Implement WoT algorithm
This commit is contained in:
Heiko Schaefer
parent
389c99cf1b
commit
cee061d01c
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// SPDX-FileCopyrightText: 2023 Heiko Schaefer <heiko@schaefer.name>
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//
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// SPDX-License-Identifier: Apache-2.0
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package org.pgpainless.wot.dijkstra
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import org.pgpainless.wot.network.*
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import org.pgpainless.wot.query.Path
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import org.pgpainless.wot.query.Paths
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import org.slf4j.Logger
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import org.slf4j.LoggerFactory
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import kotlin.math.max
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// The trust amount that is considered "fully trusted"
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private const val FULLY_TRUSTED = 120
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class Query(private val rawNetwork: Network,
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private val roots: Roots,
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private val certificationNetwork: Boolean) {
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private val logger: Logger = LoggerFactory.getLogger(Query::class.java)
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/**
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* Authenticate the binding "targetFpr <-> targetUserid".
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*
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* Performs an OpenPGP "Web of Trust" query, following the semantics and approach described in
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* https://gitlab.com/sequoia-pgp/sequoia-wot/-/blob/main/spec/sequoia-wot.md
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*
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* Searches for enough paths to satisfy `targetTrustAmount`, if available.
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*/
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fun authenticate(targetFpr: Fingerprint, targetUserid: String, targetTrustAmount: Int): Paths {
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logger.debug("Authenticating <{}, '{}'>\nRoots: {}", targetFpr, targetUserid, roots)
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// Wrap the raw Network in a WotNetwork
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val network = WotNetwork(rawNetwork, certificationNetwork)
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// FIXME: add roots to the WotNetwork? -> handle their trust amounts from there (-> drop suppressIssuer?)
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if (!certificationNetwork) {
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// We're building a regular authentication network.
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// Set trust amount cap for any root that is not FULLY_TRUSTED
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roots.roots()
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.filter { it.amount != FULLY_TRUSTED }
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.forEach { network.capCertificate(it.fingerprint, it.amount) }
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// FIXME: If A is a fully trusted root, and B is a root at trust amount 40, should B's amount in
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// the path A -> B -> C be capped?
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//
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// Theory: The cap should only be in effect for paths in which B serves as a root.
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// -> Make a test case for this, and fix it. (How?)
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// (Maybe the order in which paths are usually found will shadow this problem most of the time?)
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}
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val paths = Paths()
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// Perform a (partial, until the targetTrustAmount is reached) run of the Ford-Fulkerson algorithm
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// (https://en.wikipedia.org/wiki/Ford%E2%80%93Fulkerson_algorithm):
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//
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// Find a path, subtract that path from the network, then loop and search for more paths, if any.
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while (paths.amount < targetTrustAmount) {
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val authPaths = backwardPropagate(network, targetFpr, targetUserid)
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// Pick one of the paths returned by backwardPropagate(), first by trust amount, then by length.
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// We subtract that path from the network and search again, if we haven't yet reached 'targetTrustAmount'.
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val bestPath = roots.fingerprints()
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.mapNotNull { authPaths[it] } // Only consider paths that start at a root.
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.maxWithOrNull(compareBy(
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{ it.second }, // We want paths with the *largest* trust amount,
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{ -it.first.length }, // and of these, the *shortest* path.
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{ it.first.root.fingerprint } // Break ties based on the fingerprint of the root.
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))
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if (bestPath != null) {
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val (path, amount) = bestPath
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assert(path.length > 1) // We don't support paths without an edge!
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network.suppressPath(path, amount) // Subtract the path from the residual network
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paths.add(path, amount) // Add the path to the set of results
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} else {
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// We made no progress in this iteration, there are no more paths to be found. We're done.
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break
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}
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}
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return paths
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}
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// FIXME: This should not be public, but is currently needed for the `BackPropagationTest` suite.
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fun backwardPropagate(targetFpr: Fingerprint, targetUserid: String): HashMap<Fingerprint, Pair<Path, Int>> {
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val network = WotNetwork(rawNetwork, false) // these tests want authentication networks
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return backwardPropagate(network, targetFpr, targetUserid)
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}
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/**
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* Finds a path in the network from one or multiple `roots` that
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* authenticates the target binding.
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*
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* If `roots` is empty, authenticated paths starting from any node
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* are returned.
