Network Working Group M. Kuehlewind
Internet-Draft ETH Zurich
Intended status: Informational T. Pauly
Expires: September 6, 2018 C. Wood
Apple Inc.
March 05, 2018
Separating Crypto Negotiation and Communication
draft-kuehlewind-taps-crypto-sep-02
Abstract
Secure transport protocols often consist of three logically distinct
components: transport, control (handshake), and record protection.
Typically, such a protocol contains a single module that is
responsible for all three functions. However, in many cases, this
coupling is unnecessary. For example, while cryptographic context
and endpoint capabilities need to be known before encrypted
application data can be sent on a specific transport connection,
there is otherwise no technical constraint that a cryptographic
handshake must be performed on said connection. This document
recommends a logical separation between transport, control, and
record components of secure transport protocols. We compare existing
protocols such as Transport Layer Security, QUIC, and IKEv2+ESP in
the context of this logical separation.
Status of This Memo
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This Internet-Draft will expire on September 6, 2018.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Interfaces . . . . . . . . . . . . . . . . . . . . . 4
3.1. Control-Transport Interface . . . . . . . . . . . . . . . 5
3.1.1. Passive Configuration Interface . . . . . . . . . . . 5
3.1.2. Active Control and Introspection Interface . . . . . 6
3.2. Control-Record Interface . . . . . . . . . . . . . . . . 6
3.3. Transport-Record Interface . . . . . . . . . . . . . . . 6
4. Existing Mappings . . . . . . . . . . . . . . . . . . . . . . 7
5. Benefits of Separation . . . . . . . . . . . . . . . . . . . 8
5.1. Reducing Connection Latency . . . . . . . . . . . . . . . 9
5.2. Protocol Flexibility . . . . . . . . . . . . . . . . . . 9
5.3. Protocol Capability Negotiation . . . . . . . . . . . . . 10
6. Transport Service Architecture Integration . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Secure transport protocols are generally composed of three pieces:
1. A transport protocol to handle the transfer of data.
2. A record protocol to frame, encrypt and/or authenticate data
3. A control protocol to perform cryptographic handshakes, negotiate
shared secrets, and maintain state during the lifetime of
cryptographic session including session resumption and key
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refreshment. (In the context of TLS, the control protocol is
called the handshake protocol.)
For ease of deployment and standardization, among other reasons,
these constituents are often tightly coupled. For example, in TLS
[RFC5246], the control protocol depends on the record protocol, and
vice versa. However, more recent transport protocols such as QUIC
[I-D.ietf-quic-tls] keep these pieces separate. For example, QUIC
uses TLS to negotiate secrets, and exports those secrets to encrypt
packets independent of TLS.
Separating these pieces is important, as new secure transport
protocols increasingly rely on session resumption mechanisms where
cryptographic context can be resumed to transmit application data
with the first packet without delay for connection setup and
negotiation. In the case where there is no cryptographic context
available when an application expresses the need to transmit data to
a certain endpoint, it must first run the control protocol on a
transport connection before being able to transmit application data.
If the control protocol can be separated from the other components,
then it can use another transport connection to establish secrets
without blocking the application's main transport connection. This
also opens up the possibility to run the control protocol well in
advance of the need to send application data, to avoid unnecessary
delays. For example, a client system could maintain a database of
endpoints it is likely to communicate with, and establish keying
material with a control protocol at periodic intervals to ensure
fresh keys for new transport connections.
[I-D.moskowitz-sse] proposes a similar approach. However while
[I-D.moskowitz-sse] proposes a new protocol to negotiate and maintain
long-term cryptographic sessions, this document relies on the use of
existing protocols and only discusses requirements for the evolution
of these protocols and exchange of information within one endpoint
locally.
2. Terminology
o Transport Protocol: A protocol that can transport messages between
two endpoints. This may represent the service offered to
applications to allow them to send and receive data before
encryption; and also represent the protocol that can transmit
control data and encrypted records.
o Control Protocol: A protocol that performs a cryptographic
handshake and, in addition, can validate and authenticate
endpoints, encrypt and authenticate its negotiation, and
ultimately generate keying material.
