Independent Submission D. Thakore

Independent Submission D. Thakore
Request for Comments: 7562 CableLabs
Category: Informational July 2015
ISSN: 2070-1721

Transport Layer Security (TLS) Authorization Using
Digital Transmission Content Protection (DTCP) Certificates

Abstract

This document specifies the use of Digital Transmission Content
Protection (DTCP) certificates as an authorization data type in the
authorization extension for the Transport Layer Security (TLS)
protocol. This is in accordance with the guidelines for
authorization extensions as specified in RFC 5878. As with other TLS
extensions, this authorization data can be included in the client and
server hello messages to confirm that both parties support the
desired authorization data types. If supported by both the client
and the server, DTCP certificates are exchanged in the supplemental
data TLS handshake message as specified in RFC 4680. This
authorization data type extension is in support of devices containing
DTCP certificates issued by the Digital Transmission Licensing
Administrator (DTLA).

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7562.

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Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.

Table of Contents

1. Introduction ....................................................3
1.1. Applicability Statement ....................................3
1.2. Conventions ................................................4
2. Overview ........................................................4
2.1. Overview of DTCP Certificates ..............................4
2.2. Overview of SupplementalData Handshake .....................5
2.3. Overview of Authorization Extensions .......................5
2.4. Overview of SupplementalData Usage for Authorization .......6
3. DTCP Authorization Data Format ..................................6
3.1. DTCP Authorization Type ....................................6
3.2. DTCP Authorization Data ....................................6
3.3. Usage Rules for Clients to Exchange DTCP
Authorization Data .........................................7
3.4. Usage Rules for Servers to Exchange DTCP
Authorization Data .........................................8
3.5. TLS Message Exchange with dtcp_authz_data ..................8
3.6. Alert Messages .............................................9
4. IANA Considerations ............................................10
5. Security Considerations ........................................10
6. References .....................................................11
6.1. Normative References ......................................11
6.2. Informative References ....................................12
Appendix A. Alternate Double Handshake Example ....................13
Acknowledgements ..................................................15
Author's Address ..................................................15

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1. Introduction

The Transport Layer Security (TLS) protocol (see TLS 1.0 [RFC2246],
TLS 1.1 [RFC4346], and TLS1 .2 [RFC5246]) is being used in an ever
increasing variety of operational environments, the most common among
which is its use in securing HTTP traffic [RFC2818]. [RFC5878]
introduces extensions that enable TLS to operate in environments
where authorization information needs to be exchanged between the
client and the server before any protected data is exchanged. The
use of these TLS authorization extensions is especially attractive
since it allows the client and server to determine the type of
protected data to exchange based on the authorization information
received in the extensions.

A substantial number of deployed consumer electronics devices, such
as televisions, tablets, game consoles, set-top boxes, and other
multimedia devices, contain Digital Transmission Content Protection
[DTCP] certificates issued by [DTLA]. These DTCP certificates enable
secure transmission of premium audiovisual content between devices
over various types of links (e.g., DTCP over IP [DTCP-IP]). These
DTCP certificates can also be used to verify device functionality
(e.g., supported device features).

This document describes the format and necessary identifiers to
exchange DTCP certificates within the supplemental data message (see
[RFC4680]) while negotiating a TLS session. The DTCP certificates
are then used independent of their use for content protection (e.g.,
to verify supported features) and the corresponding DTCP
Authentication and Key Exchange (AKE) protocol. This communication
allows either the client, the server, or both to perform certain
actions or provide specific services. The actual semantics of the
authorization decision by the client/server are beyond the scope of
this document. The DTCP certificate, which is not an X.509
certificate, can be cryptographically tied to the X.509 certificate
being used during the TLS tunnel establishment by an Elliptic Curve
Digital Signature Algorithm (EC-DSA) [DTCP] signature.

