diff --git a/data/acltreebuilder.go b/data/acltreebuilder.go
index 99289d41..1308ec43 100644
--- a/data/acltreebuilder.go
+++ b/data/acltreebuilder.go
@@ -36,7 +36,7 @@ func (tb *ACLTreeBuilder) Init() {
 }
 
 func (tb *ACLTreeBuilder) Build() (*Tree, error) {
-	heads := tb.thread.Heads()
+	heads := tb.thread.MaybeHeads()
 	aclHeads, err := tb.getACLHeads(heads)
 	if err != nil {
 		return nil, err
diff --git a/data/document.go b/data/document.go
index 45eaa6a4..4fbabcac 100644
--- a/data/document.go
+++ b/data/document.go
@@ -64,6 +64,8 @@ func (d *Document) Update(changes ...*threadmodels.RawChange) (DocumentState, Up
 		treeChange := d.treeBuilder.changeCreator(ch.Id, aclChange)
 		treeChanges = append(treeChanges, treeChange)
 
+		// this already sets MaybeHeads to include new changes
+		// TODO: change this behaviour as non-obvious, because it is not evident from the interface
 		err = d.thread.AddChange(ch)
 		if err != nil {
 			return nil, UpdateResultNoAction, fmt.Errorf("change with id %s cannot be added: %w", ch.Id, err)
@@ -99,6 +101,11 @@ func (d *Document) Update(changes ...*threadmodels.RawChange) (DocumentState, Up
 		return res, UpdateResultRebuild, fmt.Errorf("could not add changes to state, rebuilded")
 	}
 
+	// setting all heads
+	d.thread.SetHeads(d.docContext.fullTree.Heads())
+	// this should be the entrypoint when we build the document
+	d.thread.SetMaybeHeads(d.docContext.fullTree.Heads())
+
 	return newState, UpdateResultAppend, nil
 }
 
@@ -151,6 +158,11 @@ func (d *Document) build(fromStart bool) (DocumentState, error) {
 		return nil, err
 	}
 
+	// setting all heads
+	d.thread.SetHeads(d.docContext.fullTree.Heads())
+	// this should be the entrypoint when we build the document
+	d.thread.SetMaybeHeads(d.docContext.fullTree.Heads())
+
 	return d.docContext.docState, nil
 }
 
diff --git a/data/threadbuilder/invalidsnapshotexample.yml b/data/threadbuilder/invalidsnapshotexample.yml
index eb49fa95..fff71365 100644
--- a/data/threadbuilder/invalidsnapshotexample.yml
+++ b/data/threadbuilder/invalidsnapshotexample.yml
@@ -117,6 +117,6 @@ graph:
     baseSnapshot: A.1.2
     aclHeads: [A.1.2]
     treeHeads: [A.1.2]
-heads:
+maybeHeads:
   - A.1.3
   - B.1.2
diff --git a/data/threadbuilder/keychain.go b/data/threadbuilder/keychain.go
index 69051c79..be3119ab 100644
--- a/data/threadbuilder/keychain.go
+++ b/data/threadbuilder/keychain.go
@@ -14,11 +14,13 @@ type SymKey struct {
 }
 
 type Keychain struct {
-	SigningKeys         map[string]threadmodels.SigningPrivKey
-	EncryptionKeys      map[string]threadmodels.EncryptionPrivKey
-	ReadKeys            map[string]*SymKey
-	GeneratedIdentities map[string]string
-	coder               *threadmodels.Ed25519SigningPubKeyDecoder
+	SigningKeys           map[string]threadmodels.SigningPrivKey
+	SigningKeysByIdentity map[string]threadmodels.SigningPrivKey
+	EncryptionKeys        map[string]threadmodels.EncryptionPrivKey
+	ReadKeys              map[string]*SymKey
+	ReadKeysByHash        map[uint64]*SymKey
+	GeneratedIdentities   map[string]string
+	coder                 *threadmodels.Ed25519SigningPubKeyDecoder
 }
 
 func NewKeychain() *Keychain {
@@ -71,6 +73,7 @@ func (k *Keychain) AddSigningKey(name string) {
 	if err != nil {
 		panic(err)
 	}
+	k.SigningKeysByIdentity[res] = newPrivKey
 	k.GeneratedIdentities[name] = res
 }
 
