diff --git a/crypto/cross_sign.go b/crypto/cross_sign.go
index 0f25c89a..a158af8a 100644
--- a/crypto/cross_sign.go
+++ b/crypto/cross_sign.go
@@ -15,6 +15,28 @@ import (
 
 func (mach *OlmMachine) storeCrossSigningKeys(crossSigningKeys map[id.UserID]mautrix.CrossSigningKeys, deviceKeys map[id.UserID]map[id.DeviceID]mautrix.DeviceKeys) {
 	for userID, userKeys := range crossSigningKeys {
+		currentKeys, err := mach.CryptoStore.GetCrossSigningKeys(userID)
+		if err != nil {
+			mach.Log.Error("Error fetching current cross-signing keys of user %v: %v", userID, err)
+		}
+		if currentKeys != nil {
+			for curKeyUsage, curKey := range currentKeys {
+				// got a new key with the same usage as an existing key
+				for _, newKeyUsage := range userKeys.Usage {
+					if newKeyUsage == curKeyUsage {
+						if _, ok := userKeys.Keys[id.NewKeyID(id.KeyAlgorithmEd25519, curKey.String())]; !ok {
+							// old key is not in the new key map so we drop signatures made by it
+							if count, err := mach.CryptoStore.DropSignaturesByKey(userID, curKey); err != nil {
+								mach.Log.Error("Error deleting old signatures: %v", err)
+							} else {
+								mach.Log.Debug("Dropped %v signatures made by key `%v` (%v) as it has been replaced", count, curKey, curKeyUsage)
+							}
+						}
+					}
+				}
+			}
+		}
+
 		for _, key := range userKeys.Keys {
 			for _, usage := range userKeys.Usage {
 				mach.Log.Debug("Storing cross-signing key for %v: %v (type %v)", userID, key, usage)
@@ -36,7 +58,7 @@ func (mach *OlmMachine) storeCrossSigningKeys(crossSigningKeys map[id.UserID]mau
 						}
 					}
 
-					mach.Log.Debug("Verifying %v with: %v %v %v", userKeys, signUserID, signKeyName, signingKey)
+					mach.Log.Debug("Verifying with key %v of user %v", signingKey, signUserID)
 					if verified, err := olm.VerifySignatureJSON(userKeys, signUserID, signKeyName, signingKey); err != nil {
 						mach.Log.Error("Error while verifying cross-signing keys: %v", err)
 					} else {
diff --git a/crypto/devicelist.go b/crypto/devicelist.go
index 47813764..bb4e45df 100644
--- a/crypto/devicelist.go
+++ b/crypto/devicelist.go
@@ -121,7 +121,6 @@ func (mach *OlmMachine) fetchKeys(users []id.UserID, sinceToken string, includeU
 		mach.Log.Warn("Didn't get any keys for user %s", userID)
 	}
 
-	// TODO delete old signatures by previous x-signing keys if they have been updated
 	mach.storeCrossSigningKeys(resp.MasterKeys, resp.DeviceKeys)
 	mach.storeCrossSigningKeys(resp.SelfSigningKeys, resp.DeviceKeys)
 	mach.storeCrossSigningKeys(resp.UserSigningKeys, resp.DeviceKeys)
diff --git a/crypto/sql_store.go b/crypto/sql_store.go
index 9089d3a2..4d276f0a 100644
--- a/crypto/sql_store.go
+++ b/crypto/sql_store.go
@@ -664,3 +664,16 @@ func (store *SQLCryptoStore) IsKeySignedBy(userID id.UserID, key id.Ed25519, sig
 	_, ok := sigs[signerKey]
 	return ok, nil
 }
+
+// DropSignaturesByKey deletes the signatures made by the given user and key from the store. It returns the number of signatures deleted.
+func (store *SQLCryptoStore) DropSignaturesByKey(userID id.UserID, key id.Ed25519) (int64, error) {
+	res, err := store.DB.Exec("DELETE FROM crypto_cross_signing_signatures WHERE signer_user_id=$1 AND signer_key=$2", userID, key)
+	if err != nil {
+		return 0, err
+	}
+	count, err := res.RowsAffected()
+	if err != nil {
+		return 0, err
+	}
+	return count, nil
+}
diff --git a/crypto/store.go b/crypto/store.go
index 7d0b81da..18f81669 100644
--- a/crypto/store.go
+++ b/crypto/store.go
@@ -151,6 +151,8 @@ type Store interface {
 	GetSignaturesForKeyBy(id.UserID, id.Ed25519, id.UserID) (map[id.Ed25519]string, error)
 	// IsKeySignedBy returns whether a cross-signing or device key is signed by the given signer.
 	IsKeySignedBy(id.UserID, id.Ed25519, id.UserID, id.Ed25519) (bool, error)
+	// DropSignaturesByKey deletes the signatures made by the given user and key from the store. It returns the number of signatures deleted.
+	DropSignaturesByKey(id.UserID, id.Ed25519) (int64, error)
 }
 
 type messageIndexKey struct {
@@ -569,3 +571,20 @@ func (gs *GobStore) IsKeySignedBy(userID id.UserID, key id.Ed25519, signerID id.
 	_, ok := sigs[signerKey]
 	return ok, nil
 }
+
+func (gs *GobStore) DropSignaturesByKey(userID id.UserID, key id.Ed25519) (int64, error) {
+	var count int64
+	gs.lock.RLock()
+	for _, userSigs := range gs.KeySignatures {
+		for _, keySigs := range userSigs {
+			if signedBySigner, ok := keySigs[userID]; ok {
+				if _, ok := signedBySigner[key]; ok {
+					count++
+					delete(signedBySigner, key)
+				}
+			}
+		}
+	}
+	gs.lock.RUnlock()
+	return count, nil
+}
diff --git a/crypto/utils/utils.go b/crypto/utils/utils.go
index 2a013170..652b2d33 100644
--- a/crypto/utils/utils.go
+++ b/crypto/utils/utils.go
@@ -47,7 +47,7 @@ func GenAttachmentA256CTR() (key [AESCTRKeyLength]byte, iv [AESCTRIVLength]byte)
 		panic(err)
 	}
 
-	// For some reason we leave the 8 last bytes empty even though AES256-CTR has a 16-byte block size.
+	// The last 8 bytes of the IV act as the counter in AES-CTR, which means they're left empty here
 	_, err = rand.Read(iv[:8])
 	if err != nil {
 		panic(err)