Bind Noise sessions to claimed peer identities

bitchat/Noise/NoiseSecurityConstants.swift

@@ -15,6 +15,9 @@ enum NoiseSecurityConstants {
     // Maximum handshake message size
     static let maxHandshakeMessageSize = 2048 // 2KB to accommodate XX pattern
 
+    // Noise XX message 1 contains only the initiator's 32-byte ephemeral key.
+    static let xxInitialMessageSize = 32
+    
     // Session timeout - sessions older than this should be renegotiated
     static let sessionTimeout: TimeInterval = 86400 // 24 hours
     

bitchat/Noise/NoiseSessionError.swift

@@ -11,4 +11,5 @@ enum NoiseSessionError: Error, Equatable {
     case notEstablished
     case sessionNotFound
     case alreadyEstablished
+    case peerIdentityMismatch
 }

bitchat/Noise/NoiseSessionManager.swift

@@ -13,6 +13,11 @@ import BitFoundation
 
 final class NoiseSessionManager {
     private var sessions: [PeerID: NoiseSession] = [:]
+    /// A responder rehandshake must not evict a working transport session
+    /// before the candidate proves that its authenticated static key belongs
+    /// to the claimed wire ID. Candidates therefore live outside `sessions`
+    /// until the XX handshake completes and the binding is validated.
+    private var responderCandidates: [PeerID: NoiseSession] = [:]
     private let sessionFactory: (PeerID, NoiseRole) -> NoiseSession
     private let managerQueue = DispatchQueue(label: "chat.bitchat.noise.manager", attributes: .concurrent)
     
@@ -54,6 +59,9 @@ final class NoiseSessionManager {
             if let session = sessions.removeValue(forKey: peerID) {
                 session.reset() // Clear sensitive data before removing
             }
+            if let candidate = responderCandidates.removeValue(forKey: peerID) {
+                candidate.reset()
+            }
         }
     }
 
@@ -62,7 +70,11 @@ final class NoiseSessionManager {
             for (_, session) in sessions {
                 session.reset()
             }
+            for (_, candidate) in responderCandidates {
+                candidate.reset()
+            }
             sessions.removeAll()
+            responderCandidates.removeAll()
         }
     }
     
@@ -79,6 +91,7 @@ final class NoiseSessionManager {
             // Remove any existing non-established session
             if let existingSession = sessions[peerID], !existingSession.isEstablished() {
                 _ = sessions.removeValue(forKey: peerID)
+                existingSession.reset()
             }
             
             // Create new initiator session
@@ -91,6 +104,7 @@ final class NoiseSessionManager {
             } catch {
                 // Clean up failed session
                 _ = sessions.removeValue(forKey: peerID)
+                session.reset()
                 SecureLogger.error(.handshakeFailed(peerID: peerID.id, error: error.localizedDescription))
                 throw error
             }
@@ -100,39 +114,50 @@ final class NoiseSessionManager {
     func handleIncomingHandshake(from peerID: PeerID, message: Data) throws -> Data? {
         // Process everything within the synchronized block to prevent race conditions
         return try managerQueue.sync(flags: .barrier) {
-            var shouldCreateNew = false
-            var existingSession: NoiseSession? = nil
+            let session: NoiseSession
+            let isReplacementCandidate: Bool
 