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*/
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private fun backwardPropagate(network: WotNetwork, targetFpr: Fingerprint, targetUserid: String):
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HashMap<Fingerprint, Pair<Path, Int>> {
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logger.debug("Query.backwardPropagate <{}, '{}'>\nRoots: {}", targetFpr, targetUserid, roots)
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// If the Network tells us we can't use this node as a target, return early
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val target = network.isValidTarget(targetFpr, targetUserid) ?: return HashMap()
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// Perform Dijkstra's shortest path algorithm using a priority queue.
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// https://en.wikipedia.org/wiki/Dijkstra's_algorithm#Using_a_priority_queue
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// We are processing the OpenPGP certification graph in the backwards direction
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// (working from the target binding to the trust roots).
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// Note: The first step in the (reverse) paths we consider is always a certification of a User ID binding.
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// All other steps must be delegations with the minimum depth appropriate to the path's length.
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val prev: HashMap<Fingerprint, ForwardPointer> = HashMap()
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val dist: HashMap<Fingerprint, Cost> = HashMap()
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val queue: PairPriorityQueue<Fingerprint, Cost> = PairPriorityQueue()
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// Does a self-sig for the target exist?
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val selfSig = network.getSelfSig(targetFpr, targetUserid)
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val selfSigAmount = if (selfSig != null) network.getEffectiveTrustAmount(selfSig) else 0
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if (selfSig != null && selfSigAmount > 0) {
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// The first step of the (backwards-facing) path:
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// Arriving indirectly to the targetNode's User ID, via a delegation plus a self-signed binding
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val cost = Cost(1, selfSigAmount)
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prev[targetFpr] = ForwardPointer(selfSig)
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dist[targetFpr] = cost
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queue.insertOrUpdate(targetFpr, cost)
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} else {
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// The first step of the (backwards-facing) path:
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// We will arrive directly at the targetNode's relevant User ID, via a third party certification
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val cost = Cost(0, FULLY_TRUSTED)
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prev[targetFpr] = ForwardPointer(null)
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dist[targetFpr] = cost
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queue.insertOrUpdate(targetFpr, cost)
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}
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// Process the priority queue until it is empty.
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while (true) {
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// To be safe, we're not using the cost from the priority queue (which is available in `pop().second`).
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// It could have been updated in the meantime (?)
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val signeeFpr = queue.pop()?.first ?: break
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logger.debug("Processing signee {}", signeeFpr)
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val root = roots.get(signeeFpr)
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if ((root != null) && (root.amount >= FULLY_TRUSTED)) {
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logger.debug(" Skipping signee that is a fully trusted root")
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continue
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}
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val signee = network.nodeByFpr(signeeFpr)!! // We expect that the signee exists in the Network
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// Get the signee's current forward pointer (the edge that currently points at the signee)
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val signeeFp: ForwardPointer = prev[signeeFpr]!!
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// ... and the current cost/"distance" from the signee to the target
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val signeeCost = dist[signeeFpr]!!
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logger.debug(" Current forward pointer: {}", signeeFp.next?.target)
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logger.debug(" Current cost to target: {}", signeeCost)
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// Get certifications that point at signeeFpr
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// (with the necessary minimum depth, and matching the targetUserid to the certification regex, if any)
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// We are considering two different possibilities: extending the existing path, or effectively
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// *replacing* the existing path. The latter happens if our path currently consists of a self-sig,
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// but we're "replacing" that path with a third party sig.
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// XX: ask for "depth=n-1" to be able to replace a terminating self-sig with a depth 0 third party sig.
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// This requires additional checking below. Can this be simplified?