@@ -87,6 +90,10 @@ func (k *Keychain) AddReadKey(name string) {
 		Hash: hasher.Sum64(),
 		Key:  key,
 	}
+	k.ReadKeysByHash[hasher.Sum64()] = &SymKey{
+		Hash: hasher.Sum64(),
+		Key:  key,
+	}
 }
 
 func (k *Keychain) AddKey(key string) {
diff --git a/data/threadbuilder/threadbuilder.go b/data/threadbuilder/threadbuilder.go
index 92424ab1..c1de52b5 100644
--- a/data/threadbuilder/threadbuilder.go
+++ b/data/threadbuilder/threadbuilder.go
@@ -20,36 +20,17 @@ type threadChange struct {
 	readKey *SymKey
 	signKey threadmodels.SigningPrivKey
 
-	changesData *pb.PlainTextChangeData
+	changesDataDecrypted []byte
 }
 
 type ThreadBuilder struct {
 	threadId   string
 	allChanges map[string]*threadChange
 	heads      []string
+	maybeHeads []string
 	keychain   *Keychain
 }
 
-func (t *ThreadBuilder) AddChange(change *threadmodels.RawChange) error {
-	//TODO implement me
-	panic("implement me")
-	return nil
-}
-
-func (t *ThreadBuilder) MaybeHeads() []string {
-	//TODO implement me
-	panic("implement me")
-}
-
-func (t *ThreadBuilder) SetMaybeHeads(heads []string) {
-	//TODO implement me
-	panic("implement me")
-}
-
-func (t *ThreadBuilder) SetHeads(heads []string) {
-	//TODO implement me
-}
-
 func NewThreadBuilder(keychain *Keychain) *ThreadBuilder {
 	return &ThreadBuilder{
 		allChanges: make(map[string]*threadChange),
@@ -92,17 +73,58 @@ func (t *ThreadBuilder) Heads() []string {
 	return t.heads
 }
 
+func (t *ThreadBuilder) AddChange(change *threadmodels.RawChange) error {
+	aclChange := new(pb.ACLChange)
+	var err error
+
+	// TODO: think what should we do with such cases, because this can be used by attacker to break our tree
+	if err = proto.Unmarshal(change.Payload, aclChange); err != nil {
+		return fmt.Errorf("could not unmarshall changes")
+	}
+	var changesData []byte
+
+	// get correct readkey
+	readKey := t.keychain.ReadKeysByHash[aclChange.CurrentReadKeyHash]
+	if aclChange.ChangesData != nil {
+		changesData, err = readKey.Key.Decrypt(aclChange.ChangesData)
+		if err != nil {
+			return fmt.Errorf("failed to decrypt changes data: %w", err)
+		}
+	}
+
+	// get correct signing key
+	signKey := t.keychain.SigningKeysByIdentity[aclChange.Identity]
+	t.maybeHeads = append(t.maybeHeads, change.Id)
+
+	t.allChanges[change.Id] = &threadChange{
+		ACLChange:            aclChange,
+		id:                   change.Id,
+		readKey:              readKey,
+		signKey:              signKey,
+		changesDataDecrypted: changesData,
+	}
+	return nil
+}
+
+func (t *ThreadBuilder) MaybeHeads() []string {
+	return t.maybeHeads
+}
+
+func (t *ThreadBuilder) SetMaybeHeads(heads []string) {
+	// we should copy here instead of just setting the value
+	t.maybeHeads = heads
+}
+
+func (t *ThreadBuilder) SetHeads(heads []string) {
+	// we should copy here instead of just setting the value
+	t.heads = heads
+}
+
 func (t *ThreadBuilder) GetChange(ctx context.Context, recordID string) (*threadmodels.RawChange, error) {
 	rec := t.allChanges[recordID]
 
-	var encrypted []byte
-	if rec.changesData != nil {
-		m, err := proto.Marshal(rec.changesData)
-		if err != nil {
-			panic("should be able to marshal data!")
-		}
-
-		encrypted, err = rec.readKey.Key.Encrypt(m)
+	if rec.changesDataDecrypted != nil {
+		encrypted, err := rec.readKey.Key.Encrypt(rec.changesDataDecrypted)
 		if err != nil {
 			panic("should be able to encrypt data with read key!")
 		}
@@ -162,9 +184,9 @@ func (t *ThreadBuilder) Parse(thread *YMLThread) {
 			}
 		}
 		if len(ch.Changes) > 0 || ch.Snapshot != nil {
-			newChange.changesData = &pb.PlainTextChangeData{}
+			changesData := &pb.PlainTextChangeData{}
 			if ch.Snapshot != nil {
-				newChange.changesData.Snapshot = t.parseChangeSnapshot(ch.Snapshot)
+				changesData.Snapshot = t.parseChangeSnapshot(ch.Snapshot)
 			}
 			if len(ch.Changes) > 0 {
 				var changeContents []*pb.PlainTextChangeContent
@@ -172,8 +194,13 @@ func (t *ThreadBuilder) Parse(thread *YMLThread) {
 					aclChangeContent := t.parseDocumentChange(ch)
 					changeContents = append(changeContents, aclChangeContent)
 				}
-				newChange.changesData.Content = changeContents
+				changesData.Content = changeContents
 			}
+			m, err := proto.Marshal(changesData)
+			if err != nil {
+				return
+			}
+			newChange.changesDataDecrypted = m
 		}
 		aclChange.CurrentReadKeyHash = k.Hash
 		newChange.ACLChange = aclChange
@@ -421,4 +448,5 @@ func (t *ThreadBuilder) parseGraph(thread *YMLThread) {
 