-            if let existing = sessions[peerID] {
-                // If we have an established session, the peer must have cleared their session
-                // for a good reason (e.g., decryption failure, restart, etc.)
-                // We should accept the new handshake to re-establish encryption
-                if existing.isEstablished() {
-                    SecureLogger.info("Accepting handshake from \(peerID) despite existing session - peer likely cleared their session", category: .session)
-                    _ = sessions.removeValue(forKey: peerID)
-                    shouldCreateNew = true
+            if let candidate = responderCandidates[peerID] {
+                // A fresh XX message 1 supersedes an incomplete candidate,
+                // but never the established session it is trying to replace.
+                if message.count == NoiseSecurityConstants.xxInitialMessageSize {
+                    candidate.reset()
+                    let replacement = sessionFactory(peerID, .responder)
+                    responderCandidates[peerID] = replacement
+                    session = replacement
                 } else {
-                    // If we're in the middle of a handshake and receive a new initiation,
-                    // reset and start fresh (the other side may have restarted)
-                    if existing.getState() == .handshaking && message.count == 32 {
-                        _ = sessions.removeValue(forKey: peerID)
-                        shouldCreateNew = true
-                    } else {
-                        existingSession = existing
-                    }
+                    session = candidate
+                }
+                isReplacementCandidate = true
+            } else if let existing = sessions[peerID] {
+                if existing.isEstablished() {
+                    SecureLogger.info(
+                        "Validating replacement handshake from \(peerID) while preserving the established session",
+                        category: .session
+                    )
+                    let candidate = sessionFactory(peerID, .responder)
+                    responderCandidates[peerID] = candidate
+                    session = candidate
+                    isReplacementCandidate = true
+                } else if existing.getState() == .handshaking,
+                          message.count == NoiseSecurityConstants.xxInitialMessageSize {
+                    // No established transport state exists to preserve. A
+                    // fresh initiation replaces the incomplete handshake.
+                    _ = sessions.removeValue(forKey: peerID)
+                    existing.reset()
+                    let replacement = sessionFactory(peerID, .responder)
+                    sessions[peerID] = replacement
+                    session = replacement
+                    isReplacementCandidate = false
+                } else {
+                    session = existing
+                    isReplacementCandidate = false
                 }
             } else {
-                shouldCreateNew = true
-            }
-            
-            // Get or create session
-            let session: NoiseSession
-            if shouldCreateNew {
                 let newSession = sessionFactory(peerID, .responder)
                 sessions[peerID] = newSession
                 session = newSession
-            } else {
-                session = existingSession!
+                isReplacementCandidate = false
             }
             
             // Process the handshake message within the synchronized block
@@ -141,18 +166,40 @@ final class NoiseSessionManager {
                 
                 // Check if session is established after processing
                 if session.isEstablished() {
-                    if let remoteKey = session.getRemoteStaticPublicKey() {
-                        // Schedule callback outside the synchronized block to prevent deadlock
-                        DispatchQueue.global().async { [weak self] in
-                            self?.onSessionEstablished?(peerID, remoteKey)
+                    guard let remoteKey = session.getRemoteStaticPublicKey(),
+                          authenticatedRemoteKey(remoteKey, matches: peerID) else {
+                        throw NoiseSessionError.peerIdentityMismatch
+                    }
+
+                    if isReplacementCandidate {
+                        _ = responderCandidates.removeValue(forKey: peerID)
+                        let previous = sessions.updateValue(session, forKey: peerID)
+                        if let previous, previous !== session {
+                            previous.reset()
                         }
                     }
+
+                    // Schedule callback outside the synchronized block to prevent deadlock
+                    DispatchQueue.global().async { [weak self] in
+                        self?.onSessionEstablished?(peerID, remoteKey)
+                    }
                 }
                 
                 return response
             } catch {
-                // Reset the session on handshake failure so next attempt can start fresh
-                _ = sessions.removeValue(forKey: peerID)
+                // A failed candidate is discarded without touching the
+                // established session. Ordinary failed handshakes retain the
+                // historical cleanup behavior.
+                if isReplacementCandidate {
+                    if let storedCandidate = responderCandidates[peerID],
+                       storedCandidate === session {
+                        _ = responderCandidates.removeValue(forKey: peerID)
+                    }
+                } else if let storedSession = sessions[peerID],
+                          storedSession === session {
+                    _ = sessions.removeValue(forKey: peerID)
+                }
+                session.reset()
                 
                 // Schedule callback outside the synchronized block to prevent deadlock
                 DispatchQueue.global().async { [weak self] in
@@ -165,6 +212,24 @@ final class NoiseSessionManager {
         }
     }
 