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val curMinLen = max(0, signeeCost.length - 1)
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val ecs = network.certificationsForSignee(signeeFpr, targetUserid, curMinLen)
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logger.debug(" Checking {} certifications for {}:", ecs.size, signee.toString())
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for (ec in ecs) {
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logger.debug(" Certification by {}", ec.issuer.fingerprint)
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val amount = network.getEffectiveTrustAmount(ec)
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if (amount == 0) {
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logger.debug(" Skipping (effective trust amount is 0)")
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continue
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}
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if (signeeFpr == targetFpr && ec.userId != targetUserid
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// Matching User ID only matters for the last hop
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&& signeeCost.length == 0) {
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logger.debug(" Certification is for the wrong user id ({})", ec.userId)
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continue
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}
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val altCost = if (ec.userId == targetUserid) {
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// This path replaces a direct signature
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Cost(1, amount)
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} else {
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// XX: temp hack, see above
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if (ec.trustDepth < signeeCost.length) {
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logger.debug(" Certification does not allow enough depth ({}, needed: {}), skipping",
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ec.trustDepth, signeeCost.length)
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continue
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}
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signeeCost.extendBy(amount)
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}
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logger.debug(" Cost to target via {}: {}", ec.target.fingerprint, altCost)
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val issuerFpr = ec.issuer.fingerprint
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val currentCost: Cost? = dist[issuerFpr]
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// If we haven't visited this node before, or the new cost is preferable, store or update pointer+cost
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if (currentCost == null || altCost < currentCost) {
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logger.debug(" Setting forward pointer for {}: {}", ec.issuer, ec.target)
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if (currentCost != null)
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logger.debug(" (Replaces previous path with cost {})", currentCost)
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prev[issuerFpr] = ForwardPointer(ec)
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dist[issuerFpr] = altCost
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}
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if (currentCost == null) {
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// We haven't seen this node before -> queue it for processing
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logger.debug(" Queuing node {}", ec.issuer)
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queue.insertOrUpdate(issuerFpr, altCost)
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}
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}
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}
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return reconstructPaths(network, targetUserid, prev, target, dist)
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}
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private fun reconstructPaths(network: WotNetwork, targetUserid: String, bestNextNode: HashMap<Fingerprint, ForwardPointer>, target: Node, dist: HashMap<Fingerprint, Cost>): HashMap<Fingerprint, Pair<Path, Int>> {
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// Follow the forward pointers and reconstruct the paths.
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val paths: HashMap<Fingerprint, Pair<Path, Int>> = HashMap()
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bestNextNode.entries
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// If roots were specified, only reconstruct paths for roots
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.filter { roots.roots().isEmpty() || roots.isRoot(it.key) }
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// Don't consider nodes that specify no "next" edge
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.filter { it.value.next != null }
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.forEach { (issuerFpr, fp) ->
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// Next is guaranteed to be non-null by the filter above
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val issuer = fp.next!!.issuer
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logger.trace("Recovering path starting at {}", network.nodeByFpr(issuerFpr))
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val path = assemblePath(target, targetUserid, issuer, bestNextNode)
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val amount = dist[issuer.fingerprint]!!.amount
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logger.debug("Authenticated <{}, {}>:\n{}", target.fingerprint, targetUserid, path)
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paths[issuerFpr] = Pair(path, amount)
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}
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return paths
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}
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private fun assemblePath(target: Node, targetUserid: String,
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issuer: Node, bestNextNode: HashMap<Fingerprint, ForwardPointer>): Path {
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var fp = bestNextNode[issuer.fingerprint]!!
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// Path starts at the root (issuer); the last edge points to the target.
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val p = Path(issuer)
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while (true) {
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val ec = fp.next ?: break
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p.append(ec)
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if (ec.userId == targetUserid) {
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// We've arrived (the target node may have an extra self-sig forward pointer to itself, but we don't
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// want to collect that edge).
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break
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}
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fp = bestNextNode[ec.target.fingerprint]!!
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}
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assert(p.certifications.isNotEmpty())
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logger.trace("\nAssembled path from {} to <{} <-> {}>:\n {}",
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issuer.fingerprint, targetUserid, target.fingerprint,
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p.certifications.withIndex().joinToString("\n ") { (i, c) -> "$i: $c" })
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return p
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}
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}
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@ -0,0 +1,149 @@
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// SPDX-FileCopyrightText: 2023 Heiko Schaefer <heiko@schaefer.name>
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//
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// SPDX-License-Identifier: Apache-2.0
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package org.pgpainless.wot.dijkstra
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import org.pgpainless.wot.network.*
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import org.pgpainless.wot.query.Path
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/**
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* A wrapper for `Network` that performs the following functions:
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*
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* - Expose an (experimental) API that simplifies the implementation of the WoT algorithm (in `Query`)
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* - TODO: polish, and possibly upstream into `Network`
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*
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* - Form residual networks for the Ford-Fulkerson algorithm via `suppressPath()`
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* - `getEffectiveTrustAmount()` returns effective trust amounts of certifications
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* (taking the residual network into account)
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*/
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internal class WotNetwork(private val network: Network, private val certificationNetwork: Boolean) {
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fun nodeByFpr(fpr: Fingerprint): Node? = network.nodes[fpr]
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fun getSelfSig(targetFpr: Fingerprint, targetUserid: String): EdgeComponent? {
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val target = nodeByFpr(targetFpr)
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if (target?.userIds?.get(targetUserid) != null) {
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// Return a synthesized self-binding.
