diff --git a/bramble-api/src/main/java/org/briarproject/bramble/api/transport/TransportConstants.java b/bramble-api/src/main/java/org/briarproject/bramble/api/transport/TransportConstants.java
index af9d09731dcb40aa8549797a7573037161f0d3c5..cf5502a5ff212c523cfef42c659bbc8cf26f8534 100644
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/transport/TransportConstants.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/transport/TransportConstants.java
@@ -7,7 +7,7 @@ public interface TransportConstants {
 	/**
 	 * The current version of the transport protocol.
 	 */
-	int PROTOCOL_VERSION = 3;
+	int PROTOCOL_VERSION = 4;
 
 	/**
 	 * The length of the pseudo-random tag in bytes.
@@ -80,4 +80,32 @@ public interface TransportConstants {
 	 * The size of the reordering window.
 	 */
 	int REORDERING_WINDOW_SIZE = 32;
+
+	/**
+	 * Label for deriving Alice's initial tag key from the master secret.
+	 */
+	String ALICE_TAG_LABEL = "org.briarproject.bramble.transport/ALICE_TAG_KEY";
+
+	/**
+	 * Label for deriving Bob's initial tag key from the master secret.
+	 */
+	String BOB_TAG_LABEL = "org.briarproject.bramble.transport/BOB_TAG_KEY";
+
+	/**
+	 * Label for deriving Alice's initial header key from the master secret.
+	 */
+	String ALICE_HEADER_LABEL =
+			"org.briarproject.bramble.transport/ALICE_HEADER_KEY";
+
+	/**
+	 * Label for deriving Bob's initial header key from the master secret.
+	 */
+	String BOB_HEADER_LABEL =
+			"org.briarproject.bramble.transport/BOB_HEADER_KEY";
+
+	/**
+	 * Label for deriving the next period's key in key rotation.
+	 */
+	String ROTATE_LABEL = "org.briarproject.bramble.transport/ROTATE";
+
 }
diff --git a/bramble-core/src/main/java/org/briarproject/bramble/crypto/TransportCryptoImpl.java b/bramble-core/src/main/java/org/briarproject/bramble/crypto/TransportCryptoImpl.java
index 52b51723332d345cc8fd8f947205adc4814ea921..ea992491716e6f0e6f57231703086857aacefc6b 100644
--- a/bramble-core/src/main/java/org/briarproject/bramble/crypto/TransportCryptoImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/crypto/TransportCryptoImpl.java
@@ -13,6 +13,11 @@ import org.spongycastle.crypto.Digest;
 
 import javax.inject.Inject;
 
+import static org.briarproject.bramble.api.transport.TransportConstants.ALICE_HEADER_LABEL;
+import static org.briarproject.bramble.api.transport.TransportConstants.ALICE_TAG_LABEL;
+import static org.briarproject.bramble.api.transport.TransportConstants.BOB_HEADER_LABEL;
+import static org.briarproject.bramble.api.transport.TransportConstants.BOB_TAG_LABEL;
+import static org.briarproject.bramble.api.transport.TransportConstants.ROTATE_LABEL;
 import static org.briarproject.bramble.api.transport.TransportConstants.TAG_LENGTH;
 import static org.briarproject.bramble.util.ByteUtils.INT_16_BYTES;
 import static org.briarproject.bramble.util.ByteUtils.INT_64_BYTES;
@@ -21,15 +26,6 @@ import static org.briarproject.bramble.util.ByteUtils.MAX_32_BIT_UNSIGNED;
 
 class TransportCryptoImpl implements TransportCrypto {
 
-	// KDF labels for tag key derivation
-	private static final String A_TAG = "ALICE_TAG_KEY";
-	private static final String B_TAG = "BOB_TAG_KEY";
-	// KDF labels for header key derivation
-	private static final String A_HEADER = "ALICE_HEADER_KEY";
-	private static final String B_HEADER = "BOB_HEADER_KEY";
-	// KDF label for key rotation
-	private static final String ROTATE = "ROTATE";
-
 	private final CryptoComponent crypto;
 
 	@Inject
@@ -93,19 +89,21 @@ class TransportCryptoImpl implements TransportCrypto {
 	private SecretKey rotateKey(SecretKey k, long rotationPeriod) {
 		byte[] period = new byte[INT_64_BYTES];
 		ByteUtils.writeUint64(rotationPeriod, period, 0);
-		return crypto.deriveKey(ROTATE, k, period);
+		return crypto.deriveKey(ROTATE_LABEL, k, period);
 	}
 
 	private SecretKey deriveTagKey(SecretKey master, TransportId t,
 			boolean alice) {
+		String label = alice ? ALICE_TAG_LABEL : BOB_TAG_LABEL;
 		byte[] id = StringUtils.toUtf8(t.getString());
-		return crypto.deriveKey(alice ? A_TAG : B_TAG, master, id);
+		return crypto.deriveKey(label, master, id);
 	}
 
 	private SecretKey deriveHeaderKey(SecretKey master, TransportId t,
 			boolean alice) {
+		String label = alice ? ALICE_HEADER_LABEL : BOB_HEADER_LABEL;
 		byte[] id = StringUtils.toUtf8(t.getString());
-		return crypto.deriveKey(alice ? A_HEADER : B_HEADER, master, id);
+		return crypto.deriveKey(label, master, id);
 	}
 
 	@Override