+    /// Mesh handshakes normally use a 16-hex wire ID. Full Noise-key IDs are
+    /// also accepted by internal callers when they exactly match the static
+    /// key. Non-wire identifiers remain available to protocol test harnesses;
+    /// BLE packet ingress always supplies a short hexadecimal ID.
+    private func authenticatedRemoteKey(
+        _ remoteKey: Curve25519.KeyAgreement.PublicKey,
+        matches claimedPeerID: PeerID
+    ) -> Bool {
+        let rawKey = remoteKey.rawRepresentation
+        if claimedPeerID.isShort {
+            return PeerID(publicKey: rawKey) == claimedPeerID
+        }
+        if let claimedNoiseKey = claimedPeerID.noiseKey {
+            return claimedNoiseKey == rawKey
+        }
+        return true
+    }
+    
     // MARK: - Encryption/Decryption
     
     func encrypt(_ plaintext: Data, for peerID: PeerID) throws -> Data {

bitchat/Services/BLE/BLENoisePacketHandler.swift

@@ -49,7 +49,11 @@ final class BLENoisePacketHandler {
         self.environment = environment
     }
 
-    func handleHandshake(_ packet: BitchatPacket, from peerID: PeerID) {
+    /// Returns true when the handshake message was processed successfully.
+    /// Callers use this to distinguish an authenticated replacement completion
+    /// from a rejected candidate while an older session remains established.
+    @discardableResult
+    func handleHandshake(_ packet: BitchatPacket, from peerID: PeerID) -> Bool {
         let env = environment
         // Use NoiseEncryptionService for handshake processing
         if PeerID(hexData: packet.recipientID) == env.localPeerID() {
@@ -72,14 +76,26 @@ final class BLENoisePacketHandler {
 
                 // Session establishment will trigger onPeerAuthenticated callback
                 // which will send any pending messages at the right time
+                return true
+            } catch NoiseSessionError.peerIdentityMismatch {
+                // The candidate was already discarded by the session manager.
+                // Do not let a spoofed claimed ID trigger a fresh outbound
+                // handshake or recreate state for the attacker-selected ID.
+                SecureLogger.warning(
+                    "Rejected Noise handshake whose static key does not match \(peerID.id.prefix(8))…",
+                    category: .security
+                )
+                return false
             } catch {
                 SecureLogger.error("Failed to process handshake: \(error)")
                 // Try initiating a new handshake
                 if !env.hasNoiseSession(peerID) {
                     env.initiateHandshake(peerID)
                 }
+                return false
             }
         }
+        return false
     }
 
     func handleEncrypted(_ packet: BitchatPacket, from peerID: PeerID) {

bitchat/Services/BLE/BLEService.swift

@@ -1618,7 +1618,44 @@ final class BLEService: NSObject {
         }
     }
 
-    private func handleLeave(_: BitchatPacket, from peerID: PeerID) {
+    /// Accept a leave only when the claimed sender proves possession of the
+    /// signing key bound by a verified announce. The persisted identity cache
+    /// keeps delayed/relayed leaves verifiable after the live registry entry
+    /// has aged out.
+    private func handleLeave(_ packet: BitchatPacket, from peerID: PeerID) -> Bool {
+        let registrySigningKey = collectionsQueue.sync {
+            peerRegistry.info(for: peerID)?.signingPublicKey
+        }
+        let verifiedViaRegistry = registrySigningKey.map {
+            noiseService.verifyPacketSignature(packet, publicKey: $0)
+        } ?? false
+        let verifiedViaPersistedIdentity = !verifiedViaRegistry
+            && identityManager.getCryptoIdentitiesByPeerIDPrefix(peerID).contains { identity in
+                PeerID(publicKey: identity.publicKey) == peerID
+                    && identity.signingPublicKey.map {
+                        noiseService.verifyPacketSignature(packet, publicKey: $0)
+                    } == true
+            }
+
+        guard verifiedViaRegistry || verifiedViaPersistedIdentity else {
+            SecureLogger.warning(
+                "🚫 Dropping leave with missing/invalid signature for claimed sender \(peerID.id.prefix(8))…",
+                category: .security
+            )
+            return false
+        }
+
+        // A valid departure retires transport state too; otherwise
+        // canDeliverSecurely could remain true for a peer we just removed.
+        noiseService.clearSession(for: peerID)
+        readLinkState { _ in
+            let departedLinks = noiseAuthenticatedLinkOwners.compactMap { link, owner in
+                owner == peerID ? link : nil
+            }
+            for link in departedLinks {
+                noiseAuthenticatedLinkOwners.removeValue(forKey: link)
+            }
+        }
         _ = collectionsQueue.sync(flags: .barrier) {
             // Remove the peer when they leave
             peerRegistry.remove(peerID)
@@ -1635,6 +1672,7 @@ final class BLEService: NSObject {
             self.deliverTransportEvent(.peerDisconnected(peerID))
             self.deliverTransportEvent(.peerListUpdated(currentPeerIDs))
         }
+        return true
     }
     private func sendAnnounce(forceSend: Bool = false) {
         // Throttle announces to prevent flooding
@@ -2336,6 +2374,12 @@ extension BLEService {
         }
     }
 