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// XX: This EC should be generated during network generation.
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return EdgeComponent(target, target, targetUserid, network.referenceTime.timestamp,
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null, true, 120, Depth.limited(0), RegexSet.wildcard())
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} else
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return null
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}
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/**
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* Return list of EdgeComponents that point to `fpr`.
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*
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* Doesn't currently return self-bindings.
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*
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* `targetUserid` is currently only evaluated to check matching with regex scoped delegations.
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* Doesn't currently filter out EdgeComponents that point to `fpr`, but a different User ID
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* (FIXME: filter by target User ID, if fpr == target, and target user id != null, then simplify Query?)
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*
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* In authentication network mode:
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* - only return EdgeComponents whose depth allows authentication of a path that is `curLen` long.
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* - only return EdgeComponents whose regexes match `targetUserid`.
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*/
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fun certificationsForSignee(fpr: Fingerprint, targetUserid: String, curLen: Int): List<EdgeComponent> {
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val edges = network.reverseEdges[fpr] ?: return listOf()
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val ec = edges.map { edge ->
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edge.components.map { it.value }.flatten()
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}
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.flatten()
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.filter {
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if (!certificationNetwork) {
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// Authentication network mode: honor depth limitation and regexes
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it.trustDepth >= curLen && it.regexes.matches(targetUserid)
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} else {
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// Certification network mode: Keep certifications of any depth, and ignore regex scoping
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true
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}
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}
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return ec
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}
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fun isValidTarget(targetFpr: Fingerprint, targetUserid: String): Node? {
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// Node must be in the network.
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val target = nodeByFpr(targetFpr) ?: return null
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// Target may not be expired at the reference time.
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if ((target.expirationTime != null) &&
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(target.expirationTime <= network.referenceTime.timestamp)) {
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return null
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}
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// Target may not be revoked at the reference time.
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if (target.revocationState.isEffective(network.referenceTime)) {
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return null
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}
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// The target doesn't need to have self-signed the User ID to authenticate the User ID.
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// But if the target has revoked it, then it can't be authenticated.
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val targetUa: RevocationState? = target.userIds[targetUserid]
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if (targetUa != null && targetUa.isEffective(network.referenceTime)) {
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return null
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}
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return target
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}
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// Modifiers are processed in order by getEffectiveTrustAmount() [first cap, then suppress]
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private val capCertificate: HashMap<Fingerprint, Int> = HashMap()
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private val suppressPath: HashMap<Pair<Fingerprint, Fingerprint>, Int> = HashMap()
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/**
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* Get effective trust amount for a certification:
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* Cap if the root has limited trust, and take into account additional constraints of the residual network.
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*/
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fun getEffectiveTrustAmount(ec: EdgeComponent): Int {
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// Start from trust amount on the certification
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var amount = ec.trustAmount
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// Cap to issuer's `capCertificate`, if set.
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// (Used in case of less than fully trusted roots)
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capCertificate[ec.issuer.fingerprint]?.let {
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if (it < amount) {
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amount = it
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}
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}
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// Suppress by certificate (for Ford-Fulkerson residual network)
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suppressPath[Pair(ec.issuer.fingerprint, ec.target.fingerprint)]?.let {
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if (amount > it) {
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amount -= it
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} else {
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amount = 0
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}
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}
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return amount
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}
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/** Limit a certificate's initial trust amount to `amount` */
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fun capCertificate(fingerprint: Fingerprint, amount: Int) {
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capCertificate[fingerprint] = amount
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}
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/**
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* Add suppression rules for all certifications along the specified path:
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* Each edge is suppressed by `amountToSuppress`.
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*/
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fun suppressPath(path: Path, amountToSuppress: Int) {
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if (amountToSuppress == 0) return
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assert(amountToSuppress <= 120)
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for (c in path.certifications) {
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val curAmount = suppressPath[Pair(c.issuer.fingerprint, c.target.fingerprint)] ?: 0
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val newAmount = curAmount + amountToSuppress
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assert(newAmount <= 120)
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suppressPath[Pair(c.issuer.fingerprint, c.target.fingerprint)] = newAmount
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}
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}
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}
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