Network Working Group                                           T. Pauly
Internet-Draft                                                Apple Inc.
Intended status: Informational                                   K. Rose
Expires: July 7, 2018                          Akamai Technologies, Inc.
                                                                 C. Wood
                                                              Apple Inc.
                                                        January 03, 2018

                A Survey of Transport Security Protocols


   This document provides a survey of commonly used or notable network
   security protocols, with a focus on how they interact and integrate
   with applications and transport protocols.  Its goal is to supplement
   efforts to define and catalog transport services [RFC8095] by
   describing the interfaces required to add security protocols.  It
   examines Transport Layer Security (TLS), Datagram Transport Layer
   Security (DTLS), Quick UDP Internet Connections with TLS (QUIC +
   TLS), MinimalT, CurveCP, tcpcrypt, Internet Key Exchange with
   Encapsulating Security Protocol (IKEv2 + ESP), SRTP (with DTLS), and
   WireGuard.  This survey is not limited to protocols developed within
   the scope or context of the IETF.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 7, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Transport Security Protocol Descriptions  . . . . . . . . . .   5
     3.1.  TLS . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Protocol Description  . . . . . . . . . . . . . . . .   5
       3.1.2.  Protocol Features . . . . . . . . . . . . . . . . . .   6
       3.1.3.  Protocol Dependencies . . . . . . . . . . . . . . . .   6
     3.2.  DTLS  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Protocol Description  . . . . . . . . . . . . . . . .   7
       3.2.2.  Protocol Features . . . . . . . . . . . . . . . . . .   7
       3.2.3.  Protocol Dependencies . . . . . . . . . . . . . . . .   8
     3.3.  QUIC with TLS . . . . . . . . . . . . . . . . . . . . . .   8
       3.3.1.  Protocol Description  . . . . . . . . . . . . . . . .   8
       3.3.2.  Protocol Features . . . . . . . . . . . . . . . . . .   9
       3.3.3.  Protocol Dependencies . . . . . . . . . . . . . . . .   9
     3.4.  MinimalT  . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.4.1.  Protocol Description  . . . . . . . . . . . . . . . .   9
       3.4.2.  Protocol Features . . . . . . . . . . . . . . . . . .  10
       3.4.3.  Protocol Dependencies . . . . . . . . . . . . . . . .  10
     3.5.  CurveCP . . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.5.1.  Protocol Description  . . . . . . . . . . . . . . . .  11
       3.5.2.  Protocol Features . . . . . . . . . . . . . . . . . .  12
       3.5.3.  Protocol Dependencies . . . . . . . . . . . . . . . .  12
     3.6.  tcpcrypt  . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.6.1.  Protocol Description  . . . . . . . . . . . . . . . .  12
       3.6.2.  Protocol Features . . . . . . . . . . . . . . . . . .  13
       3.6.3.  Protocol Dependencies . . . . . . . . . . . . . . . .  13
     3.7.  IKEv2 with ESP  . . . . . . . . . . . . . . . . . . . . .  14
       3.7.1.  Protocol descriptions . . . . . . . . . . . . . . . .  14
       3.7.2.  Protocol features . . . . . . . . . . . . . . . . . .  15
       3.7.3.  Protocol dependencies . . . . . . . . . . . . . . . .  16
     3.8.  WireGuard . . . . . . . . . . . . . . . . . . . . . . . .  16
       3.8.1.  Protocol description  . . . . . . . . . . . . . . . .  16
       3.8.2.  Protocol features . . . . . . . . . . . . . . . . . .  17
       3.8.3.  Protocol dependencies . . . . . . . . . . . . . . . .  17
     3.9.  SRTP (with DTLS)  . . . . . . . . . . . . . . . . . . . .  17

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       3.9.1.  Protocol descriptions . . . . . . . . . . . . . . . .  18
       3.9.2.  Protocol features . . . . . . . . . . . . . . . . . .  18
       3.9.3.  Protocol dependencies . . . . . . . . . . . . . . . .  18
   4.  Common Transport Security Features  . . . . . . . . . . . . .  19
     4.1.  Mandatory Features  . . . . . . . . . . . . . . . . . . .  19
       4.1.1.  Handshake . . . . . . . . . . . . . . . . . . . . . .  19
       4.1.2.  Record  . . . . . . . . . . . . . . . . . . . . . . .  19
     4.2.  Optional Features . . . . . . . . . . . . . . . . . . . .  19
       4.2.1.  Handshake . . . . . . . . . . . . . . . . . . . . . .  19
       4.2.2.  Record  . . . . . . . . . . . . . . . . . . . . . . .  20
   5.  Transport Security Protocol Interfaces  . . . . . . . . . . .  20
     5.1.  Configuration Interfaces  . . . . . . . . . . . . . . . .  20
     5.2.  Handshake Interfaces  . . . . . . . . . . . . . . . . . .  21
     5.3.  Record Interfaces . . . . . . . . . . . . . . . . . . . .  22
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   This document provides a survey of commonly used or notable network
   security protocols, with a focus on how they interact and integrate
   with applications and transport protocols.  Its goal is to supplement
   efforts to define and catalog transport services [RFC8095] by
   describing the interfaces required to add security protocols.  It
   examines Transport Layer Security (TLS), Datagram Transport Layer
   Security (DTLS), Quick UDP Internet Connections with TLS (QUIC +
   TLS), MinimalT, CurveCP, tcpcrypt, Internet Key Exchange with
   Encapsulating Security Protocol (IKEv2 + ESP), SRTP (with DTLS), and
   WireGuard.  This survey is not limited to protocols developed within
   the scope or context of the IETF.

   For each protocol, this document provides a brief description, the
   security features it provides, and the dependencies it has on the
   underlying transport.  This is followed by defining the set of
   transport security features shared by these protocols.  Finally, we
   distill the application and transport interfaces provided by the
   transport security protocols.

2.  Terminology

   The following terms are used throughout this document to describe the
   roles and interactions of transport security protocols:

   o  Transport Feature: a specific end-to-end feature that the
      transport layer provides to an application.  Examples include

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      confidentiality, reliable delivery, ordered delivery, message-
      versus-stream orientation, etc.

   o  Transport Service: a set of Transport Features, without an
      association to any given framing protocol, which provides a
      functionality to an application.

   o  Transport Protocol: an implementation that provides one or more
      different transport services using a specific framing and header
      format on the wire.  A Transport Protocol services an application.

   o  Application: an entity that uses a transport protocol for end-to-
      end delivery of data across the network (this may also be an upper
      layer protocol or tunnel encapsulation).

   o  Security Feature: a specific feature that a network security layer
      provides to applications.  Examples include authentication,
      encryption, key generation, session resumption, and privacy.  A
      feature may be considered to be Mandatory or Optional to an
      application's implementation.

   o  Security Protocol: a defined network protocol that implements one
      or more security features.  Security protocols may be used
      alongside transport protocols, and in combination with one another
      when appropriate.

   o  Handshake Protocol: a security protocol that performs a handshake
      to validate peers and establish a shared cryptographic key.

   o  Record Protocol: a security protocol that allows data to be
      encrypted in records or datagrams based on a shared cryptographic

   o  Session: an ephemeral security association between applications.

   o  Connection: the shared state of two or more endpoints that
      persists across messages that are transmitted between these
      endpoints.  A connection is a transient participant of a session,
      and a session generally lasts between connection instances.

   o  Connection Mobility: a property of a connection that allows it to
      be multihomed or resilient across network interface or address

   o  Peer: an endpoint application party to a session.

   o  Client: the peer responsible for initiating a session.

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   o  Server: the peer responsible for responding to a session

3.  Transport Security Protocol Descriptions

   This section contains descriptions of security protocols that
   currently used to protect data being sent over a network.

   For each protocol, we describe the features it provides and its
   dependencies on other protocols.

3.1.  TLS

   TLS (Transport Layer Security) [RFC5246] is a common protocol used to
   establish a secure session between two endpoints.  Communication over
   this session "prevents eavesdropping, tampering, and message
   forgery."  TLS consists of a tightly coupled handshake and record
   protocol.  The handshake protocol is used to authenticate peers,
   negotiate protocol options, such as cryptographic algorithms, and
   derive session-specific keying material.  The record protocol is used
   to marshal (possibly encrypted) data from one peer to the other.
   This data may contain handshake messages or raw application data.

3.1.1.  Protocol Description

   TLS is the composition of a handshake and record protocol
   [I-D.ietf-tls-tls13].  The record protocol is designed to marshal an
   arbitrary, in-order stream of bytes from one endpoint to the other.
   It handles segmenting, compressing (when enabled), and encrypting
   data into discrete records.  When configured to use an AEAD
   algorithm, it also handles nonce generation and encoding for each
   record.  The record protocol is hidden from the client behind a byte
   stream-oriented API.

   The handshake protocol serves several purposes, including: peer
   authentication, protocol option (key exchange algorithm and
   ciphersuite) negotiation, and key derivation.  Peer authentication
   may be mutual.  However, commonly, only the server is authenticated.
   X.509 certificates are commonly used in this authentication step,
   though other mechanisms, such as raw public keys [RFC7250], exist.
   The client is not authenticated unless explicitly requested by the
   server with a CertificateRequest handshake message.

   The handshake protocol is also extensible.  It allows for a variety
   of extensions to be included by either the client or server.  These
   extensions are used to specify client preferences, e.g., the
   application-layer protocol to be driven with the TLS connection
   [RFC7301], or signals to the server to aid operation, e.g., the

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   server name [RFC6066].  Various extensions also exist to tune the
   parameters of the record protocol, e.g., the maximum fragment length

   Alerts are used to convey errors and other atypical events to the
   endpoints.  There are two classes of alerts: closure and error
   alerts.  A closure alert is used to signal to the other peer that the
   sender wishes to terminate the connection.  The sender typically
   follows a close alert with a TCP FIN segment to close the connection.
   Error alerts are used to indicate problems with the handshake or
   individual records.  Most errors are fatal and are followed by
   connection termination.  However, warning alerts may be handled at
   the discretion of each respective implementation.

   Once a session is disconnected all session keying material must be
   torn down, unless resumption information was previously negotiated.
   TLS supports stateful and stateless resumption.  (Here, the state
   refers to the information requirements for the server.  It is assumed
   that the client must always store some state information in order to
   resume a session.)

3.1.2.  Protocol Features

   o  Key exchange and ciphersuite algorithm negotiation.

   o  Stateful and stateless session resumption.

   o  Certificate- and raw public-key-based authentication.

   o  Mutual client and server authentication.

   o  Byte stream confidentiality and integrity.

   o  Extensibility via well-defined extensions.

   o  0-RTT data support (in TLS 1.3 only).

   o  Application-layer protocol negotiation.

   o  Transparent data segmentation.

3.1.3.  Protocol Dependencies

   o  TCP for in-order, reliable transport.

   o  (Optionally) A PKI trust store for certificate validation.

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3.2.  DTLS

   DTLS (Datagram Transport Layer Security) [RFC6347] is based on TLS,
   but differs in that it is designed to run over UDP instead of TCP.
   Since UDP does not guarantee datagram ordering or reliability, DTLS
   modifies the protocol to make sure it can still provide the same
   security guarantees as TLS.  DTLS was designed to be as close to TLS
   as possible, so this document will assume that all properties from
   TLS are carried over except where specified.

3.2.1.  Protocol Description

   DTLS is modified from TLS to account for packet loss and reordering
   that occur when operating over a datagram-based transport, i.e., UDP.
   Each message is assigned an explicit sequence number to be used to
   reorder on the receiving end.  This removes the inter-record
   dependency and allows each record to be decrypt in isolation of the
   rest.  However, DTLS does not deviate from TLS in that in still
   provides in-order delivery of data to the application.

   With respect to packet loss, if one peer has sent a handshake message
   and has not yet received its expected response, it will retransmit
   the handshake message after a configurable timeout.

   To account for long records that cannot fit within a single UDP
   datagram, DTLS supports fragmentation of records across datagrams,
   keeping track of fragment offsets and lengths in each datagram.  The
   receiving peer must re-assemble records before decrypting.

   DTLS relies on UDP's port numbers to allow peers with multiple DTLS
   sessions between them to demultiplex 'streams' of encrypted packets
   that share a single TLS session.

   Since datagrams may be replayed, DTLS provides anti-replay detection
   based on a window of acceptable sequence numbers [RFC4303].

3.2.2.  Protocol Features

   o  Anti-replay protection between datagrams.

   o  Basic reliability for handshake messages.

   o  See also the features from TLS.

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3.2.3.  Protocol Dependencies

   o  Since DTLS runs over an unreliable, unordered datagram transport,
      it does not require any reliability features.

   o  DTLS contains its own length, so although it runs over a datagram
      transport, it does not rely on the transport protocol supporting

   o  UDP for port numbers used for demultiplexing.

   o  Path MTU discovery.

3.3.  QUIC with TLS

   QUIC (Quick UDP Internet Connections) is a new transport protocol
   that runs over UDP, and was originally designed with a tight
   integration with its security protocol and application protocol
   mappings.  The QUIC transport layer itself provides support for data
   confidentiality and integrity.  This requires keys to be derived with
   a separate handshake protocol.  A mapping for QUIC over TLS 1.3
   [I-D.ietf-quic-tls] has been specified to provide this handshake.

3.3.1.  Protocol Description

   Since QUIC integrates TLS with its transport, it relies on specific
   integration points between its security and transport sides.
   Specifically, these points are:

   o  Starting the handshake to generate keys and provide authentication
      (and providing the transport for the handshake).

   o  Client address validation.

   o  Key ready events from TLS to notify the QUIC transport.

   o  Exporting secrets from TLS to the QUIC transport.

   The QUIC transport layer support multiple streams over a single
   connection.  The first stream is reserved specifically for a TLS
   connection.  The TLS handshake, along with further records, are sent
   over this stream.  This TLS connection follows the TLS standards and
   inherits the security properties of TLS.  The handshake generates
   keys, which are then exported to the rest of the QUIC connection, and
   are used to protect the rest of the streams.

   Initial QUIC messages (packets) are encrypted using "fixed" keys
   derived from the QUIC version and public packet information

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   (Connection ID).  Packets are later encrypted using keys derived from
   the TLS traffic secret upon handshake completion.  The TLS 1.3
   handshake for QUIC is used in either a single-RTT mode or a fast-open
   zero-RTT mode.  When zero-RTT handshakes are possible, the encryption
   first transitions to use the zero-RTT keys before using single-RTT
   handshake keys after the next TLS flight.

3.3.2.  Protocol Features

   o  Handshake properties of TLS.

   o  Multiple encrypted streams over a single connection without head-
      of-line blocking.

   o  Packet payload encryption and complete packet authentication (with
      the exception of the Public Reset packet, which is not

3.3.3.  Protocol Dependencies

   o  QUIC transport relies on UDP.

   o  QUIC transport relies on TLS 1.3 for authentication and initial
      key derivation.

   o  TLS within QUIC relies on a reliable stream abstraction for its

3.4.  MinimalT

   MinimalT is a UDP-based transport security protocol designed to offer
   confidentiality, mutual authentication, DoS prevention, and
   connection mobility [MinimalT].  One major goal of the protocol is to
   leverage existing protocols to obtain server-side configuration
   information used to more quickly bootstrap a connection.  MinimalT
   uses a variant of TCP's congestion control algorithm.

3.4.1.  Protocol Description

   MinimalT is a secure transport protocol built on top of a widespread
   directory service.  Clients and servers interact with local directory
   services to (a) resolve server information and (b) public ephemeral
   state information, respectively.  Clients connect to a local resolver
   once at boot time.  Through this resolver they recover the IP
   address(es) and public key(s) of each server to which they want to

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   Connections are instances of user-authenticated, mobile sessions
   between two endpoints.  Connections run within tunnels between hosts.
   A tunnel is a server-authenticated container that multiplexes
   multiple connections between the same hosts.  All connections in a
   tunnel share the same transport state machine and encryption.  Each
   tunnel has a dedicated control connection used to configure and
   manage the tunnel over time.  Moreover, since tunnels are independent
   of the network address information, they may be reused as both ends
   of the tunnel move about the network.  This does however imply that
   the connection establishment and packet encryption mechanisms are

   Before a client connects to a remote service, it must first establish
   a tunnel to the host providing or offering the service.  Tunnels are
   established in 1-RTT using an ephemeral key obtained from the
   directory service.  Tunnel initiators provide their own ephemeral key
   and, optionally, a DoS puzzle solution such that the recipient
   (server) can verify the authenticity of the request and derive a
   shared secret.  Within a tunnel, new connections to services may be

3.4.2.  Protocol Features

   o  0-RTT forward secrecy for new connections.

   o  DoS prevention by client-side puzzles.

   o  Tunnel-based mobility.

   o  (Transport Feature) Connection multiplexing between hosts across
      shared tunnels.

   o  (Transport Feature) Congestion control state is shared across
      connections between the same host pairs.

3.4.3.  Protocol Dependencies

   o  A DNS-like resolution service to obtain location information (an
      IP address) and ephemeral keys.

   o  A PKI trust store for certificate validation.

3.5.  CurveCP

   CurveCP [CurveCP] is a UDP-based transport security protocol from
   Daniel J.  Bernstein.  Unlike other transport security protocols, it
   is based entirely upon highly efficient public key algorithms.  This

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   removes many pitfalls associated with nonce reuse and key

3.5.1.  Protocol Description

   CurveCP is a UDP-based transport security protocol.  It is built on
   three principal features: exclusive use of public key authenticated
   encryption of packets, server-chosen cookies to prohibit memory and
   computation DoS at the server, and connection mobility with a client-
   chosen ephemeral identifier.

   There are two rounds in CurveCP.  In the first round, the client
   sends its first initialization packet to the server, carrying its
   (possibly fresh) ephemeral public key C', with zero-padding encrypted
   under the server's long-term public key.  The server replies with a
   cookie and its own ephemeral key S' and a cookie that is to be used
   by the client.  Upon receipt, the client then generates its second
   initialization packet carrying: the ephemeral key C', cookie, and an
   encryption of C', the server's domain name, and, optionally, some
   message data.  The server verifies the cookie and the encrypted
   payload and, if valid, proceeds to send data in return.  At this
   point, the connection is established and the two parties can

   The use of only public-key encryption and authentication, or
   "boxing", is done to simplify problems that come with symmetric key
   management and synchronization.  For example, it allows the sender of
   a message to be in complete control of each message's nonce.  It does
   not require either end to share secret keying material.  And it
   allows ephemeral public keys to be associated with connections (or

   The client and server do not perform a standard key exchange.
   Instead, in the initial exchange of packets, the each party provides
   its own ephemeral key to the other end.  The client can choose a new
   ephemeral key for every new connection.  However, the server must
   rotate these keys on a slower basis.  Otherwise, it would be trivial
   for an attacker to force the server to create and store ephemeral
   keys with a fake client initialization packet.

   Unlike TCP, the server employs cookies to enable source validation.
   After receiving the client's initial packet, encrypted under the
   server's long-term public key, the server generates and returns a
   stateless cookie that must be echoed back in the client's following
   message.  This cookie is encrypted under the client's ephemeral
   public key.  This stateless technique prevents attackers from
   hijacking client initialization packets to obtain cookie values to
   flood clients.  (A client would detect the duplicate cookies and

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   reject the flooded packets.)  Similarly, replaying the client's
   second packet, carrying the cookie, will be detected by the server.

   CurveCP supports a weak form of client authentication.  Clients are
   permitted to send their long-term public keys in the second
   initialization packet.  A server can verify this public key and, if
   untrusted, drop the connection and subsequent data.

   Unlike some other protocols, CurveCP data packets only leave the
   ephemeral public key, i.e., the connection ID, and the per-message
   nonce in the clear.  Everything else is encrypted.

3.5.2.  Protocol Features

   o  Forward-secure data encryption and authentication.

   o  Per-packet public-key encryption.

   o  1-RTT session bootstrapping.

   o  Connection mobility based on a client-chosen ephemeral identifier.

   o  Connection establishment message padding to prevent traffic

   o  Sender-chosen explicit nonces, e.g., based on a sequence number.

3.5.3.  Protocol Dependencies

   o  An unreliable transport protocol such as UDP.

3.6.  tcpcrypt

   Tcpcrypt is a lightweight extension to the TCP protocol to enable
   opportunistic encryption with hooks available to the application
   layer for implementation of endpoint authentication.

3.6.1.  Protocol Description

   Tcpcrypt extends TCP to enable opportunistic encryption between the
   two ends of a TCP connection [I-D.ietf-tcpinc-tcpcrypt].  It is a
   family of TCP encryption protocols (TEP), distinguished by key
   exchange algorithm.  The use of a TEP is negotiated with a TCP option
   during the initial TCP handshake via the mechanism described by TCP
   Encryption Negotiation Option (ENO) [I-D.ietf-tcpinc-tcpeno].  In the
   case of initial session establishment, once a tcpcrypt TEP has been
   negotiated the key exchange occurs within the data segments of the
   first few packets exchanged after the handshake completes.  The

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   initiator of a connection sends a list of supported AEAD algorithms,
   a random nonce, and an ephemeral public key share.  The responder
   typically chooses a mutually-supported AEAD algorithm and replies
   with this choice, its own nonce, and ephemeral key share.  An initial
   shared secret is derived from the ENO handshake, the tcpcrypt
   handshake, and the initial keying material resulting from the key
   exchange.  The traffic encryption keys on the initial connection are
   derived from the shared secret.  Connections can be re-keyed before
   the natural AEAD limit for a single set of traffic encryption keys is

   Each tcpcrypt session is associated with a ladder of resumption IDs,
   each derived from the respective entry in a ladder of shared secrets.
   These resumption IDs can be used to negotiate a stateful resumption
   of the session in a subsequent connection, resulting in use of a new
   shared secret and traffic encryption keys without requiring a new key
   exchange.  Willingness to resume a session is signaled via the ENO
   option during the TCP handshake.  Given the length constraints
   imposed by TCP options, unlike stateless resumption mechanisms (such
   as that provided by session tickets in TLS) resumption in tcpcrypt
   requires the maintenance of state on the server, and so successful
   resumption across a pool of servers implies shared state.

   Owing to middlebox ossification issues, tcpcrypt only protects the
   payload portion of a TCP packet.  It does not encrypt any header
   information, such as the TCP sequence number.

   Tcpcrypt exposes a universally-unique connection-specific session ID
   to the application, suitable for application-level endpoint
   authentication either in-band or out-of-band.

3.6.2.  Protocol Features

   o  Forward-secure TCP payload encryption and integrity protection.

   o  Session caching and address-agnostic resumption.

   o  Connection re-keying.

   o  Application-level authentication primitive.

3.6.3.  Protocol Dependencies

   o  TCP

   o  TCP Encryption Negotiation Option (ENO)

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3.7.  IKEv2 with ESP

   IKEv2 [RFC7296] and ESP [RFC4303] together form the modern IPsec
   protocol suite that encrypts and authenticates IP packets, either as
   for creating tunnels (tunnel-mode) or for direct transport
   connections (transport-mode).  This suite of protocols separates out
   the key generation protocol (IKEv2) from the transport encryption
   protocol (ESP).  Each protocol can be used independently, but this
   document considers them together, since that is the most common

3.7.1.  Protocol descriptions  IKEv2

   IKEv2 is a control protocol that runs on UDP port 500.  Its primary
   goal is to generate keys for Security Associations (SAs).  It first
   uses a Diffie-Hellman key exchange to generate keys for the "IKE SA",
   which is a set of keys used to encrypt further IKEv2 messages.  It
   then goes through a phase of authentication in which both peers
   present blobs signed by a shared secret or private key, after which
   another set of keys is derived, referred to as the "Child SA".  These
   Child SA keys are used by ESP.

   IKEv2 negotiates which protocols are acceptable to each peer for both
   the IKE and Child SAs using "Proposals".  Each proposal may contain
   an encryption algorithm, an authentication algorithm, a Diffie-
   Hellman group, and (for IKE SAs only) a pseudorandom function
   algorithm.  Each peer may support multiple proposals, and the most
   preferred mutually supported proposal is chosen during the handshake.

   The authentication phase of IKEv2 may use Shared Secrets,
   Certificates, Digital Signatures, or an EAP (Extensible
   Authentication Protocol) method.  At a minimum, IKEv2 takes two round
   trips to set up both an IKE SA and a Child SA.  If EAP is used, this
   exchange may be expanded.

   Any SA used by IKEv2 can be rekeyed upon expiration, which is usually
   based either on time or number of bytes encrypted.

   There is an extension to IKEv2 that allows session resumption

   MOBIKE is a Mobility and Multihoming extension to IKEv2 that allows a
   set of Security Associations to migrate over different addresses and
   interfaces [RFC4555].

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   When UDP is not available or well-supported on a network, IKEv2 may
   be encapsulated in TCP [I-D.ietf-ipsecme-tcp-encaps].  ESP

   ESP is a protocol that encrypts and authenticates IP and IPv6
   packets.  The keys used for both encryption and authentication can be
   derived from an IKEv2 exchange.  ESP Security Associations come as
   pairs, one for each direction between two peers.  Each SA is
   identified by a Security Parameter Index (SPI), which is marked on
   each encrypted ESP packet.

   ESP packets include the SPI, a sequence number, an optional
   Initialization Vector (IV), payload data, padding, a length and next
   header field, and an Integrity Check Value.

   From [RFC4303], "ESP is used to provide confidentiality, data origin
   authentication, connectionless integrity, an anti-replay service (a
   form of partial sequence integrity), and limited traffic flow

   Since ESP operates on IP packets, it is not directly tied to the
   transport protocols it encrypts.  This means it requires little or no
   change from transports in order to provide security.

   ESP packets are sent directly over IP, except when a NAT is present,
   in which case they are sent on UDP port 4500, or via TCP
   encapsulation [I-D.ietf-ipsecme-tcp-encaps].