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o Record Protocol: A protocol that can use keying material to
transform messages. A record will generally add a frame around
application data, and authenticate and/or encrypt the data.
o Keying Material: A shared secret from which pre-shared keys can be
derived and subsequently used to encrypt and authenticate data,
generated by a control protocol and used by a record protocol.
3. Protocol Interfaces
In traditional models in which the protocols are not separated out
into the three elements of control, record, and transport protocols,
there are two basic approaches to the interactions:
1. The transport protocol provides data to the security protocol and
gets back an encrypted version of the data to be sent (control
and record protocols are combined).
2. The security protocol provides keying material to the transport
protocol, and the transport protocol is responsible for
encrypting data (transport and record protocols are combined).
By teasing apart all three portions as separate protocols, there end
up being six interface points:
Application Data
| ^
| |
+----V----+-----+ (1) +---------------+
| +----------------> |
| Transport | | Control |
| <----------------+ |
+-+-----^-------+ (2) +-----+-----^---+
| | | |
| |(6) (3)| |
| | | |(4)
| | +---------------+ | |
| +--------+ <-----+ |
|(5) | Record | |
+--------------> +-----------+
+---------------+
Figure 1: Secure Transport Protocol Components and Interactions
1. A transport protocol depends upon a control protocol to establish
keying material to protect application data being sent through
the transport. The main interface it relies upon is starting the
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control channel, or handshake, or ensuring that the material is
ready.
2. A control protocol depends upon a transport protocol in order to
send and receive negotiation messages with the remote peer.
3. A control protocol sends its keying material and cryptographic
context to the record protocol to use.
4. A record protocol may signal state expiration events to a control
protocol.
5. A transport protocol uses a record protocol to send and receive
application data.
6. A record protocol uses a transport protocol to send and receive
encrypted data.
3.1. Control-Transport Interface
Note that for the purposes of this interface description, it is
assumed that the application is primarily interacting with the
transport protocol, and thus the control protocol interacts with the
application primarily through the abstraction of the transport
protocol. Since security protocol interfaces often require pre-
connection and active behavior on behalf of clients, we further
categorize the following interfaces based on whether they are meant
for passive configuration or active control.
3.1.1. Passive Configuration Interface
o Start negotiation: The interface MUST provide an indication to
start the protocol handshake for key negotiation, and have a way
to be notified when the handshake is complete.
o Identity constraints: The interface MUST allow the application to
constrain the identities that it will accept a connection to, such
as the hostname it expects to be provided in certificate SAN.
o Local identities: The interface MUST allow the local identity to
be set via a raw private key or interface to one to perform
cryptographic operations such as signing and decryption.
o Caching domain and lifetime: The application SHOULD be able to
specify the instances of the protocol that can share cached keys,
as well as the lifetime of cached resources.
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o Pre-shared keying material: The application SHOULD be able to
specify pre-share keying material to use to bootstrap connections.
The control protocol can pass this directly to the record protocol
for use.
o The protocol SHOULD allow applications to negotiate application
protocols and related information.
o The protocol SHOULD allow applications to specify negotiable
cryptographic algorithm suites.
3.1.2. Active Control and Introspection Interface
o State changes: The interface SHOULD provide a way for the
transport to be notified of important state changes during the
protocol execution and session lifetime, e.g., when the handshake
begins, ends, or when a key update occurs.
o Validation: The interface MUST provide a way for the application
to participate in the endpoint authentication and validation,
which can either be specified as parameters to define how the
peer's authentication can be validated, or when the protocol
provides the authentication information for the application to
inspect directly.
o The protocol SHOULD expose the peer's identity information during
and after connection establishment.
3.2. Control-Record Interface
o Key export: The interface MUST provide a way to export keying
material from a control protocol to a record protocol with well-
defined cryptographic properties, e.g., "forward-secure."
o Key lifetime and rotation: The interface MUST provide a way for
the control protocol to define key lifetime bounds in terms of
_time_ or _bytes encrypted_ and, additionally, provide a way to
forcefully update cryptographic session keys at will. The record
protocol MUST be able to signal back to the control protocol that
a lifetime has been reached and that rotation is required. These
values SHOULD be configurable by the application.