+    func _test_isNoiseAuthenticatedCentral(_ centralUUID: String, for peerID: PeerID) -> Bool {
+        bleQueue.sync {
+            noiseAuthenticatedLinkOwners[.central(centralUUID)] == peerID
+        }
+    }
+
     func _test_seedConnectedPeer(_ peerID: PeerID, nickname: String) {
         collectionsQueue.sync(flags: .barrier) {
             peerRegistry.upsert(BLEPeerInfo(
@@ -4785,7 +4829,9 @@ extension BLEService {
             handleMeshPong(packet, from: senderID)
 
         case .leave:
-            handleLeave(packet, from: senderID)
+            // A forged leave must neither evict the claimed peer nor spread
+            // to downstream nodes.
+            guard handleLeave(packet, from: senderID) else { return }
 
         case .none:
             SecureLogger.warning("⚠️ Unknown message type: \(packet.type)", category: .session)
@@ -5426,8 +5472,14 @@ extension BLEService {
 
     private func handleNoiseHandshake(_ packet: BitchatPacket, from peerID: PeerID) {
         let wasEstablished = noiseService.hasEstablishedSession(with: peerID)
-        noisePacketHandler.handleHandshake(packet, from: peerID)
-        if !wasEstablished, noiseService.hasEstablishedSession(with: peerID) {
+        let processed = noisePacketHandler.handleHandshake(packet, from: peerID)
+        let isEstablished = noiseService.hasEstablishedSession(with: peerID)
+        // XX message 1 is exactly the unauthenticated 32-byte ephemeral key.
+        // While replacing an existing session, do not authenticate its ingress
+        // link until a later message completes and validates the candidate.
+        let completedAuthenticatedHandshake = !wasEstablished
+            || packet.payload.count != NoiseSecurityConstants.xxInitialMessageSize
+        if processed, isEstablished, completedAuthenticatedHandshake {
             markNoiseAuthenticatedIngressLink(for: packet, peerID: peerID)
         }
     }

bitchatTests/BLEServiceCoreTests.swift

@@ -99,6 +99,95 @@ struct BLEServiceCoreTests {
         #expect(ble.currentPeerSnapshots().isEmpty)
     }
 
