p2p/discover: new distance metric based on sha3(id)

The previous metric was pubkey1^pubkey2, as specified in the Kademlia
paper. We missed that EC public keys are not uniformly distributed.
Using the hash of the public keys addresses that. It also makes it
a bit harder to generate node IDs that are close to a particular node.

p2p/discover/table.go

@@ -7,20 +7,24 @@
 package discover
 
 import (
+	"crypto/rand"
 	"net"
 	"sort"
 	"sync"
 	"time"
 
+	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/logger"
 	"github.com/ethereum/go-ethereum/logger/glog"
 )
 
 const (
-	alpha               = 3              // Kademlia concurrency factor
-	bucketSize          = 16             // Kademlia bucket size
-	nBuckets            = nodeIDBits + 1 // Number of buckets
+	alpha      = 3  // Kademlia concurrency factor
+	bucketSize = 16 // Kademlia bucket size
+	hashBits   = len(common.Hash{}) * 8
+	nBuckets   = hashBits + 1 // Number of buckets
+
 	maxBondingPingPongs = 10
 )
 
@@ -116,21 +120,23 @@ func (tab *Table) Bootstrap(nodes []*Node) {
 // Lookup performs a network search for nodes close
 // to the given target. It approaches the target by querying
 // nodes that are closer to it on each iteration.
-func (tab *Table) Lookup(target NodeID) []*Node {
+// The given target does not need to be an actual node
+// identifier.
+func (tab *Table) Lookup(targetID NodeID) []*Node {
 	var (
+		target         = crypto.Sha3Hash(targetID[:])
 		asked          = make(map[NodeID]bool)
 		seen           = make(map[NodeID]bool)
 		reply          = make(chan []*Node, alpha)
 		pendingQueries = 0
 	)
-	// don't query further if we hit the target or ourself.
+	// don't query further if we hit ourself.
 	// unlikely to happen often in practice.
-	asked[target] = true
 	asked[tab.self.ID] = true
 
 	tab.mutex.Lock()
 	// update last lookup stamp (for refresh logic)
-	tab.buckets[logdist(tab.self.ID, target)].lastLookup = time.Now()
+	tab.buckets[logdist(tab.self.sha, target)].lastLookup = time.Now()
 	// generate initial result set
 	result := tab.closest(target, bucketSize)
 	tab.mutex.Unlock()
@@ -143,7 +149,7 @@ func (tab *Table) Lookup(target NodeID) []*Node {
 				asked[n.ID] = true
 				pendingQueries++
 				go func() {
-					r, _ := tab.net.findnode(n.ID, n.addr(), target)
+					r, _ := tab.net.findnode(n.ID, n.addr(), targetID)
 					reply <- tab.bondall(r)
 				}()
 			}
@@ -166,17 +172,16 @@ func (tab *Table) Lookup(target NodeID) []*Node {
 
 // refresh performs a lookup for a random target to keep buckets full.
 func (tab *Table) refresh() {
-	ld := -1 // logdist of chosen bucket
-	tab.mutex.Lock()
-	for i, b := range tab.buckets {
-		if i > 0 && b.lastLookup.Before(time.Now().Add(-1*time.Hour)) {
-			ld = i
-			break
-		}
-	}
-	tab.mutex.Unlock()
-
-	result := tab.Lookup(randomID(tab.self.ID, ld))
+	// The Kademlia paper specifies that the bucket refresh should
+	// perform a refresh in the least recently used bucket. We cannot
+	// adhere to this because the findnode target is a 512bit value
+	// (not hash-sized) and it is not easily possible to generate a
+	// sha3 preimage that falls into a chosen bucket.
+	//
+	// We perform a lookup with a random target instead.
+	var target NodeID
+	rand.Read(target[:])
+	result := tab.Lookup(target)
 	if len(result) == 0 {
 		// Pick a batch of previously know seeds to lookup with
 		seeds := tab.db.querySeeds(10)
@@ -196,7 +201,7 @@ func (tab *Table) refresh() {
 
 // closest returns the n nodes in the table that are closest to the
 // given id. The caller must hold tab.mutex.
-func (tab *Table) closest(target NodeID, nresults int) *nodesByDistance {
+func (tab *Table) closest(target common.Hash, nresults int) *nodesByDistance {
 	// This is a very wasteful way to find the closest nodes but
 	// obviously correct. I believe that tree-based buckets would make
 	// this easier to implement efficiently.
@@ -278,7 +283,8 @@ func (tab *Table) bond(pinged bool, id NodeID, addr *net.UDPAddr, tcpPort uint16
 	}
 	tab.mutex.Lock()
 	defer tab.mutex.Unlock()
-	if b := tab.buckets[logdist(tab.self.ID, n.ID)]; !b.bump(n) {
+	b := tab.buckets[logdist(tab.self.sha, n.sha)]
+	if !b.bump(n) {
 		tab.pingreplace(n, b)
 	}
 	return n, nil
@@ -346,7 +352,7 @@ outer:
 			// don't add self.
 			continue
 		}
-		bucket := tab.buckets[logdist(tab.self.ID, n.ID)]
+		bucket := tab.buckets[logdist(tab.self.sha, n.sha)]
 		for i := range bucket.entries {
 			if bucket.entries[i].ID == n.ID {
 				// already in bucket
@@ -375,13 +381,13 @@ func (b *bucket) bump(n *Node) bool {
 // distance to target.
 type nodesByDistance struct {
 	entries []*Node
-	target  NodeID
+	target  common.Hash
 }
 
 // push adds the given node to the list, keeping the total size below maxElems.
 func (h *nodesByDistance) push(n *Node, maxElems int) {
 	ix := sort.Search(len(h.entries), func(i int) bool {
-		return distcmp(h.target, h.entries[i].ID, n.ID) > 0
+		return distcmp(h.target, h.entries[i].sha, n.sha) > 0
 	})
 	if len(h.entries) < maxElems {
 		h.entries = append(h.entries, n)