diff --git a/src/connection.zig b/src/connection.zig
index 1cb0b99..ebc8501 100644
--- a/src/connection.zig
+++ b/src/connection.zig
@@ -438,10 +438,101 @@ pub const Connection = struct {
     }
 
     fn connectViaRelay(self: *Connection, relay_addr: []const u8) !void {
-        _ = relay_addr;
-        // TODO: Implementar conexión via relay
-        _ = self;
-        return Error.RelayFailed;
+        // Parsear la URL del relay: relay://host:port/device_id
+        if (!std.mem.startsWith(u8, relay_addr, "relay://")) {
+            return Error.RelayFailed;
+        }
+
+        const rest = relay_addr[8..]; // Quitar "relay://"
+
+        // Buscar el último / para separar host:port de device_id
+        const path_sep = std.mem.lastIndexOf(u8, rest, "/") orelse return Error.RelayFailed;
+        const host_port = rest[0..path_sep];
+
+        // Parsear host:port
+        var host_end = host_port.len;
+        var port: u16 = relay.RELAY_PORT;
+
+        if (std.mem.lastIndexOf(u8, host_port, ":")) |colon| {
+            host_end = colon;
+            port = std.fmt.parseInt(u16, host_port[colon + 1 ..], 10) catch relay.RELAY_PORT;
+        }
+
+        const host = host_port[0..host_end];
+
+        // Parsear IP del host
+        var octets: [4]u8 = undefined;
+        var octet_idx: usize = 0;
+        var current: u16 = 0;
+
+        for (host) |c| {
+            if (c == '.') {
+                if (octet_idx >= 4) return Error.RelayFailed;
+                octets[octet_idx] = @intCast(current);
+                octet_idx += 1;
+                current = 0;
+            } else if (c >= '0' and c <= '9') {
+                current = current * 10 + (c - '0');
+                if (current > 255) return Error.RelayFailed;
+            } else {
+                // Hostname - por ahora no soportado (necesita DNS)
+                return Error.RelayFailed;
+            }
+        }
+
+        if (octet_idx != 3) return Error.RelayFailed;
+        octets[3] = @intCast(current);
+
+        const addr = std.net.Address.initIp4(octets, port);
+
+        // Crear cliente relay
+        var relay_client = relay.RelayClient.init(self.allocator, self.device_id);
+        errdefer relay_client.deinit();
+
+        // Conectar al servidor relay
+        relay_client.connect(addr) catch return Error.RelayFailed;
+
+        // Unirse al pool del relay
+        relay_client.joinRelay() catch return Error.RelayFailed;
+
+        // Solicitar conexión al dispositivo target
+        relay_client.requestConnection(self.device_id) catch return Error.RelayFailed;
+
+        // Esperar a que la sesión esté conectada (con timeout)
+        const deadline = std.time.milliTimestamp() + 10000; // 10 segundos
+        while (std.time.milliTimestamp() < deadline) {
+            if (relay_client.state == .connected) break;
+            if (relay_client.state == .@"error") return Error.RelayFailed;
+
+            // Recibir y procesar mensajes
+            var recv_buf: [1024]u8 = undefined;
+            if (relay_client.socket) |sock| {
+                // Non-blocking receive
+                const tv = std.posix.timeval{ .sec = 0, .usec = 100000 }; // 100ms
+                std.posix.setsockopt(sock, std.posix.SOL.SOCKET, std.posix.SO.RCVTIMEO, std.mem.asBytes(&tv)) catch {};
+
+                const n = std.posix.recv(sock, &recv_buf, 0) catch continue;
+                if (n > 0) {
+                    relay_client.processMessage(recv_buf[0..n]) catch {};
+                }
+            }
+        }
+
+        if (relay_client.state != .connected) {
+            return Error.RelayFailed;
+        }
+
+        // Guardar socket del relay como socket de conexión
+        self.socket = relay_client.socket;
+        relay_client.socket = null; // Transferir ownership
+
+        self.state = .connected;
+        self.connected_at = std.time.timestamp();
+        self.last_activity = self.connected_at;
+        self.connection_method = .relay;
+
+        // Inicializar buffer de recepción
+        self.recv_buffer = RecvBuffer.init(self.allocator) catch null;
     }
 
     fn performTlsHandshake(self: *Connection) !void {
@@ -692,22 +783,21 @@ pub const P2P = struct {
         const self = allocator.create(P2P) catch return Error.OutOfMemory;
         errdefer allocator.destroy(self);
 
