objects in the att field (including nested objects) MUST NOT contain duplicate keys and MUST have their keys ordered lexicographically with two steps:

Sort by byte value.
If a string starts with another, the shorter string comes first (e.g. msg/send comes before msg/send-to)

This is the same as the Array.sort() method in JavaScript. In the example below, crud/delete must appear before crud/update and other/action, similarly msg/receive must appear before msg/send.

The following is a non-normative example of a ReCap Capability Object with att and prf:

{
   "att":{
      "https://example.com/pictures/":{
         "crud/delete": [{}],
         "crud/update": [{}],
         "other/action": [{}]
      },
      "mailto:[email protected]":{
          "msg/receive": [{
              "max_count": 5,
              "templates": ["newsletter", "marketing"]
          }],
          "msg/send": [{ "to": "[email protected]" }, { "to": "[email protected]" }]
      }
   },
   "prf":["bafybeigk7ly3pog6uupxku3b6bubirr434ib6tfaymvox6gotaaaaaaaaa"]
}

In the example above, the Relying Party is authorized to perform the actions crud/update, crud/delete and other/action on resource https://example.com/pictures/ without limitations for any. Additionally the Relying Party is authorized to perform actions msg/send and msg/recieve on resource mailto:[email protected], where msg/send is limited to sending to [email protected] or [email protected] and msg/recieve is limited to a maximum of 5 and templates newsletter or marketing. Note, the Relying Party can invoke each action individually and independently from each other in the RS. Additionally the ReCap Capability Object contains some additional information that the RS will need during verification. The responsibility for defining the structure and semantics of this data lies with the RS. These action and restriction semantics are examples not intended to be universally understood. The Nota Bene objects appearing in the array associated with ability strings represent restrictions on use of an ability. An empty object implies that the action can be performed with no restrictions, but an empty array with no objects implies that there is no way to use this ability in a valid way.

It is expected that RS implementers define which resources they want to expose through ReCap Details Objects and which actions they want to allow users to invoke on them.

This example is expected to transform into the following recap-transformed-statement (for URI of https://example.com):

I further authorize the stated URI to perform the following actions on my behalf: (1) 'crud': 'delete', 'update' for 'https://example.com/pictures/'. (2) 'other': 'action' for 'https://example.com/pictures/'. (3) 'msg': 'receive', 'send' for 'mailto:[email protected]'.

This example is also expected to transform into the following recap-uri:

urn:recap:eyJhdHQiOnsiaHR0cHM6Ly9leGFtcGxlLmNvbS9waWN0dXJlcy8iOnsiY3J1ZC9kZWxldGUiOlt7fV0sImNydWQvdXBkYXRlIjpbe31dLCJvdGhlci9hY3Rpb24iOlt7fV19LCJtYWlsdG86dXNlcm5hbWVAZXhhbXBsZS5jb20iOnsibXNnL3JlY2VpdmUiOlt7Im1heF9jb3VudCI6NSwidGVtcGxhdGVzIjpbIm5ld3NsZXR0ZXIiLCJtYXJrZXRpbmciXX1dLCJtc2cvc2VuZCI6W3sidG8iOiJzb21lb25lQGVtYWlsLmNvbSJ9LHsidG8iOiJqb2VAZW1haWwuY29tIn1dfX0sInByZiI6WyJ6ZGo3V2o2Rk5TNHJVVWJzaUp2amp4Y3NOcVpkRENTaVlSOHNLUVhmb1BmcFNadUF3Il19

Merging Capability Objects

Any two Recap objects can be merged together by recursive concatenation of their field elements as long as the ordering rules of the field contents is followed. For example, two recap objects:

{
  "att": {
    "https://example1.com": {
      "crud/read": [{}]
    }
  },
  "prf": ["bafyexample1"]
}

{
  "att": {
    "https://example1.com": {
      "crud/update": [{
        "max_times": 1
      }]
    },
    "https://example2.com": {
      "crud/delete": [{}]
    }
  },
  "prf": ["bafyexample2"]
}

combine into:

{
  "att": {
    "https://example1.com": {
      "crud/read": [{}],
      "crud/update": [{
        "max_times": 1
      }]
    },
    "https://example2.com": {
      "crud/delete": [{}]
    }
  },
  "prf": ["bafyexample1", "bafyexample2"]
}

ReCap Translation Algorithm

After applying the ReCap Translation Algorithm on a given SIWE message that MAY include a pre-defined statement, the recap-transformed-statement in a ReCap SIWE message MUST conform to the following ABNF:

recap-transformed-statement = statement recap-preamble 1*(" " recap-statement-entry ".")
   ; see ERC-4361 for definition of input-statement
recap-preamble = "I further authorize the stated URI to perform the following actions on my behalf:"
recap-statement-entry = "(" number ") " action-namespace ": " 
                          action-name *("," action-name) "for"
                          recap-resource
   ; see RFC8259 for definition of number
ability-namespace = string
   ; see RFC8259 for definition of string
ability-name = string
   ; see RFC8259 for definition of string
recap-resource = string
   ; see RFC8259 for definition of string

The following algorithm or an algorithm that produces the same output MUST be performed to generate the SIWE ReCap Transformed Statement.

Inputs:

Let recap-uri be a ReCap URI, which represents the ReCap Capabilities that are to be encoded in the SIWE message, and which contains a ReCap Details Object which conforms to the ReCap Details Object Schema.
[Optional] Let statement be the statement field of the input SIWE message conforming to ERC-4361. Algorithm:
Let recap-transformed-statement be an empty string value.
If statement is present, do the following:
- Append the value of the statement field of siwe to recap-transformed-statement.
- Append a single space character " " to recap-transformed-statement.
Append the following string to recap-transformed-statement: "I further authorize the stated URI to perform the following actions on my behalf:".
Let numbering be an integer starting with 1.
Let attenuations be the att field of the ReCap Details Object
For each key and value pair in attenuations (starting with the first entry), perform the following:
- Let resource be the key and abilities be the value
- Group the keys of the abilities object by their ability-namespace
- For each ability-namespace, perform the following:
  - Append the string concatenation of " (", numbering, ")" to recap-transformed-statement.
  - Append the string concatenation of ', ability-namespace, ': to recap-transformed-statement.
  - For each ability-name in the ability-namespace group, perform the following:
    - Append the string concatenation of ', ability-name, ' to recap-transformed-statement
    - If not the final ability-name, append , to recap-transformed-statement
  - Append for ', resource, '. to recap-transformed-statement
  - Increase numbering by 1
Return recap-transformed-statement.

ReCap Verification Algorithm

The following algorithm or an algorithm that produces the same output MUST be performed to verify a SIWE ReCap.