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, Expiry, unix timestamp of when the jwt is no longer valid.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"),
ucans.capability.parse({
with: "wnfs://boris.fission.name/public/photos/",
can: "wnfs/OVERWRITE"
}),
{
with: ucans.capability.resourcePointer.parse("wnfs://boris.fission.name/public/photos/vacation/"),
can: ucans.capability.ability.parse("wnfs/REVISE")
},
{
with: { scheme: "wnfs", hierPart: "//boris.fission.name/public/photos/vacation/" },
can: { namespace: "wnfs", segments: [ "REVISE" ] }
}
]
})
// 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)
// Comparing capabilities
const a = {
with: { scheme: "scheme", hierPart: "hierPart" },
can: { namespace: "namespace", segments: [ "a", "B" ] }
}
const b = {
with: { scheme: "SCHEME", hierPart: "hierPart" },
can: { namespace: "NAMESPACE", segments: [ "A", "b" ] }
}
ucans.capability.isEqual(a, b)
ucans.capability.resourcePointer.isEqual(a.with, b.with)
ucans.capability.ability.isEqual(a.can, b.can)
Sponsors
Sponsors that contribute developer time or resources to this implementation of UCANs: