Internet DRAFT - draft-friel-tls-atls
draft-friel-tls-atls
Network Working Group O. Friel
Internet-Draft R. Barnes
Intended status: Informational M. Pritikin
Expires: February 23, 2022 Cisco
H. Tschofenig
Arm Ltd.
M. Baugher
Consultant
August 22, 2021
Application-Layer TLS
draft-friel-tls-atls-05
Abstract
This document specifies how TLS and DTLS can be used at the
application layer for the purpose of establishing secure end-to-end
encrypted communication security.
Encodings for carrying TLS and DTLS payloads are specified for HTTP
and CoAP to improve interoperability. While the use of TLS and DTLS
is straight forward we present multiple deployment scenarios to
illustrate the need for end-to-end application layer encryption and
the benefits of reusing a widely deployed and readily available
protocol. Application software architectures for building, and
network architectures for deploying application layer TLS are
outlined.
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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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 February 23, 2022.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Application Layer End-to-End Security Use Cases . . . . . . . 4
3.1. Constrained Devices . . . . . . . . . . . . . . . . . . . 4
3.2. Bootstrapping Devices . . . . . . . . . . . . . . . . . . 6
4. ATLS Goals . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Architecture Overview . . . . . . . . . . . . . . . . . . . . 7
5.1. Application Architecture . . . . . . . . . . . . . . . . 7
5.2. Functional Design . . . . . . . . . . . . . . . . . . . . 13
5.3. Network Architecture . . . . . . . . . . . . . . . . . . 15
6. ATLS Session Establishment . . . . . . . . . . . . . . . . . 16
7. ATLS over CoAP Transport . . . . . . . . . . . . . . . . . . 18
8. ATLS over HTTP Transport . . . . . . . . . . . . . . . . . . 19
8.1. Protocol Summary . . . . . . . . . . . . . . . . . . . . 20
8.2. Content-Type Header . . . . . . . . . . . . . . . . . . . 20
8.3. HTTP Status Codes . . . . . . . . . . . . . . . . . . . . 20
8.4. ATLS Session Tracking . . . . . . . . . . . . . . . . . . 20
8.5. Session Establishment and Key Exporting . . . . . . . . . 21
8.6. Illustrative ATLS over HTTP Session Establishment . . . . 21
9. Key Exporting and Application Data Encryption . . . . . . . . 22
9.1. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.2. COSE . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10. TLS Ciphersuite to COSE/OSCORE Algorithm Mapping . . . . . . 24
11. TLS Extensions . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. The "oscore_connection_id" Extension . . . . . . . . . . 24
11.2. The "cose_ext" Extension . . . . . . . . . . . . . . . . 25
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
12.1. "oscore_connection_id" TLS extension . . . . . . . . . . 26
12.2. TLS Ciphersuite to OSCORE/COSE Algorithm Mapping . . . . 26
12.3. .well-known URI Registry . . . . . . . . . . . . . . . . 27
12.4. Media Types Registry . . . . . . . . . . . . . . . . . . 27
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12.5. HTTP Content-Formats Registry . . . . . . . . . . . . . 28
12.6. CoAP Content-Formats Registry . . . . . . . . . . . . . 28
12.7. TLS Key Extractor Label . . . . . . . . . . . . . . . . 28
13. Security Considerations . . . . . . . . . . . . . . . . . . . 28
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
14.1. Normative References . . . . . . . . . . . . . . . . . . 29
14.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Pseudo Code . . . . . . . . . . . . . . . . . . . . 32
A.1. OpenSSL . . . . . . . . . . . . . . . . . . . . . . . . . 32
A.2. Java JSSE . . . . . . . . . . . . . . . . . . . . . . . . 34
Appendix B. ATLS and HTTP CONNECT . . . . . . . . . . . . . . . 36
Appendix C. Alternative Approaches to Application Layer End-to-
End Security . . . . . . . . . . . . . . . . . . . . 39
C.1. Noise . . . . . . . . . . . . . . . . . . . . . . . . . . 39
C.2. Signal . . . . . . . . . . . . . . . . . . . . . . . . . 40
C.3. Google ALTS . . . . . . . . . . . . . . . . . . . . . . . 40
C.4. Ephemeral Diffie-Hellman Over COSE (EDHOC) . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
There are multiple scenarios where there is a need for application
layer end-to-end security between clients and application services.
Two examples include:
o Constrained devices connecting via gateways to application
services, where different transport layer protocols may be in use
on either side of the gateway, with the gateway transcoding
between the different transport layer protocols.
o Bootstrapping devices that must connect to HTTP application
services across untrusted TLS interception middleboxes
These two scenarios are described in more detail in Section 3.
This document describes how clients and applications can leverage
standard TLS software stacks to establish secure end-to-end encrypted
connections at the application layer. TLS [RFC5246] [RFC8446] or
DTLS [RFC6347] [I-D.ietf-tls-dtls13] can be used and this document is
agnostic to the versions being used. There are multiple advantages
to reuse of existing TLS software stacks for establishment of
application layer secure connections. These include:
o many clients and application services already include a TLS
software stack, so there is no need to include yet another
software stack in the software build
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o no need to define a new cryptographic negotiation, authentication,
and key exchange protocol between clients and services
o provides standards based PKI mutual authentication between clients
and services
o no need to train software developers on how to use a new
cryptographic protocols or libraries
o automatically benefit from new cipher suites by simply upgrading
the TLS software stack
o automatically benefit from new features, bugfixes, etc. in TLS
software stack upgrades
When TLS or DTLS is used at the application layer we refer to it as
Application-Layer TLS, or ATLS. There is, however, no difference to
TLS versions used over connection-oriented transports, such as TCP or
SCTP. The same is true for DTLS. The difference is mainly in its
use and the requirements placed on the underlying transport.
This document defines how ATLS can be used over HTTP [RFC7230]
[RFC7540] and over CoAP [RFC7252]. This document does not preclude
the use of other transports. However, defining how ATLS can be
established over [ZigBee], [Bluetooth], etc. is beyond the scope of
this document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Application-Layer TLS is referred to as ATLS throughout this
document.
3. Application Layer End-to-End Security Use Cases
This section describes describes a few end-to-end use cases in more
detail.
3.1. Constrained Devices
Two constrained device use cases are outlined here.
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3.1.1. Constrained Device Connecting over a Non-IP Network
There are industry examples of smart lighting systems where
luminaires are connected using ZigBee to a gateway. A server
connects to the gateway using CoAP over DTLS. The server can control
the luminaires by sending messages and commands via the gateway. The
gateway has full access to all messages sent between the luminaires
and the server.
A generic use case similar to the smart lighting system outlined
above has an IoT device talking ZigBee, Bluetooth Low Energy,
LoRaWAN, NB-IoT, etc. to a gateway, with the gateway in turn talking
CoAP over DTLS or another protocol to a server located in the cloud
or on-premise. This is illustrated in Figure 1.
There are scenarios where certain messages sent between the IoT
device and the server must not be exposed to the gateway function.
Additionally, the two endpoints may not have visibility to and no
guarantees about what transport layer security and encryption is
enforced across all hops end-to-end as they only have visibility to
their immediate next hop. ATLS addresses these concerns.