1.1. Applicability Statement

DTCP-enabled consumer electronics devices (e.g., televisions, game
consoles) use DTCP certificates for secure transmission of
audiovisual content. The AKE protocol defined in [DTCP] is used to
exchange DTCP certificates and allows a device to be identified and
authenticated based on the information in the DTCP certificate.
However, these DTCP-enabled devices offer additional functionality
(e.g., via HTML5 User Agents or web-enabled applications) that is
distinct from its capability to transmit and play audiovisual
content. The mechanism outlined in this document allows a DTCP-

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enabled consumer electronics device to authenticate and authorize
using its DTCP certificate when accessing services over the internet;
for example, web applications on televisions that can enable value-
added services. This is anticipated to be very valuable since there
are a considerable number of such devices. The reuse of well-known
web security will also keep such communication consistent with
existing standards and best practices.

1.2. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2. Overview

2.1. Overview of DTCP Certificates

DTCP certificates issued by [DTLA] to DTLA-compliant devices come in
three general variations (see Section 4.2.3.1 of [DTCP]):

o Restricted Authentication device certificate format (Format 0):
Typically issued to devices with limited computation resources.

o Baseline Full Authentication device certificate format (Format 1):
This is the most commonly issued certificate format. Format 1
certificates include a unique DeviceID and device EC-DSA public/
private key pair generated by the DTLA. (See Section 4.3 of
[DTCP]).

o Extended Full Authentication device certificate format (Format 2):
This is issued to devices that possess additional functions (e.g.,
additional channel ciphers, specific device properties). The
presence of these additional functions is indicated by the device
capability mask as specified in Section 4.2.3.2 of [DTCP]. Format
2 certificates also include a unique DeviceID and device EC-DSA
public/private key pair generated by the DTLA (see Section 4.3 of
[DTCP]).

The mechanism specified in this document allows only Formats 1 and 2
DTCP certificates to be exchanged in the supplemental data message
since it requires the use of the EC-DSA private key associated with
the certificate.

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2.2. Overview of SupplementalData Handshake

Figure 1 illustrates the exchange of the SupplementalData message
during the TLS handshake as specified in [RFC4680] (repeated here for
convenience):

Client Server

ClientHello (with extensions) -------->

ServerHello(with extensions)
SupplementalData*
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone

SupplementalData*
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data

* Indicates optional or situation-dependent messages that are
not always sent.

[] Indicates that ChangeCipherSpec is an independent TLS
protocol content type; it is not a TLS handshake message.

Figure 1: TLS Handshake Message Exchange with SupplementalData

2.3. Overview of Authorization Extensions

[RFC5878] defines two authorization extension types that are used in
the ClientHello and ServerHello messages and are repeated below for
convenience:

enum {
client_authz(7), server_authz(8), (65535)
} ExtensionType;

A client uses the client_authz and server_authz extensions in the
ClientHello message to indicate that it will send client
authorization data and receive server authorization data,

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respectively, in the SupplementalData messages. A server uses the
extensions in a similar manner in its ServerHello message. [RFC5878]
also establishes a registry that is maintained by IANA to register
authorization data formats. This document defines a new
authorization data type for both the client_authz and server_authz
extensions and allows the client and server to exchange DTCP
certificates in the SupplementalData message.

2.4. Overview of SupplementalData Usage for Authorization

Section 3 of [RFC5878] specifies the syntax of the supplemental data
message when carrying the authz_data message that is negotiated in
the client_authz and/or server_authz types. This document defines a
new authorization data format that is used in the authz_data message
when sending DTCP Authorization Data.

3. DTCP Authorization Data Format

3.1. DTCP Authorization Type

The DTCP Authorization type definition in the TLS Authorization Data
Formats registry is:

dtcp_authorization(66);

3.2. DTCP Authorization Data

The DTCP Authorization Data is used when the AuthzDataFormat type is
dtcp_authorization. The syntax of the authorization data is:

struct {
opaque random_bytes[32];
} RandomNonce;

struct {
opaque RandomNonce nonce;
opaque DTCPCert<0..2^24-1>;
opaque ASN.1Cert<0..2^24-1>;
opaque signature<0..2^16-1>;
} dtcp_authz_data;

RandomNonce is generated by the server and consists of 32 bytes
generated by a high-quality, secure random number generator. The
client always sends back the server-generated RandomNonce in its
dtcp_authz_data structure. The RandomNonce helps the server in
detecting replay attacks. A client can detect replay attacks by

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associating the ASN.1 certificate in the dtcp_authz_data structure
with the certificate received in the Certificate message of the TLS
handshake, so a separate nonce for the client is not required.