 func (t *ThreadBuilder) parseHeads(thread *YMLThread) {
 	t.heads = thread.Heads
+	t.maybeHeads = thread.MaybeHeads
 }
diff --git a/data/threadbuilder/threadbuildergraph_nix.go b/data/threadbuilder/threadbuildergraph_nix.go
index 1ea60a5a..d3d188b7 100644
--- a/data/threadbuilder/threadbuildergraph_nix.go
+++ b/data/threadbuilder/threadbuildergraph_nix.go
@@ -36,13 +36,13 @@ func (t *ThreadBuilder) Graph() (string, error) {
 		style := "solid"
 		if r.GetAclData() != nil {
 			style = "filled"
-		} else if r.changesData != nil {
+		} else if r.changesDataDecrypted != nil {
 			style = "dashed"
 		}
 
 		var chSymbs []string
-		if r.changesData != nil {
-			for _, chc := range r.changesData.Content {
+		if r.changesDataDecrypted != nil {
+			for _, chc := range r.changesDataDecrypted.Content {
 				tp := fmt.Sprintf("%T", chc.Value)
 				tp = strings.Replace(tp, "ChangeContentValueOf", "", 1)
 				res := ""
diff --git a/data/threadbuilder/userjoinexample.yml b/data/threadbuilder/userjoinexample.yml
index d41665ca..883ba5e1 100644
--- a/data/threadbuilder/userjoinexample.yml
+++ b/data/threadbuilder/userjoinexample.yml
@@ -99,5 +99,5 @@ graph:
     baseSnapshot: A.1.1
     aclHeads: [B.1.1]
     treeHeads: [B.1.1]
-heads:
+maybeHeads:
   - "A.1.3"
diff --git a/data/threadbuilder/userremovebeforeexample.yml b/data/threadbuilder/userremovebeforeexample.yml
index fb214099..fe4d2b6e 100644
--- a/data/threadbuilder/userremovebeforeexample.yml
+++ b/data/threadbuilder/userremovebeforeexample.yml
@@ -101,6 +101,6 @@ graph:
     baseSnapshot: A.1.1
     aclHeads: [A.1.2]
     treeHeads: [A.1.2]
-heads:
+maybeHeads:
   - "A.1.3"
   - "B.1.2"
diff --git a/data/threadbuilder/userremoveexample.yml b/data/threadbuilder/userremoveexample.yml
index 51fa2d76..ab131fb5 100644
--- a/data/threadbuilder/userremoveexample.yml
+++ b/data/threadbuilder/userremoveexample.yml
@@ -101,6 +101,6 @@ graph:
     aclSnapshot: A.1.1
     aclHeads: [A.1.3]
     treeHeads: [A.1.3]
-heads:
+maybeHeads:
   - "A.1.4"
   - "B.1.2"
diff --git a/data/threadbuilder/validsnapshotexample.yml b/data/threadbuilder/validsnapshotexample.yml
index 2e3c60c6..2c3e49d0 100644
--- a/data/threadbuilder/validsnapshotexample.yml
+++ b/data/threadbuilder/validsnapshotexample.yml
@@ -124,7 +124,7 @@ graph:
     baseSnapshot: A.1.2
     aclHeads: [A.1.2]
     treeHeads: [A.1.2]
-heads:
+maybeHeads:
   - "A.1.3"
   - "B.1.2"
 
diff --git a/data/threadbuilder/ymlentities.go b/data/threadbuilder/ymlentities.go
index 8052deb4..78775758 100644
--- a/data/threadbuilder/ymlentities.go
+++ b/data/threadbuilder/ymlentities.go
@@ -94,5 +94,6 @@ type YMLThread struct {
 		TreeHeads    []string `yaml:"treeHeads"`
 	} `yaml:"graph"`
 
-	Heads []string `yaml:"heads"`
+	Heads      []string `yaml:"heads"`
+	MaybeHeads []string `yaml:"maybeHeads"`
 }