3.7.2.  Protocol features  IKEv2

   o  Encryption and authentication of handshake packets.

   o  Cryptographic algorithm negotiation.

   o  Session resumption.

   o  Mobility across addresses and interfaces.

   o  Peer authentication extensibility based on Shared Secret,
      Certificates, Digital Signatures, or EAP methods.

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   o  Data confidentiality and authentication.

   o  Connectionless integrity.

   o  Anti-replay protection.

   o  Limited flow confidentiality.

3.7.3.  Protocol dependencies  IKEv2

   o  Availability of UDP to negotiate, or implementation support for

   o  Some EAP authentication types require accessing a hardware device,
      such as a SIM card; or interacting with a user, such as password
      prompting.  ESP

   o  Since ESP is below transport protocols, it does not have any
      dependencies on the transports themselves, other than on UDP or
      TCP for NAT traversal.

3.8.  WireGuard

   WireGuard is a layer 3 protocol designed to complement or replace
   IPsec [WireGuard].  Unlike most transport security protocols, which
   rely on PKI for peer authentication, WireGuard authenticates peers
   using pre-shared public keys delivered out-of-band, each of which is
   bound to one or more IP addresses.  Moreover, as a protocol suited
   for VPNs, WireGuard offers no extensibility, negotiation, or
   cryptographic agility.

3.8.1.  Protocol description

   WireGuard is a simple VPN protocol that binds a pre-shared public key
   to one or more IP addresses.  Users configure WireGuard by
   associating peer public keys with IP addresses.  These mappings are
   stored in a CryptoKey Routing Table.  (See Section 2 of [WireGuard]
   for more details and sample configurations.)  These keys are used
   upon WireGuard packet transmission and reception.  For example, upon
   receipt of a Handshake Initiation message, receivers use the static
   public key in their CryptoKey routing table to perform necessary
   cryptographic computations.

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   WireGuard builds on Noise [Noise] for 1-RTT key exchange with
   identity hiding.  The handshake hides peer identities as per the
   SIGMA construction [SIGMA].  As a consequence of using Noise,
   WireGuard comes with a fixed set of cryptographic algorithms:

   o  x25519 [Curve25519] and HKDF [RFC5869] for ECDH and key

   o  ChaCha20+Poly1305 [RFC7539] for packet authenticated encryption.

   o  BLAKE2s [BLAKE2] for hashing.

   There is no cryptographic agility.  If weaknesses are found in any of
   these algorithms, new message types using new algorithms must be

   WireGuard is designed to be entirely stateless, modulo the CryptoKey
   routing table, which has size linear with the number of trusted
   peers.  If a WireGuard receiver is under heavy load and cannot
   process a packet, e.g., cannot spare CPU cycles for point
   multiplication, it can reply with a cookie similar to DTLS and IKEv2.
   This cookie only proves IP address ownership.  Any rate limiting
   scheme can be applied to packets coming from non-spoofed addresses.

3.8.2.  Protocol features

   o  Optional PSK-based session creation.

   o  Mutual client and server authentication.

   o  Stateful, timestamp-based replay prevention.

   o  Cookie-based DoS mitigation similar to DTLS and IKEv2.

3.8.3.  Protocol dependencies

   o  Datagram transport.

   o  Out-of-band key distribution and management.

3.9.  SRTP (with DTLS)

   SRTP - Secure RTP - is a profile for RTP that provides
   confidentiality, message authentication, and replay protection for
   data and control packets [RFC3711].  SRTP packets are encrypted using
   a session key, which is derived from a separate master key.  Master
   keys are derived and managed externally, e.g., via DTLS, as specified
   in RFC 5736 [RFC5763].

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3.9.1.  Protocol descriptions

   SRTP adds confidentiality and, optionally, integrity protection to
   SRTP packets.  This is done by encrypting RTP payloads and optionally
   appending an authentication tag (MAC) to the packet trailer.  Packets
   are encrypted using session keys, which are ultimately derived from a
   master key and some additional master salt and session salt.  SRTP
   packets carry a 2-byte sequence number to partially identify the
   unique packet index.  SRTP peers maintain a separate rollover counter
   (ROC) that is incremented whenever the sequence number wraps.  The
   sequence number and ROC together determine the packet index.  Packets
   also carry

   Numerous encryption modes are supported.  For popular modes of
   operation, e.g., AES-CTR, The (unique) initialization vector (IV)
   used for each encryption mode is a function of the RTP SSRC
   (synchronization source), packet index, and session "salting key".

   SRTP offers replay detection by keeping a Replay List of already seen
   and processed packet indices.  If a packet arrives with an index that
   matches one in the Replay List, it is silently discarded.

   DTLS [RFC5764] is commonly used as a way to perform mutually
   authentication key establishment for SRTP [RFC5763].  (Here,
   certificates marshall public keys between endpoints.  Thus, self-
   signed certificates may be used if peers do not mutually trust one
   another, as is common on the Internet.)  When DTLS is used,
   certificate fingerprints are transmitted out-of-band using SIP.
   Peers typically verify that DTLS-offered certificates match that
   which are offered over SIP.  This prevents active attacks on RTP, but
   not on the signalling (SIP) channel.