+--------+ ZigBee +---------+ CoAP/DTLS +------------+
| Device |-------------->| Gateway |------------->| Server |
+--------+ +---------+ +------------+
^ ^
| |
+-------- Device to Server -------+
Figure 1: IoT Closed Network Gateway
3.1.2. Constrained Device Connecting over IP
In this example an IoT device connecting to a gateway using a
suitable transport mechanism, such as ZigBee, CoAP, MQTT, etc. The
gateway function in turn talks HTTP over TLS (or, for example, HTTP
over QUIC) to an application service over the Internet. This is
illustrated in Figure 2.
The gateway may not be trusted and all messages between the IoT
device and the application service must be end-to-end encrypted.
Similar to the previous use case, the endpoints have no guarantees
about what level of transport layer security is enforced across all
hops. Again, ATLS addresses these concerns.
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+--------+ CoAP/DTLS +------------------+ HTTP/TLS +---------+
| Device |-------------->| Internet Gateway |------------>| Service |
+--------+ +------------------+ +---------+
^ ^
| |
+---------Device to Cloud Service ATLS Connection----------+
Figure 2: IoT Internet Gateway
3.2. Bootstrapping Devices
There are far more classes of clients being deployed on today's
networks than at any time previously. This poses challenges for
network administrators who need to manage their network and the
clients connecting to their network, and poses challenges for client
vendors and client software developers who must ensure that their
clients can connect to all required services.
One common example is where a client is deployed on a local domain
TCP/IP network that protects its perimeter using a TLS terminating
middlebox, and the client needs to establish a secure connection to a
service in a different network via the middlebox. This is
illustrated in Figure 3.
Traditionally, this has been enabled by the network administrator
deploying the necessary certificate authority trusted roots on the
client. This can be achieved at scale using standard tools that
enable the administrator to automatically push trusted roots out to
all client machines in the network from a centralized domain
controller. This works for personal computers, laptops and servers
running standard Operating Systems that can be centrally managed.
This client management process breaks for multiple classes of clients
that are being deployed today, there is no standard mechanism for
configuring trusted roots on these clients, and there is no standard
mechanism for these clients to securely traverse middleboxes.
+--------+ C->M TLS +-----------+ M->S TLS +---------+
| Client |--------------->| Middlebox |------------->| Service |
+--------+ +-----------+ +---------+
^ ^
| |
+-----------Client to Service ATLS Connection---------+
Figure 3: Bootstrapping Devices
The ATLS mechanism defined in this document enables clients to
traverse middleboxes and establish secure connections to services
across network domain boundaries. The purpose of this connection may
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simply be to facilitate a bootstrapping process, for example
[I-D.ietf-anima-bootstrapping-keyinfra], whereby the client securely
discovers the local domain certificate authorities required to
establish a trusted network layer TLS connection to the middlebox.
4. ATLS Goals
The high level goals driving the design of this mechanism are:
o enable authenticated key exchange at the application layer by
reusing existing technologies,
o ensure that ATLS packets are explicitly identified thus ensuring
that any middleboxes or gateways at the transport layer are
content aware,
o leverage TLS stacks and handshake protocols thus avoiding
introducing new software or protocol dependencies in clients and
applications
o reuse TLS [RFC5246] [RFC8446] and DTLS [RFC6347]
[I-D.ietf-tls-dtls13] specifications,
o do not mandate constraints on how the TLS stack is configured or
used,
o be forward compatible with future TLS versions including new
developments such as compact TLS [I-D.rescorla-tls-ctls], and
o ensure that the design is as simple as possible.
5. Architecture Overview
5.1. Application Architecture
TLS software stacks allow application developers to 'unplug' the
default network socket transport layer and read and write TLS records
directly from byte buffers. This enables application developers to
use ATLS, extract the raw TLS record bytes from the bottom of the TLS
stack, and transport these bytes over any suitable transport. The
TLS software stacks can generate byte streams of full TLS flights,
which may include multiple TLS records. Additionally, TLS software
stacks support Keying Material Exporters [RFC5705] and allow
applications to export keying material from established TLS sessions.
This keying material can then be used by the application for
encryption of data outside the context of the TLS session. This is
illustrated in Figure 4 below.
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+------------+ +---------+
Handshake Records | | Handshake Records | |
------------------->| |------------------->| |
| | | Byte |
Unencrypted Data | TLS | Encrypted Data | |
------------------->| |------------------->| Buffers |
| Software | | |
Encrypted Data | | Unencrypted Data | |
------------------->| Stack |------------------->| |
| | +---------+
Keying Material | |
<-------------------| |
+ -----------+
Figure 4: TLS Stack Interfaces
These TLS software stack APIs enable application developers to build
the software architectures illustrated in Figure 5 and Figure 6.
In both architectures, the application creates and interacts with an
application layer TLS session in order to generate and consume raw
TLS records. The application transports these raw TLS records inside
transport layer message bodies using whatever standard transport
layer stack is suitable for the application or architecture. This
document does not place any restrictions on the choice of transport
layer and any suitable protocol such as HTTP, TCP, CoAP, ZigBee,
Bluetooth, etc. could be used.
The transport layer will typically encrypt data, and this encryption
is completely independent from any application layer encryption. The
transport stack may create a transport layer TLS session. The
application layer TLS session and transport layer TLS session can
both leverage a shared, common TLS software stack. This high level
architecture is applicable to both clients and application services.
The key differences between the architectures are as follows.
In the model illustrated in Figure 5, the application sends all
sensitive data that needs to be securely exchanged with the peer
application through the Application TLS session in order to be
encrypted and decrypted. All sensitive application data is thus
encoded within TLS records by the TLS stack, and these TLS records
are transmitted over the transport layer.
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+-------------+
| | App
| | Data +---------+
| Application |<---------->| App | +---------+
| | TLS | TLS |----->| TLS |
| | Records | Session | | Stack |
| +--->|<---------->| | +---------+
| | | +---------+ ^
| | | |?
| | | Transport +-----------+ +------------+
| | | Payload | Transport | | Transport |
| +--->|<--------->| Stack |--->| Encryption |-->Packets
+-------------+ +-----------+ +------------+
Figure 5: TLS Stack used for all data encryption
In the model illustrated in Figure 6, the application establishes an
application layer TLS session purely for the purposes of key
exchange. Therefore, the only TLS records that are sent or received
by the application layer are TLS handshake records. Once the
application layer TLS session is established, the application uses
Keying Material Exporter [RFC5705] APIs to export keying material
from the TLS stack from this application layer TLS session. The
application can then use these exported keys to derive suitable
shared encryption keys with its peer for exchange of encrypted data.
The application encrypts and decrypts sensitive data using these
shared encryption keys using any suitable cryptographic library
(which may be part of the same library that provides the TLS stack),
and transports the encrypted data directly over the transport layer.