DTCPCert is the sender's DTCP certificate. See Section 4.2.3.1 of
the DTCP Specification [DTCP].

ASN.1Cert is the sender's certificate used to establish the TLS
session, i.e., it is sent in the Certificate or ClientCertificate
message using the Certificate structure defined in Section 7.4.2 of
[RFC5246].

The DTCPCert and ASN.1Cert are variable-length vectors as specified
in Section 4.3 of [RFC5246]. Hence, the actual length precedes the
vector's contents in the byte stream. If the ASN.1Cert is not being
sent, the ASN.1Cert_length MUST be zero.

dtcp_authz_data contains the RandomNonce, the DTCP certificate, and
the optional ASN.1 certificate. This is then followed by the digital
signature covering the RandomNonce, the DTCP certificate, and the
ASN.1 certificate (if present). The signature is generated using the
private key associated with the DTCP certificate and using the
Signature Algorithm and Hash Algorithm as specified in Section 4.4 of
[DTCP]. This signature provides proof of the possession of the
private key by the sender. A sender sending its own DTCP certificate
MUST populate this field. The length of the signature field is
determined by the Signature Algorithm and Hash Algorithm as specified
in Section 4.4 of [DTCP], and so it is not explicitly encoded in the
dtcp_authz_data structure (e.g., the length will be 40 bytes for a
SHA1+ECDSA algorithm combination).

3.3. Usage Rules for Clients to Exchange DTCP Authorization Data

A client includes both the client_authz and server_authz extensions
in the extended client hello message when indicating its desire to
exchange dtcp_authorization data with the server. Additionally, the
client includes the AuthzDataFormat type specified in Section 3.1 in
the extension_data field to specify the format of the authorization
data.

A client will receive the server's dtcp_authz_data before it sends
its own dtcp_authz_data. When sending its own dtcp_authz_data
message, the client includes the same RandomNonce that it receives in
the server's dtcp_authz_data message. Clients MUST include its DTCP
certificate in the dtcp_authz_data message. A client MAY include its
ASN.1 certificate (certificate in the ClientCertificate message) in

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the ASN.1Cert field of the dtcp_authz_data to cryptographically tie
the dtcp_authz_data with its ASN.1Cert being used to establish the
TLS session (i.e., sent in the ClientCertificate message).

3.4. Usage Rules for Servers to Exchange DTCP Authorization Data

A server responds with both the client_authz and server_authz
extensions in the extended server hello message when indicating its
desire to exchange dtcp_authorization data with the client.

Additionally, the server includes the AuthzDataFormat type specified
in Section 3.1 in the extension_data field to specify the format of
the dtcp_authorization data. A client may or may not include an
ASN.1 certificate during the TLS handshake. However, the server will
not know that at the time of sending the SupplementalData message.
Hence, a server MUST generate and populate the RandomNonce in the
dtcp_authz_data message. If the client's hello message does not
contain both the client_authz and server_authz extensions with
dtcp_authorization type, the server MUST NOT include support for
dtcp_authorization data in its hello message. A server MAY include
its DTCP certificate in the dtcp_authz_data message. If the server
does not send a DTCP certificate, it will send only the RandomNonce
in its dtcp_authz_data message. If the server includes its DTCP
certificate, it MUST also include its server certificate (sent in the
TLS Certificate message) in the certs field to cryptographically tie
its dtcp_authz_data with the ASN.1 certificate used in the TLS
session being established. This also helps the client in detecting
replay attacks.