+    @Test
+    func unsignedAndBadSignatureLeaveDoNotEvictOrRelayClaimedPeer() async throws {
+        let ble = makeService()
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let mallory = NoiseEncryptionService(keychain: MockKeychain())
+        let alicePeerID = PeerID(publicKey: alice.getStaticPublicKeyData())
+        let outbound = OutboundPacketTap()
+        ble._test_onOutboundPacket = outbound.record
+
+        let unsigned = makeLeavePacket(sender: alicePeerID, marker: "unsigned")
+        ble._test_handlePacket(
+            unsigned,
+            fromPeerID: alicePeerID,
+            signingPublicKey: alice.getSigningPublicKeyData()
+        )
+
+        let unsignedRelayed = await TestHelpers.waitUntil(
+            { outbound.count(ofType: .leave) > 0 },
+            timeout: TestConstants.shortTimeout
+        )
+        #expect(!unsignedRelayed)
+        #expect(ble.currentPeerSnapshots().contains { $0.peerID == alicePeerID })
+
+        let badSignature = try #require(
+            mallory.signPacket(makeLeavePacket(sender: alicePeerID, marker: "bad-signature"))
+        )
+        ble._test_handlePacket(
+            badSignature,
+            fromPeerID: alicePeerID,
+            signingPublicKey: alice.getSigningPublicKeyData()
+        )
+
+        let badSignatureRelayed = await TestHelpers.waitUntil(
+            { outbound.count(ofType: .leave) > 0 },
+            timeout: TestConstants.shortTimeout
+        )
+        #expect(!badSignatureRelayed)
+        #expect(ble.currentPeerSnapshots().contains { $0.peerID == alicePeerID })
+    }
+
+    @Test
+    func validSignedLeaveEvictsSessionAndRelays() async throws {
+        let ble = makeService()
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let alicePeerID = PeerID(publicKey: alice.getStaticPublicKeyData())
+
+        // Establish a real session so the leave regression also verifies that
+        // stale secure-delivery state is retired, not just the peer-list row.
+        let message1 = try ble._test_noiseInitiateHandshake(with: alicePeerID)
+        let message2 = try #require(
+            try alice.processHandshakeMessage(from: ble.myPeerID, message: message1)
+        )
+        let message3 = try #require(
+            try ble._test_noiseProcessHandshakeMessage(from: alicePeerID, message: message2)
+        )
+        _ = try alice.processHandshakeMessage(from: ble.myPeerID, message: message3)
+        #expect(ble.canDeliverSecurely(to: alicePeerID))
+        let centralUUID = "central-valid-leave"
+        ble._test_bindCentral(centralUUID, to: alicePeerID)
+        ble._test_markNoiseAuthenticatedCentral(centralUUID, to: alicePeerID)
+        #expect(ble._test_isNoiseAuthenticatedCentral(centralUUID, for: alicePeerID))
+
+        let outbound = OutboundPacketTap()
+        ble._test_onOutboundPacket = outbound.record
+        let signedLeave = try #require(
+            alice.signPacket(makeLeavePacket(sender: alicePeerID, marker: "valid"))
+        )
+        ble._test_handlePacket(
+            signedLeave,
+            fromPeerID: alicePeerID,
+            signingPublicKey: alice.getSigningPublicKeyData()
+        )
+
+        let evicted = await TestHelpers.waitUntil(
+            {
+                !ble.currentPeerSnapshots().contains { $0.peerID == alicePeerID }
+                    && !ble.canDeliverSecurely(to: alicePeerID)
+                    && !ble._test_isNoiseAuthenticatedCentral(centralUUID, for: alicePeerID)
+            },
+            timeout: TestConstants.longTimeout
+        )
+        #expect(evicted)
+        let relayed = await TestHelpers.waitUntil(
+            { outbound.count(ofType: .leave) == 1 },
+            timeout: TestConstants.longTimeout
+        )
+        #expect(relayed)
+    }
+
     @Test
     func ingressAllowsRelayedSenderOnBoundLink() async throws {
         let ble = makeService()
@@ -690,6 +779,18 @@ private func makePublicPacket(content: String, sender: PeerID, timestamp: UInt64
     )
 }
 
+private func makeLeavePacket(sender: PeerID, marker: String) -> BitchatPacket {
+    BitchatPacket(
+        type: MessageType.leave.rawValue,
+        senderID: Data(hexString: sender.id) ?? Data(),
+        recipientID: nil,
+        timestamp: UInt64(Date().timeIntervalSince1970 * 1000),
+        payload: Data(marker.utf8),
+        signature: nil,
+        ttl: TransportConfig.messageTTLDefault
+    )
+}
+
 private final class PublicCaptureDelegate: BitchatDelegate {
     private let lock = NSLock()
     private(set) var publicMessages: [BitchatMessage] = []

bitchatTests/Noise/NoiseCoverageTests.swift

@@ -12,8 +12,15 @@ struct NoiseCoverageTests {
     private let bobStaticKey = Curve25519.KeyAgreement.PrivateKey()
     private let charlieStaticKey = Curve25519.KeyAgreement.PrivateKey()
 