3.3. Transport-Record Interface
o Transform data: The interface MUST provide a way to send raw
application data from the transport protocol to a record protocol
to transform it based on the keying material. This data is then
sent out by the transport protocol. The same applies for inbound
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data, in which inbound transport data is transformed by the record
protocol into raw application data.
o Reliability: The transport MUST specify if messages are
transmitted reliable and in order.
o Maximum message size (optional): The transport may specify a
maximum message size for the encrypted data if e.g. a datagram
transport is used
4. Existing Mappings
In this section we document existing mappings between common
transport security protocols and the three components described in
Section I.
o TLS/DTLS: TLS [RFC5246] and DTLS [RFC6347] is a combination of a
control (handshake) and record protocol, with a dependency on some
underlying transport.
Application (configure and I/O)
| ^
| |
+---------V-----+--------+
| Connection |
+----+----^--------------+
+----------|----|------------------------------------+
| | | --TLS-- |
| +----V----+-----+ +---------------+ |
| | +---------> | |
| | Control | | Record | |
| | (Handshake) <---------+ | |
| +---------------+ +----+------^---+ |
| | | |
+------------------------------------|------|--------+
| |
+----V------+----+
| Transport |
+----------------+
o QUIC + TLS: The emerging QUIC standard is decomposed into the
three pieces outlined in Section I [I-D.ietf-quic-tls]. TLS is
used as the control protocol running on a dedicated QUIC stream, a
QUIC-specific record protocol encrypts and encapsulates stream
frames, and the main QUIC component handles the transport of these
frames.
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Application (configure and I/O)
| ^
+-----|-----|------------------------------------+
| | | --QUIC-- |
| | | |
| +--V-----+---+ +--------------+ |
| | QUIC |------------>| TLS | |
| | (transport)| | (control) | |
| | <-------------+ | |
| ++---^--+--^-+ +--^-------+---+ |
| | | | | | | |
| | | | | | | |
| | | | | +V---------+-+ | | |
| | | | +--> Packet +--+ | |
| | | | | Protection | | |
| | | +-----+ (record) <----------+ |
| | | +------------+ |
| | | |
+---|---|----------+-----------------------------+
| |
+---V---+--------+
| Transport |
+----------------+
o IKEv2 + ESP: IKEv2 [RFC7296] is a control protocol commonly used
to establish keys for use in IPsec (often VPN) deployments. It is
already a distinct protocol from its commonly paired record
protocol, which is ESP [RFC4303]. ESP encrypts and authenticates
IP datagrams, and sends them as datagrams over a transport
mechanism such, e.g., IP or UDP.
Application (configure) Application (I/O)
| ^ | ^
+----V----+-----+ +-----V----+----+
| +---------> |
| IKEv2 | | Record |
| <---------+ |
+----+------^---+ +----+------^---+
| | | |
+----V------+------------------V------+----+
| (Unreliable) Transport |
+------------------------------------------+
5. Benefits of Separation
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5.1. Reducing Connection Latency
One of the clearest benefits of separating the control protocol from
the record protocol is that the cryptographic handshake can be
performed out-of-band from the application's data transfer. This
should essentially reduce the number of RTTs required before being
able to send data by the full length of the handshake (which is
commonly 1 or 2 RTTs in the best cases for TLS 1.2 and IKEv2,
potentially more if cookie challenges or extended authentication are
required).
To avoid long-lived transport connections that wouldn't be actively
used, and thus would be vulnerable to timeouts on NATs or firewalls,
an obvious approach to separating the control and record protocols is
to use different transport connections for the early handshake and
the data transfer. However, this approach of using separate
connections will not always save RTTs if the cryptographic handshake
and data transfer are back-to-back. Each connection may require its
own transport protocol handshake, and if the data transfer must wait
for two transport protocols to establish and the cryptographic
handshake to be finished before sending, then it may experience
higher latency. Implementations SHOULD avoid this by either allowing
the control and record protocols to share a single transport
connection or open two connections in parallel when the control
protocol has not pre-fetched keys. Latency benefits, however, can
even be achieved when ensuring that this scenario does not occur by
always having the control protocol refresh the keys whenever old ones
are near expiry.