-        // Generar o cargar Device ID
-        var device_id: DeviceId = undefined;
-        const cert_path = std.fmt.allocPrint(allocator, "{s}/cert.pem", .{config.data_dir}) catch return Error.OutOfMemory;
-        defer allocator.free(cert_path);
+        // Cargar o generar identidad persistente
+        const key_path = std.fmt.allocPrint(allocator, "{s}/identity.key", .{config.data_dir}) catch return Error.OutOfMemory;
+        defer allocator.free(key_path);
 
-        // Intentar cargar certificado existente
-        if (std.fs.openFileAbsolute(cert_path, .{})) |f| {
-            f.close();
-            // TODO: Cargar Device ID desde certificado
-            std.crypto.random.bytes(&device_id);
-        } else |_| {
-            // Generar nuevo Device ID
-            std.crypto.random.bytes(&device_id);
-            // TODO: Guardar certificado
+        // Asegurar que el directorio existe
+        if (std.fs.path.dirname(key_path)) |dir| {
+            std.fs.makeDirAbsolute(dir) catch {};
         }
 
+        const ident = identity.Identity.loadOrGenerate(key_path) catch {
+            // Si falla, generar identidad temporal (no persistente)
+            return Error.CertificateError;
+        };
+        const device_id = ident.device_id;
+
         self.* = .{
             .allocator = allocator,
             .config = config,
diff --git a/src/identity.zig b/src/identity.zig
index ebe009f..67311b3 100644
--- a/src/identity.zig
+++ b/src/identity.zig
@@ -2,9 +2,12 @@
 //!
 //! El Device ID es un identificador único de 32 bytes derivado del certificado TLS:
 //! DeviceID = SHA256(DER_encoded_certificate)
+//!
+//! La identidad se persiste en disco como un par de claves X25519.
 
 const std = @import("std");
 const crypto = @import("crypto.zig");
+const tls = @import("tls.zig");
 
 /// Longitud del Device ID en bytes
 pub const DEVICE_ID_LENGTH: usize = 32;
@@ -244,6 +247,90 @@ fn chunkify(input: *const [56]u8, output: []u8) []const u8 {
     return output[0..63];
 }
 
+// =============================================================================
+// Identidad persistente
+// =============================================================================
+
+/// Estructura de identidad persistente
+pub const Identity = struct {
+    keypair: tls.X25519KeyPair,
+    device_id: DeviceId,
+
+    /// Genera una nueva identidad aleatoria
+    pub fn generate() Identity {
+        const keypair = tls.X25519KeyPair.generate();
+        return .{
+            .keypair = keypair,
+            .device_id = deriveDeviceId(&keypair.public_key),
+        };
+    }
+
+    /// Crea identidad desde clave privada existente
+    pub fn fromPrivateKey(private_key: [32]u8) Identity {
+        const keypair = tls.X25519KeyPair.fromPrivate(private_key);
+        return .{
+            .keypair = keypair,
+            .device_id = deriveDeviceId(&keypair.public_key),
+        };
+    }
+
+    /// Guarda la identidad en un archivo
+    pub fn save(self: Identity, path: []const u8) !void {
+        const file = try std.fs.createFileAbsolute(path, .{});
+        defer file.close();
+
+        // Formato simple: 32 bytes clave privada + 32 bytes clave pública
+        try file.writeAll(&self.keypair.private_key);
+        try file.writeAll(&self.keypair.public_key);
+    }
+
+    /// Carga identidad desde archivo
+    pub fn load(path: []const u8) !Identity {
+        const file = std.fs.openFileAbsolute(path, .{}) catch |err| {
+            return switch (err) {
+                error.FileNotFound => error.IdentityNotFound,
+                else => err,
+            };
+        };
+        defer file.close();
+
+        var buf: [64]u8 = undefined;
+        const bytes_read = try file.readAll(&buf);
+        if (bytes_read < 64) return error.CorruptedIdentity;
+
+        return Identity.fromPrivateKey(buf[0..32].*);
+    }
+
+    /// Carga o genera identidad
+    pub fn loadOrGenerate(path: []const u8) !Identity {
+        return Identity.load(path) catch |err| {
+            if (err == error.IdentityNotFound) {
+                const identity = Identity.generate();
+                identity.save(path) catch {}; // Intentar guardar, ignorar errores
+                return identity;
+            }
+            return err;
+        };
+    }
+
+    /// Obtiene el Device ID como string
+    pub fn getDeviceIdString(self: Identity, buf: []u8) []const u8 {
+        return deviceIdToString(self.device_id, buf);
+    }
+};
+
+/// Deriva Device ID desde clave pública
+/// DeviceID = SHA256(public_key)
+pub fn deriveDeviceId(public_key: *const [32]u8) DeviceId {
+    return crypto.sha256(public_key);
+}
+
+/// Errores de identidad
+pub const IdentityError = error{
+    IdentityNotFound,
+    CorruptedIdentity,
+};
+
 // --- Tests ---
 
 test "device id round trip" {
@@ -277,3 +364,60 @@ test "parse with errors" {
         return;
     };
 }
+
+test "identity generate and derive" {
+    const id1 = Identity.generate();
+    const id2 = Identity.generate();
+
+    // Dos identidades diferentes
+    try std.testing.expect(!deviceIdEquals(id1.device_id, id2.device_id));
+
+    // Device ID derivado es consistente
+    const derived = deriveDeviceId(&id1.keypair.public_key);
+    try std.testing.expect(deviceIdEquals(id1.device_id, derived));
+}
+
+test "identity from private key" {
+    const id1 = Identity.generate();
+    const id2 = Identity.fromPrivateKey(id1.keypair.private_key);
+
+    // Misma clave privada = mismo Device ID
+    try std.testing.expect(deviceIdEquals(id1.device_id, id2.device_id));
+    try std.testing.expect(std.mem.eql(u8, &id1.keypair.public_key, &id2.keypair.public_key));
+}
+
+test "identity save and load" {
+    const test_path = "/tmp/zcatp2p-test-identity.key";
+
+    // Generar y guardar
+    const id1 = Identity.generate();
+    try id1.save(test_path);
+
+    // Cargar
+    const id2 = try Identity.load(test_path);
+
+    // Mismo Device ID
+    try std.testing.expect(deviceIdEquals(id1.device_id, id2.device_id));
+
+    // Limpiar
+    std.fs.deleteFileAbsolute(test_path) catch {};
+}
+
+test "identity load or generate" {
+    const test_path = "/tmp/zcatp2p-test-identity2.key";
+
+    // Asegurar que no existe
+    std.fs.deleteFileAbsolute(test_path) catch {};
+
+    // Primera vez: genera
+    const id1 = try Identity.loadOrGenerate(test_path);
+
+    // Segunda vez: carga
+    const id2 = try Identity.loadOrGenerate(test_path);
+
+    // Mismo Device ID
+    try std.testing.expect(deviceIdEquals(id1.device_id, id2.device_id));
+
+    // Limpiar
+    std.fs.deleteFileAbsolute(test_path) catch {};
+}
diff --git a/src/relay.zig b/src/relay.zig
index 5f88a0f..819a5e3 100644
--- a/src/relay.zig
+++ b/src/relay.zig
@@ -263,7 +263,87 @@ pub const RelayClient = struct {
 
         _ = try std.posix.send(self.socket.?, record_buf[0..record_len], 0);
 
-        // TODO: Procesar respuesta del servidor TLS
+        // Procesar respuesta del servidor TLS
+        try self.completeTlsHandshake(tls_conn);
+
+        self.state = .connected;
+    }
+
+    /// Completa el handshake TLS procesando respuestas del servidor
+    fn completeTlsHandshake(self: *RelayClient, tls_conn: *tls.TlsConnection) !void {
+        var recv_buf: [4096]u8 = undefined;
+        var total_received: usize = 0;
+        var handshake_complete = false;
+
+        while (!handshake_complete) {
+            // Recibir datos del socket
+            const bytes = std.posix.recv(self.socket.?, recv_buf[total_received..], 0) catch |err| {
+                return switch (err) {
+                    error.WouldBlock => continue,
+                    else => error.TlsError,
+                };
+            };
+
+            if (bytes == 0) return error.ConnectionClosed;
+            total_received += bytes;
+
+            // Procesar todos los TLS records disponibles
+            var offset: usize = 0;
+            while (offset + 5 <= total_received) {
+                // Parse TLS record header
+                const record_length = std.mem.readInt(u16, recv_buf[offset + 3 .. offset + 5][0..2], .big);
+                const full_record_len = 5 + record_length;
+
+                if (offset + full_record_len > total_received) {
+                    // Necesitamos más datos
+                    break;
+                }
+
+                // Parse y procesar el record
+                const parsed = tls.TlsRecord.parse(recv_buf[offset .. offset + full_record_len]) catch {
+                    return error.TlsError;
+                };
+                const tls_record = parsed[0];
+
+                try tls_conn.processRecord(tls_record);
+
+                // Verificar si completamos el handshake
+                if (tls_conn.state == .connected) {
+                    // Generar y enviar Client Finished
+                    var finished_buf: [128]u8 = undefined;
+                    const finished_len = try tls_conn.generateClientFinished(&finished_buf);
+
+                    // Cifrar el Finished
+                    var encrypted_finished: [256]u8 = undefined;
+                    const enc_len = try tls_conn.encrypt(finished_buf[0..finished_len], &encrypted_finished);
+
+                    // Enviar como TLS record
+                    var send_record_buf: [300]u8 = undefined;
+                    const send_record = tls.TlsRecord{
+                        .content_type = .application_data,
+                        .version = tls.ProtocolVersion.TLS_1_2,
+                        .length = @intCast(enc_len),
+                        .fragment = encrypted_finished[0..enc_len],
+                    };
+                    const send_len = send_record.encode(&send_record_buf);
+
+                    _ = try std.posix.send(self.socket.?, send_record_buf[0..send_len], 0);
+
+                    handshake_complete = true;
+                    break;
+                }
+
+                offset += full_record_len;
+            }
+
+            // Mover datos no procesados al inicio
+            if (offset > 0 and offset < total_received) {
+                std.mem.copyForwards(u8, &recv_buf, recv_buf[offset..total_received]);
+                total_received -= offset;
+            } else if (offset == total_received) {
+                total_received = 0;
+            }
+        }
     }
 
     /// Se une al pool del relay
diff --git a/src/tls.zig b/src/tls.zig
index 7f1b82c..3ccff39 100644
--- a/src/tls.zig
+++ b/src/tls.zig
@@ -569,6 +569,219 @@ pub const TlsConnection = struct {
         return 5 + enc_len;
     }
 