-    private let alicePeerID = PeerID(str: "0011223344556677")
-    private let bobPeerID = PeerID(str: "8899aabbccddeeff")
+    // Manager test dictionaries are keyed by the remote peer. Keep the
+    // historical names, but derive each wire ID from the static key that the
+    // corresponding manager authenticates during the handshake.
+    private var alicePeerID: PeerID {
+        PeerID(publicKey: bobStaticKey.publicKey.rawRepresentation)
+    }
+    private var bobPeerID: PeerID {
+        PeerID(publicKey: aliceStaticKey.publicKey.rawRepresentation)
+    }
     private let charliePeerID = PeerID(str: "fedcba9876543210")
 
     @Test("Protocol metadata and handshake patterns expose expected values")

bitchatTests/Services/BLENoisePacketHandlerTests.swift

@@ -152,6 +152,21 @@ struct BLENoisePacketHandlerTests {
         #expect(recorder.initiatedHandshakes.isEmpty)
     }
 
+    @Test
+    func peerIdentityMismatchDoesNotRecreateHandshakeState() {
+        let recorder = Recorder()
+        recorder.handshakeResult = .failure(NoiseSessionError.peerIdentityMismatch)
+        recorder.hasSession = false
+        let handler = makeHandler(recorder: recorder)
+        let packet = makeHandshakePacket(recipientID: Data(hexString: localPeerID.id))
+
+        #expect(!handler.handleHandshake(packet, from: remotePeerID))
+
+        #expect(recorder.hasSessionQueries.isEmpty)
+        #expect(recorder.initiatedHandshakes.isEmpty)
+        #expect(recorder.broadcastPackets.isEmpty)
+    }
+
     // MARK: Encrypted
 
     @Test

bitchatTests/Services/NoiseEncryptionServiceTests.swift

@@ -91,39 +91,150 @@ struct NoiseEncryptionServiceTests {
     func handshakeEncryptionAndFingerprintLifecycle() async throws {
         let alice = NoiseEncryptionService(keychain: MockKeychain())
         let bob = NoiseEncryptionService(keychain: MockKeychain())
-        let alicePeerID = PeerID(str: "0011223344556677")
-        let bobPeerID = PeerID(str: "8899aabbccddeeff")
+        let alicePeerID = PeerID(publicKey: alice.getStaticPublicKeyData())
+        let bobPeerID = PeerID(publicKey: bob.getStaticPublicKeyData())
         let recorder = AuthenticationRecorder()
 
         #expect(alice.onPeerAuthenticated == nil)
         alice.addOnPeerAuthenticatedHandler(recorder.record(peerID:fingerprint:))
         bob.onPeerAuthenticated = recorder.record(peerID:fingerprint:)
 
-        try establishSessions(alice: alice, bob: bob, alicePeerID: alicePeerID, bobPeerID: bobPeerID)
+        try establishSessions(alice: alice, bob: bob)
 
         let authenticated = await TestHelpers.waitUntil({ recorder.count >= 2 }, timeout: 5.0)
         #expect(authenticated)
-        #expect(alice.hasEstablishedSession(with: alicePeerID))
-        #expect(bob.hasEstablishedSession(with: bobPeerID))
-        #expect(alice.hasSession(with: alicePeerID))
-        #expect(bob.hasSession(with: bobPeerID))
-        #expect(alice.getPeerPublicKeyData(alicePeerID)?.count == 32)
-        #expect(bob.getPeerPublicKeyData(bobPeerID)?.count == 32)
-        #expect(alice.getPeerFingerprint(alicePeerID) != nil)
-        #expect(bob.getPeerFingerprint(bobPeerID) != nil)
+        #expect(alice.hasEstablishedSession(with: bobPeerID))
+        #expect(bob.hasEstablishedSession(with: alicePeerID))
+        #expect(alice.hasSession(with: bobPeerID))
+        #expect(bob.hasSession(with: alicePeerID))
+        #expect(alice.getPeerPublicKeyData(bobPeerID)?.count == 32)
+        #expect(bob.getPeerPublicKeyData(alicePeerID)?.count == 32)
+        #expect(alice.getPeerFingerprint(bobPeerID) != nil)
+        #expect(bob.getPeerFingerprint(alicePeerID) != nil)
 
         let plaintext = Data("secret payload".utf8)
-        let ciphertext = try alice.encrypt(plaintext, for: alicePeerID)
-        let decrypted = try bob.decrypt(ciphertext, from: bobPeerID)
+        let ciphertext = try alice.encrypt(plaintext, for: bobPeerID)
+        let decrypted = try bob.decrypt(ciphertext, from: alicePeerID)
         #expect(decrypted == plaintext)
 