3.9.2.  Protocol features

   o  Optional replay protection with tunable replay windows.

   o  Out-of-order packet receipt.

   o  (RFC5763) Mandatory mutually authenticated key exchange.

3.9.3.  Protocol dependencies

   o  External key derivation and management mechanism or protocol,
      e.g., DTLS [RFC5763].

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4.  Common Transport Security Features

   There exists a common set of features shared across the transport
   protocols surveyed in this document.  The mandatory features should
   be provided by any transport security protocol, while the optional
   features are extensions that a subset of the protocols provide.  For
   clarity, we also distinguish between handshake and record features.

4.1.  Mandatory Features

4.1.1.  Handshake

   o  Forward-secure segment encryption and authentication: Transit data
      must be protected with an authenticated encryption algorithm.

   o  Private key interface or injection: Authentication based on public
      key signatures is commonplace for many transport security

   o  Endpoint authentication: The endpoint (receiver) of a new
      connection must be authenticated before any data is sent to said

   o  Source validation: Source validation must be provided to mitigate
      server-targeted DoS attacks.  This can be done with puzzles or

4.1.2.  Record

   o  Pre-shared key support: A record protocol must be able to use a
      pre-shared key established out-of-band to encrypt individual
      messages, packets, or datagrams.

4.2.  Optional Features

4.2.1.  Handshake

   o  Mutual authentication: Transport security protocols should allow
      both endpoints to authenticate one another if needed.

   o  Application-layer feature negotiation: The type of application
      using a transport security protocol often requires features
      configured at the connection establishment layer, e.g., ALPN
      [RFC7301].  Moreover, application-layer features may often be used
      to offload the session to another server which can better handle
      the request.  (The TLS SNI is one example of such a feature.)  As
      such, transport security protocols should provide a generic

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      mechanism to allow for such application-specific features and
      options to be configured or otherwise negotiated.

   o  Configuration extensions: The protocol negotiation should be
      extensible with addition of new configuration options.

   o  Session caching and management: Sessions should be cacheable to
      enable reuse and amortize the cost of performing session
      establishment handshakes.

4.2.2.  Record

   o  Connection mobility: Sessions should not be bound to a network
      connection (or 5 tuple).  This allows cryptographic key material
      and other state information to be reused in the event of a
      connection change.  Examples of this include a NAT rebinding that
      occurs without a client's knowledge.

5.  Transport Security Protocol Interfaces

   This section describes the interface surface exposed by the security
   protocols described above, with each interface.  Note that not all
   protocols support each interface.

5.1.  Configuration Interfaces

   Configuration interfaces are used to configure the security protocols
   before a handshake begins or the keys are negotiated.

   o  Identity and Private Keys
      The application can provide its identities (certificates) and
      private keys, or mechanisms to access these, to the security
      protocol to use during handshakes.
      Protocols: TLS, DTLS, QUIC + TLS, MinimalT, CurveCP, IKEv2,
      WireGuard, SRTP

   o  Supported Algorithms (Key Exchange, Signatures and Ciphersuites)
      The application can choose the algorithms that are supported for
      key exchange, signatures, and ciphersuites.
      Protocols: TLS, DTLS, QUIC + TLS, MinimalT, tcpcrypt, IKEv2, SRTP

   o  Session Cache
      The application provides the ability to save and retrieve session
      state (tickets, keying material, server parameters) that may be
      used to resume the security session.
      Protocols: TLS, DTLS, QUIC + TLS, MinimalT

   o  Authentication Delegate

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      The application provides access to a separate module that will
      provide authentication, using EAP for example.
      Protocols: IKEv2, SRTP

5.2.  Handshake Interfaces

   Handshake interfaces are the points of interaction between a
   handshake protocol and the application, record protocol, and
   transport once the handshake is active.

   o  Send Handshake Messages
      The handshake protocol needs to be able to send messages over a
      transport to the remote peer to establish trust and negotiate
      Protocols: All (TLS, DTLS, QUIC + TLS, MinimalT, CurveCP, IKEv2,
      WireGuard, SRTP (DTLS))

   o  Receive Handshake Messages
      The handshake protocol needs to be able to receive messages from
      the remote peer over a transport to establish trust and negotiate
      Protocols: All (TLS, DTLS, QUIC + TLS, MinimalT, CurveCP, IKEv2,
      WireGuard, SRTP (DTLS))

   o  Identity Validation
      During a handshake, the security protocol will conduct identity
      validation of the peer.  This can call into the application to
      offload validation.  Protocols: All (TLS, DTLS, QUIC + TLS,
      MinimalT, CurveCP, IKEv2, WireGuard, SRTP (DTLS))

   o  Source Address Validation
      The handshake protocol may delegate validation of the remote peer
      that has sent data to the transport protocol or application.  This
      involves sending a cookie exchange to avoid DoS attacks.
      Protocols: QUIC + TLS, DTLS, WireGuard

   o  Key Update
      The handshake protocol may be instructed to update its keying
      material, either by the application directly or by the record
      protocol sending a key expiration event.
      Protocols: TLS, DTLS, QUIC + TLS, MinimalT, tcpcrypt, IKEv2

   o  Pre-Shared Key Export
      The handshake protocol will generate one or more keys to be used
      for record encryption/decryption and authentication.  These may be
      explicitly exportable to the application, traditionally limited to
      direct export to the record protocol, or inherently non-exportable

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      because the keys must be used directly in conjunction with the
      record protocol.