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+--------------+
| |
| Application |
| |
| +-------+ | +---------+
| | App | | Key Export | |
| | Data |<---|<-----------| |
| | Crypto| | | App |
| +-------+ | TLS | TLS | +---------+
| ^ | Handshake | Session |----->| TLS |
| | | Records | | | Stack |
| | +--->|<---------->| | +---------+
| | | | +---------+ ^
| | | | |?
| | | | Transport +-----------+ +------------+
| | | | Payload | Transport | | Transport |
| +----+--->|<--------->| Stack |--->| Encryption |-->Packets
+--------------+ +-----------+ +------------+
Figure 6: TLS stack used for key agreement and exporting
The choice of which application architecture to use will depend on
the overall solution architecture, and the underlying transport layer
or layers in use. While the choice of application architecture is
outside the scope of this document, some considerations are outlined
here.
o in some IoT use cases reducing the number of bytes transmitted is
important. [I-D.mattsson-lwig-security-protocol-comparison]
analyses the overhead of TLS headers compared with OSCORE
[I-D.ietf-core-object-security] illustrating the additional
overhead associated with TLS headers. The overhead varies between
the different TLS versions and also between TLS and DTLS. It may
be more appropriate to use the architecture defined in Figure 6 in
order to establish shared encryption keys, and then transport
encrypted data directly without the overhead of unwanted TLS
record headers.
o when using HTTP as a transport layer, it may be more appropriate
to use the architecture defined in Figure 6 in order to avoid any
TLS session vs. HTTP session affinity issues.
5.1.1. Application Architecture Benefits
There are several benefits to using a standard TLS software stack to
establish an application layer secure communications channel between
a client and a service. These include:
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o no need to define a new cryptographic negotiation and exchange
protocol between client and service
o automatically benefit from new cipher suites by simply upgrading
the TLS software stack
o automatically benefit from new features, bugfixes, etc. in TLS
software stack upgrades
5.1.2. ATLS Packet Identification
It is recommended that ATLS packets are explicitly identified by a
standardized, transport-specific identifier enabling any gateways and
middleboxes to identify ATLS packets. Middleboxes have to contend
with a vast number of applications and network operators have
difficulty configuring middleboxes to distinguish unencrypted but not
explicitly identified application data from end-to-end encrypted
data. This specification aims to assist network operators by
explicitly identifying ATLS packets. The HTTP and CoAP encodings
documented in Section 8 and Section 7 explicitly identify ATLS
packets.
5.1.3. ATLS Session Tracking
The ATLS application service establishes multiple ATLS sessions with
multiple clients. As TLS sessions are stateful, the application
service must be able to correlate ATLS records from different clients
across the relevant ATLS sessions. The details of how session
tracking is implemented are outside the scope of this document.
Recommendations are given in Section 8 and Section 7, but session
tracking is application and implementation specific.
5.1.4. ATLS Record Inspection
No constraints are placed on the ContentType contained within the
transported TLS records. The TLS records may contain handshake,
application_data, alert or change_cipher_spec messages. If new
ContentType messages are defined in future TLS versions, these may
also be transported using this protocol.
5.1.5. ATLS Message Routing
In many cases ATLS message routing is trival. However, there are
potentially cases where the middlebox topology is quite complex and
an example is shown in Figure 7. In this scenario multiple devices
(Client 1-3) are connected using serial communication to a gateway
(referred as middlebox A). Middlebox A communicates with another
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middlebox B over UDP/IP. Middlebox B then interacts with some
servers in the backend using CoAP over TCP.
This scenario raises the question about the ATLS message routing. In
particular, there are two questions:
o How do the middleboxes know to which IP address to address the
ATLS packet? This question arises in scenarios where clients are
communicating over non-IP transports.
o How are response messages demultiplexed?
In some scenarios it is feasible to pre-configure the destination IP
address of outgoing packets. Another other scenarios extra
information available in the ATLS message or in a shim layer has to
provide the necessary information. In the case of ATLS the use of
the Server Name Indicating (SNI) parameter in the TLS/DTLS
ClientHello message is a possibility to give middleboxes enough
information to determine the ATLS communication endpoint. This
approach is also compatible with SNI encryption.
For demultiplexing again different approaches are possible. The
simplest approach is to use separate source ports for each ATLS
session. In our example, Middlebox A allocates a dedicated socket
(with a separate source port) for outgoing UDP datagrams in order to
be able to relay a response message to the respective client.
Alternatively, it is possible to make use of a shim layer on top of
the transport that provides this extra demultiplexing capabilities.
The use of multiple UDP "sessions" (as well as different TCP
sessions) has the advantage of avoiding head-of-line blocking.
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+---------+ +---------+
| Server 1|----+-----| Server 2|
+---------+ | +---------+
|
|CoAP
|over
|TCP/TLS
|
+-----+-----+
|Middlebox B|
+-----------+
|
|
|CoAP
|over
|UDP/DTLS
|
+-----------+
+---------|Middlebox A|-----------+
| +-----------+ |
| | |
|CoAP |CoAP |CoAP
|over |over |over
|Serial |Serial |Serial
| | |
+--------+ +--------+ +--------+
|Client 1| |Client 2| |Client 3|
+--------+ +--------+ +--------+
Figure 7: Message Routing Scenario
5.1.6. Implementation
Pseudo code illustrating how to read and write TLS records directly
from byte buffers using both OpenSSL BIO functions and Java JSSE
SSLEngine is given in the appendices. A blog post by [Norrell]
outlines a similar approach to leveraging OpenSSL BIO functions, and
Oracle publish example code for leveraging [SSLEngine].
5.2. Functional Design
The functional design assumes that an authorization system has
established operational keys for authenticating endpoints. In a
layered design, this needs to be done for each layer, which may
operate in two separate authorization domains. Note that Figure 8
shows a generic setup where TLS/DTLS is used at two layers. In some
cases, use of TLS/DTLS at the application layer may be sufficient
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where lower layer security mechanisms provide protection of the
transport-specific headers.
+-------------------------------------------------------+
| +---+ +---+ |
| +--------+ |APP| |APP| +--------+ |
| |security| +---+ +---+ |security| |
| |--------+ ^ ^ |--------+ |
| |policies| | | |policies| |
| |LAYER 0 | | | |LAYER 0 | |
| +--------+ v v +--------+ |
| + +------+ APP +------+ + |
| | | TLS- |<--------->| TLS- | | |
| +----->|SERVER| LAYER |CLIENT|<-----+ |
| +------+ +------+ |
| TOP LAYER ^ ^ |
+-----------------|-------------------|-----------------+
| BOTTTOM LAYER | | |
| v v |
| +------+ TRANSPORT +------+ |
| | TLS- |<--------->| TLS- | |
| +--------+ |SERVER| LAYER |CLIENT| +--------+ |
| |security| +------+ +------+ |security| |
| |--------+ ^ ^ |--------+ |
| |policies| | | |policies| |
| |LAYER 1 +-----+ +-----+LAYER 1 | |
| +--------+ +--------+ |
| |
+-------------------------------------------------------+
Figure 8: Functional Design
The security policies of one layer are distinct from those of another
in Figure 8. They may overlap, but that is not necessary or perhaps
even likely since the key exchanges at the different layers terminate
at different endpoints and the two often have different authorization
domains.
TLS can protect IoT device-to-gateway communications "on the wire"
using the "bottom layer" of Figure 8, and it can protect application
data from the device to the application server using the "top layer."
Application and transport security each have a role to play.
Transport security restricts access to messages on the networks,
notably application headers and application-layer TLS restricts
access to the application payloads.