5.2. Protocol Flexibility
Separation of the control, record, and transport protocols also
allows for more flexible composition of protocols with one another.
If a deployment uses a control protocol like TLS, which requires a
stream-based transport protocol like TCP, separation of protocols
will allow it to use the resulting keys for record protocols that run
on datagram transport protocols like UDP.
This flexibility may be useful for implementations that are
optimizing for packet size by choosing minimal/lightweight record
protocols, while being able to use commonly supported control
protocols like TLS. One example here is the approach of a VPN tunnel
that uses ESP or Diet-ESP [I-D.mglt-ipsecme-diet-esp] to encrypt
datagrams, but uses TLS for establishing keys.
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5.3. Protocol Capability Negotiation
Enabling the use of a different transport protocol for the actual
data transmission than for the cryptographic handshakes opens also
the possibility to negotiate protocol capabilities for the data
transmission. For TLS, usually TCP is the appropriate transport
protocol to use, as it is also widely supported by endpoints.
Allowing an endpoint to indicate the support of other, new transport
protocols within the TCP connection that is used for the
cryptographic handshake, provides a dynamic transition path to enable
easy deployment of new protocols.
6. Transport Service Architecture Integration
The Transport Services Architecture ([I-D.pauly-taps-arch]) describes
a system that can provide transport security functionality behind a
common interface. Such systems and their APIs provide applications
with the ability to establish connections for sending and receiving
data. The lifetime of a connection is comprised of a pre-
establishment configuration stage, established (connected) stage, and
terminated stage. Pre-establishment properties configured include:
Local and Remote Endpoint, protocol selection properties, and
specific protocol options. Applications configure security protocols
during pre-establishment using the passive interfaces described in
Section Section 3.1. Active control interfaces are exercised during
connection establishment, i.e., from pre-establishment to established
states. Applications can query connection metadata or state
information, e.g., peer identity information, during and after
connection establishment.
7. IANA Considerations
This document has on request to IANA.
8. Security Considerations
(editor's note: this section will be added later. However, this
document discusses the use of cryptographic context for transport
connections and as such it has security relevant consideration within
the whole document.)
9. Acknowledgments
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
for Education, Research, and Innovation under contract no. 15.0268.
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This support does not imply endorsement. Thanks to Brian Trammell
for reviewing this draft.
10. Informative References
[I-D.ietf-quic-tls]
Thomson, M. and S. Turner, "Using Transport Layer Security
(TLS) to Secure QUIC", draft-ietf-quic-tls-10 (work in
progress), March 2018.
[I-D.mglt-ipsecme-diet-esp]
Migault, D., Guggemos, T., and C. Bormann, "ESP Header
Compression and Diet-ESP", draft-mglt-ipsecme-diet-esp-05
(work in progress), October 2017.
[I-D.moskowitz-sse]
Moskowitz, R., Faynberg, I., Lu, H., Hares, S., and P.
Giacomin, "Session Security Envelope", draft-moskowitz-
sse-05 (work in progress), June 2017.
[I-D.pauly-taps-arch]
Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
Perkins, C., Tiesel, P., and C. Wood, "An Architecture for
Transport Services", draft-pauly-taps-arch-00 (work in
progress), February 2018.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[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-editor.org/info/rfc5246>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[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, <https://www.rfc-editor.org/info/rfc7296>.
[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, <https://www.rfc-editor.org/info/rfc7301>.
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Authors' Addresses
Mirja Kuehlewind
ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: mirja.kuehlewind@tik.ee.ethz.ch
Tommy Pauly
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: tpauly@apple.com
Christopher A. Wood
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: cawood@apple.com
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