+    /// Procesa EncryptedExtensions (TLS 1.3)
+    pub fn processEncryptedExtensions(self: *TlsConnection, data: []const u8) !void {
+        if (self.state != .wait_encrypted_extensions) return error.UnexpectedMessage;
+
+        // Descifrar si es necesario
+        const decrypted = if (self.server_write_key != null)
+            try self.decryptHandshake(data)
+        else
+            data;
+        defer if (self.server_write_key != null) self.allocator.free(@constCast(decrypted));
+
+        // Actualizar transcript
+        self.transcript.update(decrypted);
+
+        // EncryptedExtensions tiene formato simple: type(1) + length(3) + extensions
+        // Por ahora ignoramos el contenido de las extensiones
+        self.state = .wait_certificate;
+    }
+
+    /// Procesa Certificate (TLS 1.3) - Para P2P usamos TOFU (Trust On First Use)
+    pub fn processCertificate(self: *TlsConnection, data: []const u8) !void {
+        if (self.state != .wait_certificate) return error.UnexpectedMessage;
+
+        const decrypted = if (self.server_write_key != null)
+            try self.decryptHandshake(data)
+        else
+            data;
+        defer if (self.server_write_key != null) self.allocator.free(@constCast(decrypted));
+
+        // Actualizar transcript
+        self.transcript.update(decrypted);
+
+        // Para P2P, aceptamos cualquier certificado (TOFU)
+        // El Device ID del peer se deriva de su certificado/clave pública
+        self.state = .wait_certificate_verify;
+    }
+
+    /// Procesa CertificateVerify (TLS 1.3)
+    pub fn processCertificateVerify(self: *TlsConnection, data: []const u8) !void {
+        if (self.state != .wait_certificate_verify) return error.UnexpectedMessage;
+
+        const decrypted = if (self.server_write_key != null)
+            try self.decryptHandshake(data)
+        else
+            data;
+        defer if (self.server_write_key != null) self.allocator.free(@constCast(decrypted));
+
+        // Actualizar transcript
+        self.transcript.update(decrypted);
+
+        // Para P2P, confiamos en que la firma es válida (TOFU)
+        self.state = .wait_finished;
+    }
+
+    /// Procesa Finished del servidor (TLS 1.3)
+    pub fn processServerFinished(self: *TlsConnection, data: []const u8) !void {
+        if (self.state != .wait_finished) return error.UnexpectedMessage;
+
+        const decrypted = if (self.server_write_key != null)
+            try self.decryptHandshake(data)
+        else
+            data;
+        defer if (self.server_write_key != null) self.allocator.free(@constCast(decrypted));
+
+        // Actualizar transcript
+        self.transcript.update(decrypted);
+
+        // Derivar claves de aplicación
+        try self.deriveApplicationKeys();
+
+        self.state = .connected;
+    }
+
+    /// Genera Client Finished
+    pub fn generateClientFinished(self: *TlsConnection, out: []u8) !usize {
+        const hs_secret = self.client_handshake_traffic_secret orelse return error.NotReady;
+
+        // Calculate verify_data
+        const transcript_hash = self.transcript.final();
+        self.transcript = crypto.Sha256.init();
+        self.transcript.update(&transcript_hash);
+
+        var finished_key: [32]u8 = undefined;
+        hkdfExpand(hs_secret, "tls13 finished", 32, &finished_key);
+
+        const verify_data = hmacSha256(&finished_key, &transcript_hash);
+
+        // Handshake message: type(1) + length(3) + verify_data(32)
+        var pos: usize = 0;
+        out[pos] = @intFromEnum(HandshakeType.finished);
+        pos += 1;
+        out[pos] = 0;
+        out[pos + 1] = 0;
+        out[pos + 2] = 32;
+        pos += 3;
+        @memcpy(out[pos .. pos + 32], &verify_data);
+        pos += 32;
+
+        // Actualizar transcript
+        self.transcript.update(out[0..pos]);
+
+        return pos;
+    }
+
+    fn decryptHandshake(self: *TlsConnection, data: []const u8) ![]const u8 {
+        const key = self.server_write_key orelse return data;
+        const iv = self.server_write_iv orelse return data;
+
+        var nonce: [12]u8 = iv;
+        const seq_bytes = std.mem.toBytes(std.mem.nativeToBig(u64, self.server_seq));
+        for (0..8) |i| {
+            nonce[4 + i] ^= seq_bytes[i];
+        }
+        self.server_seq += 1;
+
+        // Skip TLS record header if present
+        const ciphertext = if (data.len > 5 and data[0] == @intFromEnum(ContentType.application_data))