-        alice.clearSession(for: alicePeerID)
-        #expect(!alice.hasSession(with: alicePeerID))
-        #expect(alice.getPeerFingerprint(alicePeerID) == nil)
+        alice.clearSession(for: bobPeerID)
+        #expect(!alice.hasSession(with: bobPeerID))
+        #expect(alice.getPeerFingerprint(bobPeerID) == nil)
 
         bob.clearEphemeralStateForPanic()
-        #expect(!bob.hasSession(with: bobPeerID))
-        #expect(bob.getPeerFingerprint(bobPeerID) == nil)
+        #expect(!bob.hasSession(with: alicePeerID))
+        #expect(bob.getPeerFingerprint(alicePeerID) == nil)
+    }
+
+    @Test("Handshake rejects a claimed peer ID that does not match the authenticated static key")
+    func handshakeRejectsClaimedPeerIDStaticKeyMismatch() async throws {
+        let receiver = NoiseEncryptionService(keychain: MockKeychain())
+        let claimedAlice = NoiseEncryptionService(keychain: MockKeychain())
+        let mallory = NoiseEncryptionService(keychain: MockKeychain())
+        let receiverPeerID = PeerID(publicKey: receiver.getStaticPublicKeyData())
+        let claimedAlicePeerID = PeerID(publicKey: claimedAlice.getStaticPublicKeyData())
+        let recorder = AuthenticationRecorder()
+        receiver.addOnPeerAuthenticatedHandler(recorder.record(peerID:fingerprint:))
+
+        let message1 = try mallory.initiateHandshake(with: receiverPeerID)
+        let message2 = try #require(
+            try receiver.processHandshakeMessage(from: claimedAlicePeerID, message: message1)
+        )
+        let message3 = try #require(
+            try mallory.processHandshakeMessage(from: receiverPeerID, message: message2)
+        )
+
+        do {
+            _ = try receiver.processHandshakeMessage(from: claimedAlicePeerID, message: message3)
+            Issue.record("Expected the authenticated Mallory key to be rejected for Alice's peer ID")
+        } catch let error as NoiseSessionError {
+            #expect(error == .peerIdentityMismatch)
+        } catch {
+            Issue.record("Unexpected mismatch error: \(error)")
+        }
+
+        #expect(!receiver.hasSession(with: claimedAlicePeerID))
+        let emittedAuthentication = await TestHelpers.waitUntil(
+            { recorder.count > 0 },
+            timeout: TestConstants.shortTimeout
+        )
+        #expect(!emittedAuthentication)
+    }
+
+    @Test("Failed forged replacement preserves the established peer session")
+    func forgedReplacementPreservesEstablishedSession() async throws {
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let receiver = NoiseEncryptionService(keychain: MockKeychain())
+        let mallory = NoiseEncryptionService(keychain: MockKeychain())
+        let alicePeerID = PeerID(publicKey: alice.getStaticPublicKeyData())
+        let receiverPeerID = PeerID(publicKey: receiver.getStaticPublicKeyData())
+        let recorder = AuthenticationRecorder()
+        receiver.addOnPeerAuthenticatedHandler(recorder.record(peerID:fingerprint:))
+
+        try establishSessions(alice: alice, bob: receiver)
+        let initialAuthentication = await TestHelpers.waitUntil(
+            { recorder.count == 1 },
+            timeout: TestConstants.longTimeout
+        )
+        #expect(initialAuthentication)
+
+        let before = try alice.encrypt(Data("before".utf8), for: receiverPeerID)
+        #expect(try receiver.decrypt(before, from: alicePeerID) == Data("before".utf8))
+
+        let forgedMessage1 = try mallory.initiateHandshake(with: receiverPeerID)
+        let forgedMessage2 = try #require(
+            try receiver.processHandshakeMessage(from: alicePeerID, message: forgedMessage1)
+        )
+        // The replacement has not authenticated yet; the working Alice
+        // transport session must remain available throughout the candidate.
+        #expect(receiver.hasEstablishedSession(with: alicePeerID))
+        let forgedMessage3 = try #require(
+            try mallory.processHandshakeMessage(from: receiverPeerID, message: forgedMessage2)
+        )
+
+        do {
+            _ = try receiver.processHandshakeMessage(from: alicePeerID, message: forgedMessage3)
+            Issue.record("Expected forged replacement to fail peer binding")
+        } catch let error as NoiseSessionError {
+            #expect(error == .peerIdentityMismatch)
+        } catch {
+            Issue.record("Unexpected replacement error: \(error)")
+        }
+
+        #expect(receiver.hasEstablishedSession(with: alicePeerID))
+        let after = try alice.encrypt(Data("after".utf8), for: receiverPeerID)
+        #expect(try receiver.decrypt(after, from: alicePeerID) == Data("after".utf8))
+        let emittedReplacementAuthentication = await TestHelpers.waitUntil(
+            { recorder.count > 1 },
+            timeout: TestConstants.shortTimeout
+        )
+        #expect(!emittedReplacementAuthentication)
+    }
+
+    @Test("Valid rehandshake atomically replaces the established session")
+    func validRehandshakeReplacesEstablishedSession() throws {
+        let alice = NoiseEncryptionService(keychain: MockKeychain())
+        let receiver = NoiseEncryptionService(keychain: MockKeychain())
+        let alicePeerID = PeerID(publicKey: alice.getStaticPublicKeyData())
+        let receiverPeerID = PeerID(publicKey: receiver.getStaticPublicKeyData())
+
+        try establishSessions(alice: alice, bob: receiver)
+        alice.clearSession(for: receiverPeerID)
+
+        let message1 = try alice.initiateHandshake(with: receiverPeerID)
+        let message2 = try #require(
+            try receiver.processHandshakeMessage(from: alicePeerID, message: message1)
+        )
+        #expect(receiver.hasEstablishedSession(with: alicePeerID))
+        let message3 = try #require(
+            try alice.processHandshakeMessage(from: receiverPeerID, message: message2)
+        )
+        _ = try receiver.processHandshakeMessage(from: alicePeerID, message: message3)
+
+        #expect(alice.hasEstablishedSession(with: receiverPeerID))
+        #expect(receiver.hasEstablishedSession(with: alicePeerID))
+        let ciphertext = try alice.encrypt(Data("new session".utf8), for: receiverPeerID)
+        #expect(try receiver.decrypt(ciphertext, from: alicePeerID) == Data("new session".utf8))
     }
 