As shown in Figure 8, an application-layer message, which gets
encrypted and integrity protected and, in the generic case, the the
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resulting TLS message and headers are passed to a TLS socket at the
bottom layer, which may have a different security policy than the
application layer.
5.3. Network Architecture
An example network deployment is illustrated in Figure 9. It shows a
constrained client connecting to an application service via an
internet gateway. The client uses CoAP over DTLS to communicate with
the gateway. The gateway extracts the messages the client sent over
CoAP and sends these messages inside HTTP message bodies to the
application service. It also shows a TLS terminator deployed in
front of the application service. The client establishes a transport
layer CoAP/DTLS connection with the gateway (C->G DTLS), the gateway
in turn opens a transport layer TLS connection with the TLS
terminator deployed in front of the service (G->T TLS). The client
can ignore any certificate validation errors when it connects to the
gateway. CoAP messages are transported between the client and the
gateway, and HTTP messages are transported between the client and the
service. Finally, application layer TLS messages are exchanged
inside the CoAP and HTTP message bodies in order to establish an end-
to-end TLS session between the client and the service (C->S TLS).
+----------+ +----------+
| App Data | | App Data |
+----------+ +----------+ +----------+
| C->S TLS | | C->S TLS | | App Data |
+----------+ +----------+ +----------+
| CoAP | | HTTP | | C->S TLS |
+----------+ +----------+ +----------+
| C->G DTLS| | M->T TLS | | HTTP |
+----------+ +----------+ +----------+
| UDP | | TCP | | TCP |
+----------+ +----------+ +----------+
+--------+ +-----------+ +----------------+ +---------+
| Client |----->| Gateway |----->| TLS Terminator |---->| Service |
+--------+ +-----------+ +----------------+ +---------+
^ ^
| |
+-------------Client to Service ATLS Connection-------------+
Figure 9: Constrained Device Gateway Network Architecture
Another typical network deployment is illustrated in Figure 10. It
shows a client connecting to a service via a middlebox. It also
shows a TLS terminator deployed in front of the service. The client
establishes a transport layer TLS connection with the middlebox (C->M
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TLS), the middlebox in turn opens a transport layer TLS connection
with the TLS terminator deployed in front of the service (M->T TLS).
The client can ignore any certificate validation errors when it
connects to the middlebox. HTTP messages are transported over this
layer between the client and the service. Finally, application layer
TLS messages are exchanged inside the HTTP message bodies in order to
establish an end-to-end TLS session between the client and the
service (C->S TLS).
+----------+ +----------+
| App Data | | App Data |
+----------+ +----------+ +----------+
| C->S TLS | | C->S TLS | | App Data |
+----------+ +----------+ +----------+
| HTTP | | HTTP | | C->S TLS |
+----------+ +----------+ +----------+
| C->M TLS | | M->T TLS | | HTTP |
+----------+ +----------+ +----------+
| TCP | | TCP | | TCP |
+----------+ +----------+ +----------+
+--------+ +-----------+ +----------------+ +---------+
| Client |----->| Middlebox |----->| TLS Terminator |---->| Service |
+--------+ +-----------+ +----------------+ +---------+
^ ^
| |
+-------------Client to Service ATLS Connection-------------+
Figure 10: HTTP Middlebox Network Architecture
6. ATLS Session Establishment
Figure 11 illustrates how an ATLS session is established using the
key exporting architectural model shown in Figure 6. The number of
RTTs that take place when establishing a TLS session depends on the
version of TLS and what capabilities are enabled on the TLS software
stack. For example, a 0-RTT exchange is possible with TLS 1.3. If
applications wish to ensure a predictable number of RTTs when
establishing an application layer TLS connection, this may be
achieved by configuring the TLS software stack appropriately.
The outline is as follows:
o the client creates an ATLS session object
o the client initiates a TLS handshake on the session
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o the client extracts the TLS records for the first TLS flight (the
first RTT)
o the client sends the TLS records over the transport layer to the
server
o on receipt of the TLS flight, the server creates an ATLS session
object
o the server injects the received TLS flight into the session
o the server extracts the TLS records for the first TLS flight
response
o the server sends the TLS response records over the transport layer
to the client
o the client injects the received TLS records into its TLS session
completing the first full RTT
o the client and server repeat the above process and complete the
second RTT
o once the ATLS session is up, both sides export keying material
o both sides now can exchange data encrypted using shared keys
derived from the keying material
+-------------------------------+ +-------------------------------+
| Client | | ATLS Server |
+---------+---+-----+-+---------+ +---------+--+-----+--+---------+
| ATLS | | App | |Transport| |Transport| | App | | ATLS |
| Session | +-----+ | Stack | | Stack | +-----+ | Session |
+---------+ | +---------+ +---------+ | +---------+
| | | | | |
| | | | | |
| | | | | |
| Create | | | | |
| Session | | | | |
+ |<---------| | | | |
| | Start | | | | |
| | Handshake| | | | |
| |<---------| | | | |
| | TLS | | | | |
| | Records | Pack | | | |
| |--------->| Records | | | |
| |-------->| send packet | Unpack | |
R | | |------------>| Records | Create |
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T | | | |--------->| Session |
T | | | | |--------->|
| | | | | TLS |
1 | | | | | Records |
| | | | |--------->|
| | | | | | TLS |
| | | | | Pack | Records |
| | | | | Records |<---------|
| | | Unpack |send response|<---------| |
| | TLS | Records |<------------| | |
| | Records |<--------| | | |
+ |<---------| | | | |
| TLS | | | | |
| Records | | | | |
+ |--------->|-------->|------------>|--------->|--------->|
| | | | | | |
| | | | | Session |
R | | | | | Up |
T | | | | |<---------|
T | | | | | TLS |
| | | | | Records |
2 |<---------|<--------|<------------|<---------|<---------|
| Session | | | | |
| | Up | | | | |
+ |--------->| | | | |
| Export | | | | Export |
| Keys | | | | Keys |
|--------->| | E2E Session | |<---------|
| |<--------|-------------|--------->| |
Figure 11: ATLS Session Establishment
7. ATLS over CoAP Transport
To carry TLS messages over CoAP [RFC7252] it is recommended to use
Confirmable messages while DTLS payloads may as well use non-
confirmable messages. The exchange pattern in CoAP uses the
following style: A request from the CoAP client to the CoAP server
uses a POST with the ATLS message contained in the payload of the
request. An ATLS response is returned by the CoAP server to the CoAP
client in a 2.04 (Changed) message.
When DTLS messages are conveyed in CoAP over UDP then the DDoS
protection offered by DTLS MAY be used instead of replicating the
functionality at the CoAP layer. If TLS is conveyed in CoAP over UDP
then DDoS protection by CoAP has to be utilized. Carrying ATLS
messages in CoAP over TCP does not require any additional DDoS
protection.
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The URI path used by ATLS is "/.well-known/atls".
{{coap-example} shows a TLS 1.3 handshake inside CoAP graphically.
