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attenuation.ts
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attenuation.ts
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// https://whitepaper.fission.codes/access-control/ucan/jwt-authentication#attenuation
import { Capability, Ucan } from "./types"
import { Chained } from "./chained"
import * as util from "./util"
export interface CapabilitySemantics<A> {
/**
* Try to parse a capability into a representation used for
* delegation & returning in the `capabilities` call.
*
* If the capability doesn't seem to match the format expected
* for the capabilities with the semantics currently defined,
* return `null`.
*/
tryParsing(cap: Capability): A | null
/**
* This figures out whether a given `childCap` can be delegated from `parentCap`.
* There are three possible results with three return types respectively:
* - `A`: The delegation is possible and results in the rights returned.
* - `null`: The capabilities from `parentCap` and `childCap` are unrelated and can't be compared nor delegated.
* - `CapabilityEscalation<A>`: It's clear that `childCap` is meant to be delegated from `parentCap`, but there's a rights escalation.
*/
tryDelegating(parentCap: A, childCap: A): A | null | CapabilityEscalation<A>
// TODO builders
}
export interface CapabilityInfo {
originator: string // DID
expiresAt: number
notBefore?: number
}
export interface CapabilityWithInfo<A> {
info: CapabilityInfo
capability: A
}
export type CapabilityResult<A>
= CapabilityWithInfo<A>
| CapabilityEscalation<A>
export interface CapabilityEscalation<A> {
escalation: string // reason
capability: A // the capability that escalated rights
}
export function isCapabilityEscalation<A>(obj: unknown): obj is CapabilityEscalation<A> {
return util.isRecord(obj)
&& util.hasProp(obj, "escalation") && typeof obj.escalation === "string"
&& util.hasProp(obj, "capability")
}
export function canDelegate<A>(semantics: CapabilitySemantics<A>, capability: A, ucan: Chained): boolean {
for (const cap of capabilities(ucan, semantics)) {
if (isCapabilityEscalation(cap)) {
continue
}
const delegated = semantics.tryDelegating(cap.capability, capability)
if (isCapabilityEscalation(delegated)) {
continue
}
if (delegated != null) {
return true
}
}
return false
}
export function capabilities<A>(
ucan: Chained,
capability: CapabilitySemantics<A>,
): Iterable<CapabilityResult<A>> {
function* findParsingCaps(ucan: Ucan<never>): Iterable<CapabilityWithInfo<A>> {
const capInfo = parseCapabilityInfo(ucan)
for (const cap of ucan.payload.att) {
const parsedCap = capability.tryParsing(cap)
if (parsedCap != null) yield { info: capInfo, capability: parsedCap }
}
}
const delegate = (ucan: Ucan<never>, capabilitiesInProofs: () => Iterable<() => Iterable<CapabilityResult<A>>>) => {
return function* () {
for (const parsedChildCap of findParsingCaps(ucan)) {
let isCoveredByProof = false
for (const capabilitiesInProof of capabilitiesInProofs()) {
for (const parsedParentCap of capabilitiesInProof()) {
// pass through capability escalations from parents
if (isCapabilityEscalation(parsedParentCap)) {
yield parsedParentCap
} else {
// try figuring out whether we can delegate the capabilities from this to the parent
const delegated = capability.tryDelegating(parsedParentCap.capability, parsedChildCap.capability)
// if the capabilities *are* related, then this will be non-null
// otherwise we just continue looking
if (delegated != null) {
// we infer that the capability was meant to be delegated
isCoveredByProof = true
// it's still possible that that delegation was invalid, i.e. an escalation, though
if (isCapabilityEscalation(delegated)) {
yield delegated // which is an escalation
} else {
yield {
info: delegateCapabilityInfo(parsedChildCap.info, parsedParentCap.info),
capability: delegated
}
}
}
}
}
}
// If a capability can't be considered to be delegated by any of its proofs
// (or if there are no proofs),
// then we root its origin in the UCAN we're looking at.
if (!isCoveredByProof) {
yield parsedChildCap
}
}
}
}
return ucan.reduce(delegate)()
}
function delegateCapabilityInfo(childInfo: CapabilityInfo, parentInfo: CapabilityInfo): CapabilityInfo {
let notBefore = {}
if (childInfo.notBefore != null && parentInfo.notBefore != null) {
notBefore = { notBefore: Math.max(childInfo.notBefore, parentInfo.notBefore) }
} else if (parentInfo.notBefore != null) {
notBefore = { notBefore: parentInfo.notBefore }
} else {
notBefore = { notBefore: childInfo.notBefore }
}
return {
originator: parentInfo.originator,
expiresAt: Math.min(childInfo.expiresAt, parentInfo.expiresAt),
...notBefore,
}
}
function parseCapabilityInfo(ucan: Ucan<never>): CapabilityInfo {
return {
originator: ucan.payload.iss,
expiresAt: ucan.payload.exp,
...(ucan.payload.nbf != null ? { notBefore: ucan.payload.nbf } : {}),
}
}