diff --git a/go.mod b/go.mod
index 63f837a2..16a15862 100644
--- a/go.mod
+++ b/go.mod
@@ -8,7 +8,7 @@ require (
 	github.com/libp2p/go-libp2p-core v0.2.5
 	github.com/libp2p/go-libp2p-discovery v0.2.0
 	github.com/libp2p/go-libp2p-swarm v0.2.2
-	github.com/multiformats/go-multiaddr v0.2.0
+	github.com/multiformats/go-multiaddr v0.1.2
 	github.com/multiformats/go-multistream v0.1.0
 	github.com/whyrusleeping/timecache v0.0.0-20160911033111-cfcb2f1abfee
 )
diff --git a/go.sum b/go.sum
index 9b1654c8..e435ca9a 100644
--- a/go.sum
+++ b/go.sum
@@ -215,8 +215,6 @@ github.com/multiformats/go-multiaddr v0.1.1 h1:rVAztJYMhCQ7vEFr8FvxW3mS+HF2eY/oP
 github.com/multiformats/go-multiaddr v0.1.1/go.mod h1:aMKBKNEYmzmDmxfX88/vz+J5IU55txyt0p4aiWVohjo=
 github.com/multiformats/go-multiaddr v0.1.2 h1:HWYHNSyyllbQopmVIF5K7JKJugiah+L9/kuZKHbmNdQ=
 github.com/multiformats/go-multiaddr v0.1.2/go.mod h1:0nO36NvPpyV4QzvTLi/lafl2y95ncPj0vFwVF6k6wJ4=
-github.com/multiformats/go-multiaddr v0.2.0 h1:lR52sFwcTCuQb6bTfnXF6zA2XfyYvyd+5a9qECv/J90=
-github.com/multiformats/go-multiaddr v0.2.0/go.mod h1:0nO36NvPpyV4QzvTLi/lafl2y95ncPj0vFwVF6k6wJ4=
 github.com/multiformats/go-multiaddr-dns v0.0.1/go.mod h1:9kWcqw/Pj6FwxAwW38n/9403szc57zJPs45fmnznu3Q=
 github.com/multiformats/go-multiaddr-dns v0.0.2/go.mod h1:9kWcqw/Pj6FwxAwW38n/9403szc57zJPs45fmnznu3Q=
 github.com/multiformats/go-multiaddr-fmt v0.0.1/go.mod h1:aBYjqL4T/7j4Qx+R73XSv/8JsgnRFlf0w2KGLCmXl3Q=
diff --git a/gossipsub.go b/gossipsub.go
index 6327dfa0..d2d46f78 100644
--- a/gossipsub.go
+++ b/gossipsub.go
@@ -75,6 +75,8 @@ func (gs *GossipSubRouter) Protocols() []protocol.ID {
 func (gs *GossipSubRouter) Attach(p *PubSub) {
 	gs.p = p
 	gs.tracer = p.tracer
+	// start using the same msg ID function as PubSub for caching messages.
+	gs.mcache.ChangeMsgIdFn(p.msgID)
 	go gs.heartbeatTimer()
 }
 
diff --git a/mcache.go b/mcache.go
index 9f5e5dcd..cbb9a953 100644
--- a/mcache.go
+++ b/mcache.go
@@ -28,6 +28,7 @@ func NewMessageCache(gossip, history int) *MessageCache {
 		msgs:    make(map[string]*pb.Message),
 		history: make([][]CacheEntry, history),
 		gossip:  gossip,
+		msgID:   DefaultMsgIdFn,
 	}
 }
 
@@ -35,6 +36,11 @@ type MessageCache struct {
 	msgs    map[string]*pb.Message
 	history [][]CacheEntry
 	gossip  int
+	msgID   MsgIdFunction
+}
+
+func (mc *MessageCache) ChangeMsgIdFn(msgID MsgIdFunction) {
+	mc.msgID = msgID
 }
 
 type CacheEntry struct {
@@ -43,7 +49,7 @@ type CacheEntry struct {
 }
 
 func (mc *MessageCache) Put(msg *pb.Message) {
-	mid := msgID(msg)
+	mid := mc.msgID(msg)
 	mc.msgs[mid] = msg
 	mc.history[0] = append(mc.history[0], CacheEntry{mid: mid, topics: msg.GetTopicIDs()})
 }
diff --git a/mcache_test.go b/mcache_test.go
index 5616c218..e8a50f38 100644
--- a/mcache_test.go
+++ b/mcache_test.go
@@ -10,6 +10,7 @@ import (
 