Client Server
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/atls"
| | Content-Format: application/atls
| | Payload: ATLS (ClientHello)
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Format: application/atls
| | Payload: ATLS (ServerHello,
| | {EncryptedExtensions}, {CertificateRequest*}
| | {Certificate*}, {CertificateVerify*} {Finished})
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/atls"
| | Content-Format: application/atls
| | Payload: ATLS ({Certificate*},
| | {CertificateVerify*}, {Finished})
| |
|<---------+ Header: 2.04 Changed
| 2.04 |
| |
Figure 12: Transferring ATLS in CoAP
Note that application data can already be sent by the server in the
second message and by the client in the third message, in case of the
full TLS 1.3 handshake. In case of the 0-RTT handshake application
data can be sent earlier. To mix different media types in the same
CoAP payload the application/multipart-core content type is used.
Note also that CoAP blockwise transfer MAY be used if the payload
size, for example due to the size of the certificate chain, exceeds
the MTU size.
8. ATLS over HTTP Transport
The assumption is that the client will establish a transport layer
connection to the server for exchange of HTTP messages. The
underlying transport layer connection could be over TCP or TLS. The
client will then establish an application layer TLS connection with
the server by exchanging TLS records with the server inside HTTP
message request and response bodies.
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Note that ATLS over HTTP transport addresses a different deployment
scenario than HTTP CONNECT proxies. HTTP CONNECT proxy behaviour is
compared and contrasted with ATLS in Appendix B.
8.1. Protocol Summary
All ATLS records are transported unmodified as binary data within
HTTP message bodies. The application simply extracts the TLS records
from the TLS stack and inserts them directly into HTTP message
bodies. Each message body contains a full TLS flight, which may
contain multiple TLS records.
The client sends all ATLS records to the server in the bodies of POST
requests.
The server sends all ATLS records to the client in the bodies of 200
OK responses to the POST requests.
The URI path used by ATLS is "/.well-known/atls".
8.2. Content-Type Header
A new Content-Type header value is defined:
Content-type: application/atls
All message bodies containing ATLS records must set this Content-
Type. This enables middleboxes to readily identify ATLS payloads.
8.3. HTTP Status Codes
This document does not define any new HTTP status codes, and does not
specify additional semantics or refine existing semantics for status
codes. This is the best current practice as outlined in
[I-D.ietf-httpbis-bcp56bis].
8.4. ATLS Session Tracking
The application service needs to track multiple client application
layer TLS sessions so that it can correlate TLS records received in
HTTP message bodies with the appropriate TLS session. The
application service should use stateful cookies [RFC6265] in order to
achieve this as recommended in [I-D.ietf-httpbis-bcp56bis].
[[TODO]] An alternative approach for session tracking is to use a
RESTful model and create new resoruces to track sessions.
For example:
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Client Server
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/atls"
| | Content-Format: application/atls
| | Payload: ATLS (ClientHello)
| |
|<---------+ Header: 2.01 Created
| 2.01 | Content-Format: application/atls
| | Location-Path: /RaNdOm
| | Payload: ATLS (ServerHello,
| | {EncryptedExtensions}, {CertificateRequest*}
| | {Certificate*}, {CertificateVerify*} {Finished})
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/RaNdOm"
| | Content-Format: application/atls
| | Payload: ATLS ({Certificate*},
| | {CertificateVerify*}, {Finished})
| |
|<---------+ Header: 2.04 Changed
| 2.04 |
| |
This may align bettern with CoAP implementations.
8.5. Session Establishment and Key Exporting
It is recommended that applications using ATLS over HTTP transport
only use ATLS for session establishment and key exchange, resulting
in only 2 ATLS RTTs between the client and the application service.
Key exporting must be carried out as described in Section 9.
8.6. Illustrative ATLS over HTTP Session Establishment
A client initiates an ATLS session by sending the first TLS flight in
a POST request message body to the ATLS server.
POST /.well-known/atls
Content-Type: application/atls
<binary TLS client flight 1 records>
The server handles the request, creates an ATLS session object, and
replies by including its first TLS flight in a 200 OK message body.
The server also sets a suitable cookie for session tracking purposes.
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200 OK
Content-Type: application/atls
Set-Cookie: my-atls-cookie=my-cookie-value
<binary TLS server flight 1 records>
The client handles the server first flight TLS records and replies
with its second flight.
POST /.well-known/atls
Content-Type: application/atls
Cookie: my-atls-cookie=my-cookie-value
<binary TLS client flight 2 records>
The server handles the second flight, establishes the ATLS session,
and replies with its second flight.
200 OK
Content-Type: application/atls
<binary TLS server flight 2 records>
9. Key Exporting and Application Data Encryption
When solutions implement the architecture described in Figure 6, they
leverage [RFC5705] for exporting keys. This section describes how to
establish keying material and negotiate algorithms for OSCORE and for
COSE.
9.1. OSCORE
When the OSCORE mode has been agreed using the "oscore_connection_id"
extension defined in this document, different keys are used for DTLS/
TLS record protection and for OSCORE packet protection. These keys
are produced using a TLS exporter [RFC5705] and the exporter takes
three input values:
o a disambiguating label string,
o a per-association context value provided by the application using
the exporter, and
o a length value.
The label string for use with this specification is defined as 'atls-
oscore'. The per-association context value is empty.
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The length value is twice the size of the key size utilized by the
negotiated algorithm since the lower-half is used for the Master
Secret and the upper-half is used for the Master Salt.
For example, if a TLS/DTLS 1.2 handshake negotiated the
TLS_PSK_WITH_AES_128_CCM_8 ciphersuite then the key size utilized by
the negotiated algorithm, i.e. AES 128, is 128 bit. Hence, the key
extractor is requested to produce 2 x 128 bit keying material.
The following parameters are needed for use with OSCORE:
o Master Secret: The master secret is derived as described above.
o Sender ID: This values is negotiated using the
"oscore_connection_id" extension, as described in Section 11.1.
o Recipient ID: This values is negotiated using the
"oscore_connection_id" extension, as described in Section 11.1.
o AEAD Algorithm: This value is negotiated using the ciphersuite
exchange provided by the TLS/DTLS handshake. For example, if a
TLS/DTLS 1.2 handshake negotiated the TLS_PSK_WITH_AES_128_CCM_8
ciphersuite then the AEAD algorithm identifier is AES_128_CCM_8,
which corresponds to two COSE algorithms, which both use AES-CCM
mode with a 128-bit key, a 64-bit tag:
* AES-CCM-64-64-128
* AES-CCM-16-64-128 The difference between the two is only the
length of the nonce, which is 7-bytes in the former case and
13-bytes in the latter. In TLS/DTLS the nonce value is not
negotiated but fixed instead. Figure 13 provides the mapping
between the TLS defined ciphersuite and the COSE algorithms.
o Master Salt: The master salt is derived as described above.
o HKDF Algorithm: This value is negotiated using the ciphersuite
exchange provided by the TLS/DTLS handshake. As a default,
SHA-256 is assumed as a HKDF algorithm for algorithms using
128-bit key sizes and SHA384 for 256-bit key sizes.
o Replay Window: A default window size of 32 packets is assumed.
9.2. COSE
The key exporting procedure for COSE is similiar to the one defined
for OSCORE. The label string for use with this specification is
defined as 'atls-cose'. The per-association context value is empty.
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The length value is twice the size of the key size utilized by the
negotiated algorithm since the lower-half is used for the Master
Secret and the upper-half is used for the Master Salt.
The COSE algorithm corresponds to the ciphersuite negotiated during
the TLS/DTLS handshake with with the mapping provided in Figure 13.