      *  Explict export: TLS (for QUIC), tcpcrypt, IKEv2, DTLS (for

      *  Direct export: TLS, DTLS, MinimalT

      *  Non-exportable: CurveCP

5.3.  Record Interfaces

   Record interfaces are the points of interaction between a record
   protocol and the application, handshake protocol, and transport once
   in use.

   o  Pre-Shared Key Import
      Either the handshake protocol or the application directly can
      supply pre-shared keys for the record protocol use for encryption/
      decryption and authentication.  If the application can supply keys
      directly, this is considered explicit import; if the handshake
      protocol traditionally provides the keys directly, it is
      considered direct import; if the keys can only be shared by the
      handshake, they are considered non-importable.

      *  Explict import: QUIC, ESP

      *  Direct import: TLS, DTLS, MinimalT, tcpcrypt, WireGuard

      *  Non-importable: CurveCP

   o  Encrypt application data
      The application can send data to the record protocol to encrypt it
      into a format that can be sent on the underlying transport.  The
      encryption step may require that the application data is treated
      as a stream or as datagrams, and that the transport to send the
      encrypted records present a stream or datagram interface.

      *  Stream-to-Stream Protocols: TLS, tcpcrypt

      *  Datagram-to-Datagram Protocols: DTLS, ESP, SRTP, WireGuard

      *  Stream-to-Datagram Protocols: QUIC ((Editor's Note: This
         depends on the interface QUIC exposes to applications.))

   o  Decrypt application data
      The application can receive data from its transport to be
      decrypted using record protocol.  The decryption step may require

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      that the incoming transport data is presented as a stream or as
      datagrams, and that the resulting application data is a stream or

      *  Stream-to-Stream Protocols: TLS, tcpcrypt

      *  Datagram-to-Datagram Protocols: DTLS, ESP, SRTP, WireGuard

      *  Datagram-to-Stream Protocols: QUIC ((Editor's Note: This
         depends on the interface QUIC exposes to applications.))

   o  Key Expiration
      The record protocol can signal that its keys are expiring due to
      reaching a time-based deadline, or a use-based deadline (number of
      bytes that have been encrypted with the key).  This interaction is
      often limited to signaling between the record layer and the
      handshake layer.
      Protocols: ESP ((Editor's note: One may consider TLS/DTLS to also
      have this interface))

   o  Transport mobility
      The record protocol can be signaled that it is being migrated to
      another transport or interface due to connection mobility, which
      may reset address and state validation.
      Protocols: QUIC, MinimalT, CurveCP, ESP, WireGuard (roaming)

6.  IANA Considerations

   This document has on request to IANA.

7.  Security Considerations

   This document summarizes existing transport security protocols and
   their interfaces.  It does not propose changes to or recommend usage
   of reference protocols.

8.  Acknowledgments

   The authors would like to thank Mirja Kuehlewind, Brian Trammell,
   Yannick Sierra, Frederic Jacobs, and Bob Bradley for their input and
   feedback on earlier versions of this draft.

9.  Normative References

   [BLAKE2]   "BLAKE2 -- simpler, smaller, fast as MD5", n.d..

              "Curve25519 - new Diffie-Hellman speed records", n.d..

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   [CurveCP]  "CurveCP -- Usable security for the Internet", n.d..

              Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", draft-ietf-ipsecme-tcp-
              encaps-10 (work in progress), May 2017.

              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-08 (work in
              progress), December 2017.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-08 (work
              in progress), December 2017.

              Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
              Q., and E. Smith, "Cryptographic protection of TCP Streams
              (tcpcrypt)", draft-ietf-tcpinc-tcpcrypt-11 (work in
              progress), November 2017.

              Bittau, A., Giffin, D., Handley, M., Mazieres, D., and E.
              Smith, "TCP-ENO: Encryption Negotiation Option", draft-
              ietf-tcpinc-tcpeno-18 (work in progress), November 2017.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-22 (work in progress),
              November 2017.

              "MinimaLT -- Minimal-latency Networking Through Better
              Security", n.d..

   [Noise]    "The Noise Protocol Framework", n.d..

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,

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   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008, <https://www.rfc-

   [RFC5723]  Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
              Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
              DOI 10.17487/RFC5723, January 2010, <https://www.rfc-

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010, <https://www.rfc-

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010, <https://www.rfc-

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011, <https://www.rfc-

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <>.

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   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,

   [RFC8095]  Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
              Ed., "Services Provided by IETF Transport Protocols and
              Congestion Control Mechanisms", RFC 8095,
              DOI 10.17487/RFC8095, March 2017, <https://www.rfc-

   [SIGMA]    "SIGMA -- The 'SIGn-and-MAc' Approach to Authenticated
              Diffie-Hellman and Its Use in the IKE-Protocols", n.d..

              "WireGuard -- Next Generation Kernel Network Tunnel",

Authors' Addresses

   Tommy Pauly
   Apple Inc.
   1 Infinite Loop
   Cupertino, California 95014
   United States of America


   Kyle Rose
   Akamai Technologies, Inc.
   150 Broadway
   Cambridge, MA 02144
   United States of America


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   Christopher A. Wood
   Apple Inc.
   1 Infinite Loop
   Cupertino, California 95014
   United States of America


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