 func TestMessageCache(t *testing.T) {
 	mcache := NewMessageCache(3, 5)
+	msgID := DefaultMsgIdFn
 
 	msgs := make([]*pb.Message, 60)
 	for i := range msgs {
diff --git a/pubsub.go b/pubsub.go
index 530c47a4..b1d872d9 100644
--- a/pubsub.go
+++ b/pubsub.go
@@ -117,6 +117,9 @@ type PubSub struct {
 	seenMessagesMx sync.Mutex
 	seenMessages   *timecache.TimeCache
 
+	// function used to compute the ID for a message
+	msgID MsgIdFunction
+
 	// key for signing messages; nil when signing is disabled (default for now)
 	signKey crypto.PrivKey
 	// source ID for signed messages; corresponds to signKey
@@ -208,6 +211,7 @@ func NewPubSub(ctx context.Context, h host.Host, rt PubSubRouter, opts ...Option
 		blacklist:             NewMapBlacklist(),
 		blacklistPeer:         make(chan peer.ID),
 		seenMessages:          timecache.NewTimeCache(TimeCacheDuration),
+		msgID:                 DefaultMsgIdFn,
 		counter:               uint64(time.Now().UnixNano()),
 	}
 
@@ -240,6 +244,20 @@ func NewPubSub(ctx context.Context, h host.Host, rt PubSubRouter, opts ...Option
 	return ps, nil
 }
 
+// MsgIdFunction returns a unique ID for the passed Message, and PubSub can be customized to use any
+// implementation of this function by configuring it with the Option from WithMessageIdFn.
+type MsgIdFunction func(pmsg *pb.Message) string
+
+// WithMessageIdFn is an option to customize the way a message ID is computed for a pubsub message.
+// The default ID function is DefaultMsgIdFn (concatenate source and seq nr.),
+// but it can be customized to e.g. the hash of the message.
+func WithMessageIdFn(fn MsgIdFunction) Option {
+	return func(p *PubSub) error {
+		p.msgID = fn
+		return nil
+	}
+}
+
 // WithPeerOutboundQueueSize is an option to set the buffer size for outbound messages to a peer
 // We start dropping messages to a peer if the outbound queue if full
 func WithPeerOutboundQueueSize(size int) Option {
@@ -326,7 +344,7 @@ func WithDiscovery(d discovery.Discovery, opts ...DiscoverOpt) Option {
 // WithEventTracer provides a tracer for the pubsub system
 func WithEventTracer(tracer EventTracer) Option {
 	return func(p *PubSub) error {
-		p.tracer = &pubsubTracer{tracer: tracer, pid: p.host.ID()}
+		p.tracer = &pubsubTracer{tracer: tracer, pid: p.host.ID(), msgID: p.msgID}
 		return nil
 	}
 }
@@ -730,8 +748,8 @@ func (p *PubSub) handleIncomingRPC(rpc *RPC) {
 	p.rt.HandleRPC(rpc)
 }
 
-// msgID returns a unique ID of the passed Message
-func msgID(pmsg *pb.Message) string {
+// DefaultMsgIdFn returns a unique ID of the passed Message
+func DefaultMsgIdFn(pmsg *pb.Message) string {
 	return string(pmsg.GetFrom()) + string(pmsg.GetSeqno())
 }
 
@@ -760,7 +778,7 @@ func (p *PubSub) pushMsg(msg *Message) {
 	}
 
 	// have we already seen and validated this message?
-	id := msgID(msg.Message)
+	id := p.msgID(msg.Message)
 	if p.seenMessage(id) {
 		p.tracer.DuplicateMessage(msg)
 		return
diff --git a/trace.go b/trace.go
index 293f1660..1f980536 100644
--- a/trace.go
+++ b/trace.go
@@ -18,6 +18,7 @@ type EventTracer interface {
 type pubsubTracer struct {
 	tracer EventTracer
 	pid    peer.ID
+	msgID  MsgIdFunction
 }
 