The HKDF algorithm is negotiated using the the TLS/DTLS handshake.
As a default, SHA-256 is assumed as a HKDF algorithm for algorithms
using 128-bit key sizes and SHA384 for 256-bit key sizes.
COSE uses key ids to allow finding the appropriate security context.
Those key IDs conceptually correspond to CIDs, as described in
Section 11.2.
10. TLS Ciphersuite to COSE/OSCORE Algorithm Mapping
TLS Ciphersuite | COSE/OSCORE Algorithm
------------------+--------------------------------------------------
AES_128_CCM_8 | AES-CCM w/128-bit key, 64-bit tag, 13-byte nonce
AES_256_CCM_8 | AES-CCM w/256-bit key, 64-bit tag, 13-byte nonce
CHACHA20_POLY1305 | ChaCha20/Poly1305 w/256-bit key, 128-bit tag
AES_128_CCM | AES-CCM w/128-bit key, 128-bit tag, 13-byte nonce
AES_256_CCM | AES-CCM w/256-bit key, 128-bit tag, 13-byte nonce
AES_128_GCM | AES-GCM w/128-bit key, 128-bit tag
AES_256_GCM | AES-GCM w/256-bit key, 128-bit tag
Figure 13: TLS Ciphersuite to COSE/OSCORE Algorithm Mapping
11. TLS Extensions
11.1. The "oscore_connection_id" Extension
This document defines the "oscore_connection_id" extension, which is
used in ClientHello and ServerHello messages. It is used only for
establishing the OSCORE Sender ID and the OSCORE Recipient ID. The
OSCORE Sender ID maps to the CID provided by the server in the
ServerHello and the OSCORE Recipient ID maps to the CID provided by
the client in the ClientHello.
The negotiation mechanism follows the procedure used in
[I-D.ietf-tls-dtls-connection-id] with the exception that the
negotiated CIDs agreed with the "oscore_connection_id" extension is
only used with OSCORE and does not impact the record layer format of
the DTLS/TLS payloads nor the MAC calculation used by DTLS/TLS. As
such, this extension can be used with DTLS as well as with TLS when
those protocols are used at the application layer.
The extension type is specified as follows.
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enum {
oscore_connection_id(TBD), (65535)
} ExtensionType;
struct {
opaque cid<0..2^8-1>;
} ConnectionId;
Figure 14: The 'oscore_connection_id' Extension
Note: This extension allows a client and a server to determine
whether an OSCORE security context should be established.
11.2. The "cose_ext" Extension
This document defines the "cose_ext" extension, which is used in
ClientHello and ServerHello messages. It is used only for
establishing the key identifiers, AEAD algorithms, as well as keying
material for use with application layer protection using COSE. The
CID provided by the server in the ServerHello maps to the COSE kid
transmitted from the client to the server and the CID provided by the
client in the ClientHello maps to the COSE kid transmitted from the
server to the client.
The negotiation mechanism follows the procedure used in
[I-D.ietf-tls-dtls-connection-id] with the exception that the
negotiated CIDs agreed with the "cose_ext" extension is only used
with COSE and does not impact the record layer format of the DTLS/TLS
payloads nor the MAC calculation used by DTLS/TLS. As such, this
extension can be used with DTLS as well as with TLS when those
protocols are used at the application layer.
The extension type is specified as follows.
enum {
oscore_connection_id(TBD), (65535)
} ExtensionType;
struct {
opaque cid<0..2^8-1>;
} ConnectionId;
Figure 15: The 'cose_ext' Extension
Note: This extension allows a client and a server to determine
whether an COSE security context should be established.
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12. IANA Considerations
12.1. "oscore_connection_id" TLS extension
IANA is requested to allocate two entries to the existing TLS
"ExtensionType Values" registry, defined in [RFC5246], for
oscore_connection_id(TBD1) and cose_ext(TBD2) defined in this
document, as described in the table below.
Value Extension Name TLS 1.3 DTLS Only Recommended Reference
-----------------------------------------------------------------------
TBD1 oscore_connection_id Y N N [[This doc]]
TBD2 cose_ext Y N N [[This doc]]
Note: The "N" values in the Recommended column are set because these
extensions are intended only for specific use cases.
12.2. TLS Ciphersuite to OSCORE/COSE Algorithm Mapping
IANA is requested to create a new registry for mapping TLS
ciphersuites to SCORE/COSE algorithms
An initial mapping can be found in Figure 13.
Registration requests are evaluated after a three-week review period
on the tls-reg-review@ietf.or mailing list, on the advice of one or
more Designated Experts [RFC8126]. However, to allow for the
allocation of values prior to publication, the Designated Experts may
approve registration once they are satisfied that such a
specification will be published.
Registration requests sent to the mailing list for review should use
an appropriate subject (e.g., "Request to register an TLS - OSCORE/
COSE algorithm mapping: example"). Registration requests that are
undetermined for a period longer than 21 days can be brought to the
IESG's attention (using the iesg@ietf.org mailing list) for
resolution.
Criteria that should be applied by the Designated Experts includes
determining whether the proposed registration duplicates existing
functionality, whether it is likely to be of general applicability or
whether it is useful only for a single extension, and whether the
registration description is clear.
IANA must only accept registry updates from the Designated Experts
and should direct all requests for registration to the review mailing
list.
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12.3. .well-known URI Registry
IANA is requested to add the well-known URI 'atls' to the Well-Known
URIs registry.
o URI suffix: atls
o Change controller: IETF
o Specification document(s): [[this document]]
o Related information: None
12.4. Media Types Registry
IANA is requested to add the media type 'application/atls' to the
Media Types registry.
o Type name: application
o Subtype name: atls
o Required parameters: N/A
o Optional parameters: N/A
o Encoding considerations: binary
o Security considerations: See Security Considerations section of
this document.
o Interoperability considerations: N/A
o Published specification: [[this document]] (this document)
o Applications that use this media type: Potentially any
o Fragment identifier considerations: N/A
o Additional information:
* Magic number(s): N/A
* File extension(s): N/A
* Macintosh file type code(s): N/A
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o Person & email address to contact for further information: See
"Authors' Addresses" section.
o Intended usage: COMMON
o Restrictions on usage: N/A
o Author: See "Authors' Addresses" section.
o Change Controller: IESG
12.5. HTTP Content-Formats Registry
IANA is requested to add the media type 'application/atls' to the
HTTP Content-Formats registry.
o Media Type: application/atls
o Encoding: binary
o ID: TBD
o Reference: [[this document]]
12.6. CoAP Content-Formats Registry
IANA is requested to add the media type 'application/atls' to the
CoAP Content-Formats registry.
o Media Type: application/atls
o Encoding: binary
o ID: TBD
o Reference: [[this document]]
12.7. TLS Key Extractor Label
IANA is requested to register the "application-layer-tls" label in
the TLS Extractor Label Registry to correspond to this specification.
13. Security Considerations
This specification re-uses the TLS and DTLS and hence the security
considerations of the respective TLS/DTLS version applies. As
described in Section 5.2, implementers need to take the policy
configuration into account when applying security protection at
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various layers of the stack even if the same protocol is used since
the communiation endpoints and the security requirements are likely
going to vary.