+            data[5..]
+        else
+            data;
+
+        var input = try self.allocator.alloc(u8, 12 + ciphertext.len);
+        defer self.allocator.free(input);
+        @memcpy(input[0..12], &nonce);
+        @memcpy(input[12..], ciphertext);
+
+        return try crypto.chachaPoly1305Decrypt(&key, input, &.{}, self.allocator);
+    }
+
+    fn deriveApplicationKeys(self: *TlsConnection) !void {
+        const hs = self.handshake_secret orelse return error.NoHandshakeSecret;
+
+        // Derive-Secret for application
+        var derived_secret: [32]u8 = undefined;
+        self.deriveSecret(hs, "derived", &.{}, &derived_secret);
+
+        // Master secret (with no PSK)
+        const zero_key = [_]u8{0} ** 32;
+        const master_secret = hkdfExtract(&derived_secret, &zero_key);
+
+        // Application traffic secrets
+        const transcript_hash = self.transcript.final();
+        self.transcript = crypto.Sha256.init();
+        self.transcript.update(&transcript_hash);
+
+        var client_app_secret: [32]u8 = undefined;
+        var server_app_secret: [32]u8 = undefined;
+        self.deriveSecret(master_secret, "c ap traffic", &transcript_hash, &client_app_secret);
+        self.deriveSecret(master_secret, "s ap traffic", &transcript_hash, &server_app_secret);
+
+        self.client_traffic_secret = client_app_secret;
+        self.server_traffic_secret = server_app_secret;
+
+        // Derive application write keys
+        var client_key: [32]u8 = undefined;
+        var client_iv: [12]u8 = undefined;
+        var server_key: [32]u8 = undefined;
+        var server_iv: [12]u8 = undefined;
+
+        hkdfExpand(client_app_secret, "tls13 key", 32, &client_key);
+        hkdfExpand(client_app_secret, "tls13 iv", 12, &client_iv);
+        hkdfExpand(server_app_secret, "tls13 key", 32, &server_key);
+        hkdfExpand(server_app_secret, "tls13 iv", 12, &server_iv);
+
+        // Reset sequence numbers for application data
+        self.client_seq = 0;
+        self.server_seq = 0;
+
+        // Update keys
+        self.client_write_key = client_key;
+        self.client_write_iv = client_iv;
+        self.server_write_key = server_key;
+        self.server_write_iv = server_iv;
+    }
+
+    /// Procesa un registro TLS completo (dispatch por tipo)
+    pub fn processRecord(self: *TlsConnection, record: TlsRecord) !void {
+        switch (record.content_type) {
+            .handshake => {
+                if (record.fragment.len < 1) return error.InvalidMessage;
+                const msg_type: HandshakeType = @enumFromInt(record.fragment[0]);
+
+                switch (msg_type) {
+                    .server_hello => try self.processServerHello(record.fragment),
+                    .encrypted_extensions => try self.processEncryptedExtensions(record.fragment),
+                    .certificate => try self.processCertificate(record.fragment),
+                    .certificate_verify => try self.processCertificateVerify(record.fragment),
+                    .finished => try self.processServerFinished(record.fragment),
+                    else => {},
+                }
+            },
+            .application_data => {
+                // Datos de aplicación cifrados - descifrar y procesar handshake interno
+                if (self.server_write_key != null) {
+                    const decrypted = try self.decrypt(record.fragment);
+                    defer self.allocator.free(decrypted);
+
+                    if (decrypted.len > 0) {
+                        const inner_type: HandshakeType = @enumFromInt(decrypted[0]);
+                        switch (inner_type) {
+                            .encrypted_extensions => try self.processEncryptedExtensions(decrypted),
+                            .certificate => try self.processCertificate(decrypted),
+                            .certificate_verify => try self.processCertificateVerify(decrypted),
+                            .finished => try self.processServerFinished(decrypted),
+                            else => {},
+                        }
+                    }
+                }
+            },
+            else => {},
+        }
+    }
+
     /// Descifra datos de aplicación
     pub fn decrypt(self: *TlsConnection, record: []const u8) ![]u8 {
         const key = self.server_write_key orelse return error.NotReady;