 func (t *pubsubTracer) PublishMessage(msg *Message) {
@@ -31,7 +32,7 @@ func (t *pubsubTracer) PublishMessage(msg *Message) {
 		PeerID:    []byte(t.pid),
 		Timestamp: &now,
 		PublishMessage: &pb.TraceEvent_PublishMessage{
-			MessageID: []byte(msgID(msg.Message)),
+			MessageID: []byte(t.msgID(msg.Message)),
 			Topics:    msg.Message.TopicIDs,
 		},
 	}
@@ -50,7 +51,7 @@ func (t *pubsubTracer) RejectMessage(msg *Message, reason string) {
 		PeerID:    []byte(t.pid),
 		Timestamp: &now,
 		RejectMessage: &pb.TraceEvent_RejectMessage{
-			MessageID:    []byte(msgID(msg.Message)),
+			MessageID:    []byte(t.msgID(msg.Message)),
 			ReceivedFrom: []byte(msg.ReceivedFrom),
 			Reason:       &reason,
 		},
@@ -70,7 +71,7 @@ func (t *pubsubTracer) DuplicateMessage(msg *Message) {
 		PeerID:    []byte(t.pid),
 		Timestamp: &now,
 		DuplicateMessage: &pb.TraceEvent_DuplicateMessage{
-			MessageID:    []byte(msgID(msg.Message)),
+			MessageID:    []byte(t.msgID(msg.Message)),
 			ReceivedFrom: []byte(msg.ReceivedFrom),
 		},
 	}
@@ -89,7 +90,7 @@ func (t *pubsubTracer) DeliverMessage(msg *Message) {
 		PeerID:    []byte(t.pid),
 		Timestamp: &now,
 		DeliverMessage: &pb.TraceEvent_DeliverMessage{
-			MessageID: []byte(msgID(msg.Message)),
+			MessageID: []byte(t.msgID(msg.Message)),
 		},
 	}
 
@@ -146,7 +147,7 @@ func (t *pubsubTracer) RecvRPC(rpc *RPC) {
 		Timestamp: &now,
 		RecvRPC: &pb.TraceEvent_RecvRPC{
 			ReceivedFrom: []byte(rpc.from),
-			Meta:         traceRPCMeta(rpc),
+			Meta:         t.traceRPCMeta(rpc),
 		},
 	}
 
@@ -165,7 +166,7 @@ func (t *pubsubTracer) SendRPC(rpc *RPC, p peer.ID) {
 		Timestamp: &now,
 		SendRPC: &pb.TraceEvent_SendRPC{
 			SendTo: []byte(rpc.from),
-			Meta:   traceRPCMeta(rpc),
+			Meta:   t.traceRPCMeta(rpc),
 		},
 	}
 
@@ -184,20 +185,20 @@ func (t *pubsubTracer) DropRPC(rpc *RPC, p peer.ID) {
 		Timestamp: &now,
 		DropRPC: &pb.TraceEvent_DropRPC{
 			SendTo: []byte(rpc.from),
-			Meta:   traceRPCMeta(rpc),
+			Meta:   t.traceRPCMeta(rpc),
 		},
 	}
 
 	t.tracer.Trace(evt)
 }
 
-func traceRPCMeta(rpc *RPC) *pb.TraceEvent_RPCMeta {
+func (t *pubsubTracer) traceRPCMeta(rpc *RPC) *pb.TraceEvent_RPCMeta {
 	rpcMeta := new(pb.TraceEvent_RPCMeta)
 
 	var msgs []*pb.TraceEvent_MessageMeta
 	for _, m := range rpc.Publish {
 		msgs = append(msgs, &pb.TraceEvent_MessageMeta{
-			MessageID: []byte(msgID(m)),
+			MessageID: []byte(t.msgID(m)),
 			Topics:    m.TopicIDs,
 		})
 	}
diff --git a/validation.go b/validation.go
index bef86da3..70b4cde2 100644
--- a/validation.go
+++ b/validation.go
@@ -201,7 +201,7 @@ func (v *validation) validate(vals []*topicVal, src peer.ID, msg *Message) {
 
 	// we can mark the message as seen now that we have verified the signature
 	// and avoid invoking user validators more than once
-	id := msgID(msg.Message)
+	id := v.p.msgID(msg.Message)
 	if !v.p.markSeen(id) {
 		v.tracer.DuplicateMessage(msg)
 		return