For use in the IoT environment the considerations described in
[RFC7925] apply and other environments the guidelines in [RFC7525]
are applicable.
14. References
14.1. Normative References
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-16 (work in
progress), March 2019.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-43 (work in progress), April
2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[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>.
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[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
14.2. Informative References
[ALTS] Google, "Application Layer Transport Security", December
2017, <https://cloud.google.com/security/encryption-in-
transit/application-layer-transport-security/>.
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[Bluetooth]
Bluetooth, "Bluetooth Core Specification v5.0", 2016,
<https://www.bluetooth.com/>.
[I-D.ietf-anima-bootstrapping-keyinfra]
Pritikin, M., Richardson, M. C., Eckert, T., Behringer, M.
H., and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", draft-ietf-anima-bootstrapping-
keyinfra-45 (work in progress), November 2020.
[I-D.ietf-httpbis-bcp56bis]
Nottingham, M., "Building Protocols with HTTP", draft-
ietf-httpbis-bcp56bis-14 (work in progress), August 2021.
[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., Fossati, T., and A. Kraus,
"Connection Identifiers for DTLS 1.2", draft-ietf-tls-
dtls-connection-id-13 (work in progress), June 2021.
[I-D.mattsson-lwig-security-protocol-comparison]
Mattsson, J. and F. Palombini, "Comparison of CoAP
Security Protocols", draft-mattsson-lwig-security-
protocol-comparison-01 (work in progress), March 2018.
[I-D.rescorla-tls-ctls]
Rescorla, E., Barnes, R., and H. Tschofenig, "Compact TLS
1.3", draft-rescorla-tls-ctls-04 (work in progress), March
2020.
[I-D.selander-ace-cose-ecdhe]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
cose-ecdhe-14 (work in progress), September 2019.
[LwM2M] Open Mobile Alliance, "Lightweight Machine to Machine
Requirements", December 2017,
<http://www.openmobilealliance.org/>.
[Noise] Perrin, T., "Noise Protocol Framework", October 2017,
<http://noiseprotocol.org/>.
[Norrell] Norrell, ., "Use SSL/TLS within a different protocol with
BIO pairs", 2016,
<https://thekerneldiaries.com/2016/06/13/openssl-ssltls-
within-a-different-protocol/>.
[Signal] Open Whisper Systems, "Signal Protocol", 2016,
<https://signal.org/>.
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[SSLEngine]
Oracle, "SSLEngineSimpleDemo.java", 2004, <https://docs.or
acle.com/javase/7/docs/technotes/guides/security/jsse/
samples/sslengine/SSLEngineSimpleDemo.java>.
[ZigBee] ZigBee Alliance, "ZigBee Specification", 2012,
<http://www.zigbee.org>.
Appendix A. Pseudo Code
This appendix gives both C and Java pseudo code illustrating how to
inject and extract raw TLS records from a TLS software stack. Please
note that this is illustrative, non-functional pseudo code that does
not compile.
A.1. OpenSSL
OpenSSL provides a set of Basic Input/Output (BIO) APIs that can be
used to build a custom transport layer for TLS connections. This
appendix gives pseudo code on how BIO APIs could be used to build a
client application that completes a TLS handshake and exchanges
application data with a service.
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char inbound[MAX];
char outbound[MAX];
int rx_bytes;
SSL_CTX *ctx = SSL_CTX_new();
SSL *ssl = SSL_new(ctx);
// Create in-memory BIOs and plug in to the SSL session
BOI* bio_in = BIO_new(BIO_s_mem());
BOI* bio_out = BIO_new(BIO_s_mem());
SSL_set_bio(ssl, bio_in, bio_out);
// We are a client
SSL_set_connect_state(ssl);
// Loop through TLS flights until we are done
do {
// Calling SSL_do_handshake() will result in a full
// TLS flight being written to the BIO buffer
SSL_do_handshake(ssl);
// Read the client flight that the TLS session
// has written to memory
BIO_read(bio_out, outbound, MAX);
// POST the outbound bytes to the server using a suitable
// function. Lets assume that the server response will be
// written to the 'inbound' buffer
num_bytes = postTlsRecords(outbound, inbound);
// Write the server flight to the memory BIO so the TLS session
// can read it. The next call to SSL_do_handshake() will handle
// this received server flight
BIO_write(bio_in, inbound, num_bytes);
} while (!SSL_is_init_finished(ssl));
// Send a message to the server. Calling SSL_write() will run the
// plaintext through the TLS session and write the encrypted TLS
// records to the BIO buffer
SSL_write(ssl, "Hello World", strlen("Hello World"));
// Read the TLS records from the BIO buffer and
// POST them to the server
BIO_read(bio_out, outbound, MAX);
num_bytes = postTlsRecords(outbound, inbound);
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A.2. Java JSSE
The Java SSLEngine class "enables secure communications using
protocols such as the Secure Sockets Layer (SSL) or IETF RFC 2246
"Transport Layer Security" (TLS) protocols, but is transport
independent". This pseudo code illustrates how a server could use
the SSLEngine class to handle an inbound client TLS flight and
generate an outbound server TLS flight response.
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SSLEngine sslEngine = SSLContext.getDefault().createSSLEngine();
sslEngine.setUseClientMode(false);
sslEngine.beginHandshake();
// Lets assume 'inbound' has been populated with
// the Client 1st Flight
ByteBuffer inbound;
// 'outbound' will be populated with the
// Server 1st Flight response
ByteBuffer outbound;
// SSLEngine handles one TLS Record per call to unwrap().
// Loop until the engine is finished unwrapping.
while (sslEngine.getHandshakeStatus() ==
HandshakeStatus.NEED_UNWRAP) {
SSLEngineResult res = sslEngine.unwrap(inbound, outbound);
// SSLEngine may need additional tasks run
if (res.getHandshakeStatus() == NEED_TASK) {
Runnable run = sslEngine.getDelegatedTask();
run.run();
}
}
// The SSLEngine has now finished handling all inbound TLS Records.
// Check if it wants to generate outbound TLS Records. SSLEngine
// generates one TLS Record per call to wrap().
// Loop until the engine is finished wrapping.
while (sslEngine.getHandshakeStatus() ==
HandshakeStatus.NEED_WRAP) {
SSLEngineResult res = sslEngine.wrap(inbound, outbound);
// SSLEngine may need additional tasks run
if (res.getHandshakeStatus() == NEED_TASK) {
Runnable run = sslEngine.getDelegatedTask();
run.run();
}
}
// outbound ByteBuffer now contains a complete server flight
// containing multiple TLS Records
// Rinse and repeat!
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Appendix B. ATLS and HTTP CONNECT
It is worthwhile comparing and contrasting ATLS with HTTP CONNECT
tunneling.
First, let us introduce some terminology:
o HTTP Proxy: A HTTP Proxy operates at the application layer,
handles HTTP CONNECT messages from clients, and opens tunnels to
remote origin servers on behalf of clients. If a client
establishes a tunneled TLS connection to the origin server, the
HTTP Proxy does not attempt to intercept or inspect the HTTP
messages exchanged between the client and the server
o middlebox: A middlebox operates at the transport layer, terminates
TLS connections from clients, and originates new TLS connections
to services. A middlebox inspects all messages sent between
clients and services. Middleboxes are generally completely
transparent to applications, provided that the necessary PKI root
Certificate Authority is installed in the client's trust store.