     @Test("Encrypt without a session requests handshake and decrypt without session fails")
@@ -200,16 +311,16 @@ struct NoiseEncryptionServiceTests {
 
     private func establishSessions(
         alice: NoiseEncryptionService,
-        bob: NoiseEncryptionService,
-        alicePeerID: PeerID,
-        bobPeerID: PeerID
+        bob: NoiseEncryptionService
     ) throws {
-        let message1 = try alice.initiateHandshake(with: alicePeerID)
-        let response = try bob.processHandshakeMessage(from: bobPeerID, message: message1)
+        let alicePeerID = PeerID(publicKey: alice.getStaticPublicKeyData())
+        let bobPeerID = PeerID(publicKey: bob.getStaticPublicKeyData())
+        let message1 = try alice.initiateHandshake(with: bobPeerID)
+        let response = try bob.processHandshakeMessage(from: alicePeerID, message: message1)
         let message2 = try #require(response, "Expected handshake response")
-        let final = try alice.processHandshakeMessage(from: alicePeerID, message: message2)
+        let final = try alice.processHandshakeMessage(from: bobPeerID, message: message2)
         let message3 = try #require(final, "Expected handshake final")
-        let finalMessage = try bob.processHandshakeMessage(from: bobPeerID, message: message3)
+        let finalMessage = try bob.processHandshakeMessage(from: alicePeerID, message: message3)
         #expect(finalMessage == nil)
     }
 }