3.5. TLS Message Exchange with dtcp_authz_data

Based on the usage rules in the sections above, Figure 2 provides one
possible TLS message exchange where the client sends its DTCP
certificate to the server within the dtcp_authz_data message.

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Client Server

ClientHello (with extensions) -------->

ServerHello(with extensions)
SupplementalData(with Nonce N1)
Certificate
ServerKeyExchange*
CertificateRequest
<-------- ServerHelloDone

SupplementalData(with Data D1)
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data

N1 Indicates a Random nonce generated by server

D1 Contains dtcp_authz_data populated with the following
{(N1, DTCP Cert, Client X.509 Cert) Signature over all elements}

* Indicates optional or situation-dependent messages that are
not always sent.

[] Indicates that ChangeCipherSpec is an independent TLS
protocol content type; it is not a TLS handshake message.

Figure 2: DTCP SupplementalData Exchange

3.6. Alert Messages

This document reuses TLS Alert messages for any errors that arise
during authorization processing and reuses the AlertLevels as
specified in [RFC5878]. Additionally, the following AlertDescription
values are used to report errors in dtcp_authorization processing:

unsupported_extension:
During processing of dtcp_authorization, a client uses this when
it receives a server hello message that includes support for
dtcp_authorization in only one of client_authz or server_authz but
not in both the extensions. This message is always fatal. Note:

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Completely omitting the dtcp_authorization extension and/or
omitting the client_authz and server_authz completely is allowed
and should not constitute the reason that this alert is sent.

certificate_unknown:
During processing of dtcp_authorization, a client or server uses
this when it has received an X.509 certificate in the
dtcp_authorization data and that X.509 certificate does not match
the certificate sent in the corresponding ClientCertificate or
Certificate message.

4. IANA Considerations

This document includes an entry registered in the IANA-maintained
"TLS Authorization Data Formats" registry for dtcp_authorization(66).
This registry is defined in [RFC5878] and defines two ranges: one is
IETF Review, and the other is Specification Required. The value for
dtcp_authorization should be assigned via [RFC5226] Specification
Required. The extension defined in this document is compatible with
Data Transport Layer Security (DTLS) [RFC6347], and the registry
assignment has been marked "Y" for DTLS-OK.

5. Security Considerations

The dtcp_authorization data, as specified in this document, carries
the DTCP certificate that identifies the associated device.
Inclusion of the X.509 certificate being used to establish a TLS
Session in the dtcp_authorization data allows an application to
cryptographically tie them. However, a TLS Client is not required to
use (and may not possess) an X.509 certificate. In this case, the
dtcp_authorization data exchange is prone to a man-in-the-middle
(MITM) attack. In such situations, a TLS server MUST deny access to
the application features dependent on the DTCP certificate or use a
double handshake. The double handshake mechanism is also vulnerable
to the TLS MITM Renegotiation exploit as explained in [RFC5746]. In
order to address this vulnerability, clients and servers MUST use the
secure_renegotiation extension as specified in [RFC5746] when
exchanging dtcp_authorization data. Additionally, the renegotiation
is also vulnerable to the Triple Handshake exploit. To mitigate
this, servers MUST use the same ASN.1 certificate during
renegotiation as the one used in the initial handshake.

It should be noted that for the double handshake to succeed, any
extension (e.g., TLS Session Ticket [RFC5077]) that results in the
TLS handshake sequence being modified may result in failure to
exchange SupplementalData.

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Additionally, the security considerations specified in [RFC5878] and
[RFC5246] apply to the extension specified in this document. In
addition, the dtcp_authorization data may be carried along with other
supplemental data or some other authorization data and that
information may require additional protection. Finally, implementers
should also reference [DTCP] and [DTCP-IP] for more information
regarding DTCP certificates, their usage, and associated security
considerations.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999,
<http://www.rfc-editor.org/info/rfc2246>.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<http://www.rfc-editor.org/info/rfc4346>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.

[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.

[RFC4680] Santesson, S., "TLS Handshake Message for Supplemental
Data", RFC 4680, DOI 10.17487/RFC4680, October 2006,
<http://www.rfc-editor.org/info/rfc4680>.