HTTP Proxies and middleboxes are logically separate entities and one
or both of these may be deployed in a network.
HTTP CONNECT is used by clients to instruct a HTTP Forward Proxy
deployed in the local domain to open up a tunnel to a remote origin
server that is typically deployed in a different domain. Assuming
that TLS transport is used between both client and proxy, and proxy
and origin server, the network architecture is as illustrated in
Figure 16. Once the proxy opens the transport tunnel to the service,
the client establishes an end-to-end TLS session with the service,
and the proxy is blindly transporting TLS records (the C->S TLS
session records) between the client and the service. From the client
perspective, it is tunneling a TLS session to the service inside the
TLS session it has established to the proxy (the C->P TLS session).
No middlebox is attempting to intercept or inspect the HTTP messages
between the client and the service.
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+----------+ +----------+
| C->S HTTP| | C->S HTTP|
+----------+ +----------+
| C->S TLS | | C->S TLS |
+----------+ +----------+
| C->P TLS | | P->S TCP |
+----------+ +----------+
| C->P TCP |
+----------+
+--------+ +------------+ +---------+
| Client |----->| HTTP Proxy |----->| Service |
+--------+ +------------+ +---------+
Figure 16: HTTP Proxy transport layers
A more complex network topology where the network operator has both a
HTTP Proxy and a middlebox deployed is illustrated in Figure 17. In
this scenario, the proxy has tunneled the TLS session from the client
towards the origin server, however the middlebox is intercepting and
terminating this TLS session. A TLS session is established between
the client and the middlebox (C->M TLS), and not end-to-end between
the client and the server. It can clearly be seen that HTTP CONNECT
and HTTP Proxies serve completely different functions than
middleboxes.
Additionally, the fact that the TLS session is established between
the client and the middlebox can be problematic for two reasons:
o the middle box is inspecting traffic that is sent between the
client and the service
o the client may not have the necessary PKI root Certificate
Authority installed that would enable it to validate the TLS
connection to the middlebox. This is the scenario outlined in
Section 3.2.
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+----------+ +----------+ +----------+
| C->S HTTP| | C->S HTTP| | C->S HTTP|
+----------+ +----------+ +----------+
| C->M TLS | | C->M TLS | | M->S TLS |
+----------+ +----------+ +----------+
| C->P TLS | | P->M TCP | | M->S TCP |
+----------+ +----------+ +----------+
| C->P TCP |
+----------+
+--------+ +------------+ +-----------+ +---------+
| Client |----->| HTTP Proxy |----->| Middlebox |----->| Service |
+--------+ +------------+ +-----------+ +---------+
Figure 17: HTTP Proxy and middlebox transport layers
As HTTP CONNECT can be used to establish a tunneled TLS connection,
one hypothetical solution to this middlebox issue is for the client
to issue a HTTP CONNECT command to a HTTP Reverse Proxy deployed in
front of the origin server. This solution is not practical for
several reasons:
o if there is a local domain HTTP Forward Proxy deployed, this would
result in the client doing a first HTTP CONNECT to get past the
Forward Proxy, and then a second HTTP CONNECT to get past the
Reverse Proxy. No client or client library supports the concept
of HTTP CONNECT inside HTTP CONNECT.
o if there is no local domain HTTP Proxy deployed, the client still
has to do a HTTP CONNECT to the HTTP Reverse Proxy. This breaks
with standard and expected HTTP CONNECT operation, as HTTP CONNECT
is only ever called if there is a local domain proxy.
o clients cannot generate CONNECT from XHR in web applications.
o this would require the deployment of a Reverse Proxy in front of
the origin server, or else support of the HTTP CONNECT method in
standard web frameworks. This is not an elegant design.
o using HTTP CONNECT with HTTP 1.1 to a Reverse Proxy will break
middleboxes inspecting HTTP traffic, as the middlebox would see
TLS records when it expects to see HTTP payloads.
In contrast to trying to force HTTP CONNECT to address a problem for
which it was not designed to address, and having to address all the
issues just outlined; ATLS is specifically designed to address the
middlebox issue in a simple, easy to develop, and easy to deploy
fashion.
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o ATLS works seamlessly with HTTP Proxy deployments
o no changes are required to HTTP CONNECT semantics
o no changes are required to HTTP libraries or stacks
o no additional Reverse Proxy is required to be deployed in front of
origin servers
It is also worth noting that if HTTP CONNECT to a Reverse Proxy were
a conceptually sound solution, the solution still ultimately results
in encrypted traffic traversing the middlebox that the middlebox
cannot intercept and inspect. That is ultimately what ATLS results
in - traffic traversing the middle box that the middlebox cannot
intercept and inspect. Therefore, from a middlebox perspective, the
differences between the two solutions are in the areas of solution
complexity and protocol semantics. It is clear that ATLS is a
simpler, more elegant solution that HTTP CONNECT.
Appendix C. Alternative Approaches to Application Layer End-to-End
Security
End-to-end security at the application layer is increasing seen as a
key requirement across multiple applications and services. Some
examples of end-to-end security mechanisms are outlined here. All
the solutions outlined here have some common characteristics. The
solutions:
o do not rely on transport layer security
o define a new handshake protocol for establishment of a secure end-
to-end session
C.1. Noise
[Noise] is a framework for cryptographic protocols based on Elliptic
Curve Diffie-Hellman (ECDH) key agreement, AEAD encryption, and
BLAKE2 and SHA2 hash functions. Noise is currently used by WhatsApp,
WireGuard, and Lightning.
The current Noise protocol framework defines mechanisms for proving
possession of a private key, but does not define authentication
mechanisms. Section 14 "Security Considerations" of Noise states:
~~~ it's up to the application to determine whether the remote
party's static public key is acceptable ~~~
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C.2. Signal
The [Signal] protocol provides end-to-end encryption and uses EdDSA
signatures, Triple Diffie-Hellman handshake for shared secret
establishment, and the Double Ratchet Algorithm for key management.
It is used by Open Whisper Systems, WhatsApp and Google.
Similar to Noise, Signal does not define an authentication mechanism.
The current [X3DH] specification states in Section 4.1
"Authentication":
Methods for doing this are outside the scope of this document
C.3. Google ALTS
Google's Application Layer Transport Security [ALTS] is a mutual
authentication and transport encryption system used for securing
Remote Procedure Call (RPC) communications within Google's
infrastructure. ALTS uses an ECDH handshake protocol and a record
protocol containing AES encrypted payloads.
C.4. Ephemeral Diffie-Hellman Over COSE (EDHOC)
There is ongoing work to standardise EDHOC
[I-D.selander-ace-cose-ecdhe], which defines a SIGMA-I based
authenticated key exchange protocol using COSE and CBOR.
Authors' Addresses
Owen Friel
Cisco
Email: ofriel@cisco.com
Richard Barnes
Cisco
Email: rlb@ipv.sx
Max Pritikin
Cisco
Email: pritikin@cisco.com
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Hannes Tschofenig
Arm Ltd.
Email: hannes.tschofenig@gmx.net
Mark Baugher
Consultant
Email: mark@mbaugher.com
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