diff --git a/gjkr/member.go b/gjkr/member.go
index d21199c..3ee9c64 100644
--- a/gjkr/member.go
+++ b/gjkr/member.go
@@ -11,8 +11,8 @@ type memberIndex uint16
 // phase of the protocol.
 type member struct {
 	memberIndex memberIndex
+	sessionID   string
 	group       *group
-
 	evidenceLog evidenceLog
 
 	logger Logger
diff --git a/gjkr/message.go b/gjkr/message.go
index fb23a94..8779c82 100644
--- a/gjkr/message.go
+++ b/gjkr/message.go
@@ -17,10 +17,15 @@ import "threshold.network/roast/ephemeral"
 // within the group.
 type ephemeralPublicKeyMessage struct {
 	senderIndex memberIndex // i
+	sessionID   string
 
 	ephemeralPublicKeys map[memberIndex]*ephemeral.PublicKey // j -> Y_ij
 }
 
-func (m *ephemeralPublicKeyMessage) senderIdx() memberIndex {
+func (m *ephemeralPublicKeyMessage) getSenderIndex() memberIndex {
 	return m.senderIndex
 }
+
+func (m *ephemeralPublicKeyMessage) getSessionID() string {
+	return m.sessionID
+}
diff --git a/gjkr/message_filter.go b/gjkr/message_filter.go
index 7d553f1..1c8ea77 100644
--- a/gjkr/message_filter.go
+++ b/gjkr/message_filter.go
@@ -1,15 +1,33 @@
 package gjkr
 
+// filterForSession goes through the messages passed as a parameter and finds
+// all messages sent for the given session ID.
+func filterForSession[T interface{ getSessionID() string }](
+	sessionID string,
+	list []T,
+) []T {
+	result := make([]T, 0)
+
+	for _, msg := range list {
+		if msg.getSessionID() == sessionID {
+			result = append(result, msg)
+		}
+	}
+
+	return result
+}
+
 // findInactive goes through the messages passed as a parameter and finds all
 // inactive members for this set of messages. The function does not care if
 // the given member was already marked as inactive before. The function makes no
 // assumptions about the ordering of the list elements.
-func findInactive[T interface{ senderIdx() memberIndex }](
-	groupSize uint16, list []T,
+func findInactive[T interface{ getSenderIndex() memberIndex }](
+	groupSize uint16,
+	list []T,
 ) []memberIndex {
 	senders := make(map[memberIndex]bool)
 	for _, item := range list {
-		senders[item.senderIdx()] = true
+		senders[item.getSenderIndex()] = true
 	}
 
 	inactive := make([]memberIndex, 0)
@@ -25,16 +43,16 @@ func findInactive[T interface{ senderIdx() memberIndex }](
 // deduplicateBySender removes duplicated items for the given sender. It always
 // takes the first item that occurs for the given sender and ignores the
 // subsequent ones.
-func deduplicateBySender[T interface{ senderIdx() memberIndex }](
+func deduplicateBySender[T interface{ getSenderIndex() memberIndex }](
 	list []T,
 ) []T {
 	senders := make(map[memberIndex]bool)
 	result := make([]T, 0)
 
-	for _, item := range list {
-		if _, exists := senders[item.senderIdx()]; !exists {
-			senders[item.senderIdx()] = true
-			result = append(result, item)
+	for _, msg := range list {
+		if _, exists := senders[msg.getSenderIndex()]; !exists {
+			senders[msg.getSenderIndex()] = true
+			result = append(result, msg)
 		}
 	}
 
@@ -44,12 +62,12 @@ func deduplicateBySender[T interface{ senderIdx() memberIndex }](
 func (m *symmetricKeyGeneratingMember) preProcessMessages(
 	ephemeralPubKeyMessages []*ephemeralPublicKeyMessage,
 ) []*ephemeralPublicKeyMessage {
-	inactiveMembers := findInactive(m.group.groupSize, ephemeralPubKeyMessages)
+	forThisSession := filterForSession(m.sessionID, ephemeralPubKeyMessages)
+
+	inactiveMembers := findInactive(m.group.groupSize, forThisSession)
 	for _, ia := range inactiveMembers {
 		m.group.markMemberAsInactive(ia)
 	}
 
-	// TODO: validate session ID
-
-	return deduplicateBySender(ephemeralPubKeyMessages)
+	return deduplicateBySender(forThisSession)
 }
diff --git a/gjkr/message_filter_test.go b/gjkr/message_filter_test.go
index 7f4f6f4..74e0224 100644
--- a/gjkr/message_filter_test.go
+++ b/gjkr/message_filter_test.go
@@ -6,6 +6,24 @@ import (
 	"threshold.network/roast/internal/testutils"
 )
 
+func TestFilterForSession(t *testing.T) {
+	msg0 := &ephemeralPublicKeyMessage{sessionID: "session-2", senderIndex: 1}
+	msg1 := &ephemeralPublicKeyMessage{sessionID: "session-1", senderIndex: 1}
+	msg2 := &ephemeralPublicKeyMessage{sessionID: "session-1", senderIndex: 2}
+	msg3 := &ephemeralPublicKeyMessage{sessionID: "session-2", senderIndex: 3}
+
+	filtered := filterForSession("session-1", []*ephemeralPublicKeyMessage{
+		msg0, msg1, msg2, msg3,
+	})
+
+	testutils.AssertDeepEqual(
+		t,
+		"filtered messages",
+		[]*ephemeralPublicKeyMessage{msg1, msg2},
+		filtered,
+	)
+}
+
 func TestFindInactive(t *testing.T) {
 	var tests = map[string]struct {
 		groupSize  uint16
@@ -80,7 +98,7 @@ func TestDeduplicateBySender(t *testing.T) {
 
 			deduplicatedSenders := make([]memberIndex, 0)
 			for _, msg := range deduplicateBySender(messages) {
-				deduplicatedSenders = append(deduplicatedSenders, msg.senderIdx())
+				deduplicatedSenders = append(deduplicatedSenders, msg.getSenderIndex())
 			}
 			testutils.AssertUint16SlicesEqual(
 				t,
diff --git a/internal/testutils/assert.go b/internal/testutils/assert.go
index 8c0a0b5..e8523ce 100644
--- a/internal/testutils/assert.go
+++ b/internal/testutils/assert.go
@@ -3,6 +3,7 @@ package testutils
 import (
 	"fmt"
 	"math/big"
+	"reflect"
 	"testing"
 
 	"golang.org/x/exp/slices"
@@ -134,3 +135,19 @@ func AssertUint16SlicesEqual[T ~uint16](
 		)
 	}
 }
+
+func AssertDeepEqual(
+	t *testing.T,
+	description string,
+	expected any,
+	actual any,
+) {
+	if !reflect.DeepEqual(expected, actual) {
+		t.Errorf(
+			"unexpected %s\nexpected: %v\nactual:   %v\n",
+			description,
+			expected,
+			actual,
+		)
+	}
+}