[RFC5878] Brown, M. and R. Housley, "Transport Layer Security (TLS)
Authorization Extensions", RFC 5878, DOI 10.17487/RFC5878,
May 2010, <http://www.rfc-editor.org/info/rfc5878>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.

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[DTCP] Digital Transmission Licensing Administrator, "Digital
Transmission Content Protection Specification", Volume 1,
Informational Version,
<http://www.dtcp.com/documents/dtcp/
info-20130605-dtcp-v1-rev-1-7-ed2.pdf>.

[DTCP-IP] Digital Transmission Licensing Administrator, "Mapping
DTCP to IP", Volume 1, Supplement E, Informational
Version, <http://www.dtcp.com/documents/dtcp/
info-20130605-dtcp-v1se-ip-rev-1-4-ed3.pdf>.

6.2. Informative References

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.

[DTLA] Digital Transmission Licensing Administrator, "DTLA",
<http://www.dtcp.com>.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.

[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>.

[RFC6042] Keromytis, A., "Transport Layer Security (TLS)
Authorization Using KeyNote", RFC 6042,
DOI 10.17487/RFC6042, October 2010,
<http://www.rfc-editor.org/info/rfc6042>.

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Appendix A. Alternate Double Handshake Example

This document specifies a TLS authorization data extension that
allows TLS clients and servers to exchange DTCP certificates during a
TLS handshake exchange. In cases where the supplemental data
contains sensitive information, the double handshake technique
described in [RFC4680] can be used to provide protection for the
supplemental data information. The double handshake specified in
[RFC4680] assumes that the client knows the context of the TLS
session that is being set up and uses the authorization extensions as
needed. Figure 3 illustrates a variation of the double handshake
that addresses the case where the client may not have a priori
knowledge that it will be communicating with a server capable of
exchanging dtcp_authz_data (typical for https connections; see
[RFC2818]). In Figure 3, the client's hello messages includes the
client_authz and server_authz extensions. The server simply
establishes an encrypted TLS session with the client in the first
handshake by not indicating support for any authz extensions. The
server initiates a second handshake by sending a HelloRequest. The
second handshake will include the server's support for authz
extensions, which will result in SupplementalData being exchanged.

Alternately, it is also possible to do a double handshake where the
server sends the authorization extensions during both the first and
the second handshake. Depending on the information received in the
first handshake, the server can decide whether or not a second
handshake is needed.

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Client Server

ClientHello (w/ extensions) --------> |0
ServerHello (no authz extensions) |0
Certificate* |0
ServerKeyExchange* |0
CertificateRequest* |0
<-------- ServerHelloDone |0
Certificate* |0
ClientKeyExchange |0
CertificateVerify* |0
[ChangeCipherSpec] |0
Finished --------> |1
[ChangeCipherSpec] |0
<-------- Finished |1
<-------- HelloRequest |1
ClientHello (w/ extensions) --------> |1
ServerHello (w/ extensions) |1
SupplementalData* |1
Certificate* |1
ServerKeyExchange* |1
CertificateRequest* |1
<-------- ServerHelloDone |1
SupplementalData* |1
Certificate* |1
ClientKeyExchange |1
CertificateVerify* |1
[ChangeCipherSpec] |1
Finished --------> |2
[ChangeCipherSpec] |1
<-------- Finished |2
Application Data <-------> Application Data |2

* Indicates optional or situation-dependent messages.

Figure 3: Double Handshake to Protect SupplementalData

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Acknowledgements

The author wishes to thank Mark Brown, Sean Turner, Sumanth
Channabasappa, and the Chairs (EKR, Joe Saloway) and members of the
TLS Working Group who provided feedback and comments on one or more
revisions of this document.

This document derives its structure and much of its content from
[RFC4680], [RFC5878], and [RFC6042].

Author's Address

D. Thakore
Cable Television Laboratories, Inc.
858 Coal Creek Circle
Louisville, CO 80023
United States

Email: [email protected]

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