Ucans NPM | npm.io

ts-ucan

UCANs are JWTs that contain special keys.

At a high level, UCANs (“User Controlled Authorization Network”) are an authorization scheme ("what you can do") where users are fully in control. UCANs use DIDs ("Decentralized Identifiers") to identify users and services ("who you are").

No all-powerful authorization server or server of any kind is required for UCANs. Instead, everything a user can do is captured directly in a key or token, which can be sent to anyone who knows how to interpret the UCAN format. Because UCANs are self-contained, they are easy to consume permissionlessly, and they work well offline and in distributed systems.

UCANs work

Server → Server
Client → Server
Peer-to-peer

OAuth is designed for a centralized world, UCAN is the distributed user-controlled version.

Read more in the whitepaper: https://whitepaper.fission.codes/access-control/ucan

Structure

Header

alg, Algorithm, the type of signature.

typ, Type, the type of this data structure, JWT.

uav, UCAN version.

Payload

att, Attenuation, a list of resources and capabilities that the ucan grants.

aud, Audience, the DID of who it's intended for.

exp, Expiry, unix timestamp of when the jwt is no longer valid.

fct, Facts, an array of extra facts or information to attach to the jwt.

iss, Issuer, the DID of who sent this.

nbf, Not Before, unix timestamp of when the jwt becomes valid.

prf, Proof, an optional nested token with equal or greater privileges.

Signature

A signature (using alg) of the base64 encoded header and payload concatenated together and delimited by .

Build

ucan.build can be used to help in formatting and signing a UCAN. It takes the following parameters:

type BuildParams = {
  // from/to
  issuer: Keypair
  audience: string

  // capabilities
  capabilities?: Array<Capability>

  // time bounds
  lifetimeInSeconds?: number // expiration overrides lifetimeInSeconds
  expiration?: number
  notBefore?: number

  // proofs / other info
  facts?: Array<Fact>
  proofs?: Array<string>
  addNonce?: boolean
}

Capabilities

capabilities is an array of resource pointers and abilities:

{
  // `with` is a resource pointer in the form of a URI, which has a `scheme` and `hierPart`.
  // → "mailto:boris@fission.codes"
  with: { scheme: "mailto", hierPart: "boris@fission.codes" },

  // `can` is an ability, which always has a namespace and optional segments.
  // → "msg/SEND"
  can: { namespace: "msg", segments: [ "SEND" ] }
}

Installation

NPM:

npm install --save ucans

yarn:

yarn add ucans

Example

import * as ucans from "ucans"

// in-memory keypair
const keypair = await ucans.EdKeypair.create()
const ucan = await ucans.build({
  audience: "did:key:zabcde...", // recipient DID
  issuer: keypair, // signing key
  capabilities: [ // permissions for ucan
    {
      with: { scheme: "wnfs", hierPart: "//boris.fission.name/public/photos/" },
      can: { namespace: "wnfs", segments: [ "OVERWRITE" ] }
    },
    {
      with: { scheme: "wnfs", hierPart: "//boris.fission.name/private/4tZA6S61BSXygmJGGW885odfQwpnR2UgmCaS5CfCuWtEKQdtkRnvKVdZ4q6wBXYTjhewomJWPL2ui3hJqaSodFnKyWiPZWLwzp1h7wLtaVBQqSW4ZFgyYaJScVkBs32BThn6BZBJTmayeoA9hm8XrhTX4CGX5CVCwqvEUvHTSzAwdaR" },
      can: { namespace: "wnfs", segments: [ "APPEND" ] }
    },
    {
      with: { scheme: "mailto", hierPart: "boris@fission.codes" },
      can: { namespace: "wnfs", segments: [ "SEND" ] }
    }
  ]
})
const token = ucans.encode(ucan) // base64 jwt-formatted auth token

// You can also use your own signing function if you're bringing your own key management solution
const payload = await ucans.buildPayload(...)
const ucan = await ucans.sign(payload, keyType, signingFn)

Validating

import * as ucans from "ucans"

const ucan = ucans.build({ ... })

ucans.isExpired(ucan)
ucans.isTooEarly(ucan)
ucans.validate(ucans.encode(ucan)) // checks signature, issuer key type, and the above.

Capabilities

import * as ucans from "ucans"

// Utility functions to create capabilities
const ucan = ucans.build({
  audience: "did:key:zabcde...",
  issuer: keypair,
  capabilities: [
    ucans.capability.my("resource"),
    {
      with: ucans.capability.resourcePointer.parse("wnfs://boris.fission.name/public/photos/"),
      can: ucans.capability.ability.parse("wnfs/OVERWRITE")
    }
  ]
})

// Capability semantics
const SEMANTICS = {
  // wether or not to use the default capability structure
  // (this would parse a regular capability into a custom one)
  tryParsing: a => a,

  // can a given child capability be delegated from a parent capability?
  tryDelegating: (parentCap, childCap) => {
    if (childCap.with.scheme !== "wnfs") return null

    // we've got access to everything
    if (parentCap.with.hierPart === ucans.capability.superUser.SUPERUSER) return childCap

    // path must be the same or a path below
    if (childCap.with.hierPart.startsWith(parentCap.with.hierPart)) return childCap

    // 🚨 cannot delegate
    return null
  }
}

// Capability checking
const nowInSeconds = Math.floor(Date.now() / 1000)
const result = ucans.hasCapability(
  SEMANTICS,
  {
    info: {
      originator: keypair.did(),  // capability must have been originated from this issuer
      expiresAt: nowInSeconds,    // ucan must not have been expired before this timestamp
      notBefore: nowInSeconds     // optional
    },
    capability: {
      with: ucans.capability.resourcePointer.parse("wnfs://boris.fission.name/public/photos/vacation/"),
      can: ucans.capability.ability.parse("wnfs/REVISE")
    }
  },
  ucans.Chained.fromToken(ucans.encode(ucan))
)

if (result === false) log("UCAN does not have this capability 🚨")
else log("UCAN has the capability ✅ Info:", result.info, "Capability:", result.capability)