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draft-wouters-tls-rawkeys-00.txt
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Network Working Group P. Wouters, Ed.
Internet-Draft Xelerance
Intended status: Standards Track J. Gilmore
Expires: March 2, 2012
S. Weiler
SPARTA, Inc.
T. Kivinen
AuthenTec
H. Tschofenig
Nokia Siemens Networks
August 30, 2011
Raw Public Keys for (Datagram) Transport Layer Security
draft-wouters-tls-rawkeys-00.txt
Abstract
This document describes a mechanism for exchanging raw public keys
and their fingerprints in Transport Layer Security (TLS). The main
use case of the provided functionality is in the area of TLS and
Datagram TLS (DTLS) when used with devices in a constrained
environment, as it is the case with sensors and other embedded
devices that are constrained by memory, computational, and
communication limitations.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 March 2, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
Wouters, et al. Expires March 2, 2012 [Page 1]
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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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Changes to the Handshake Message Contents . . . . . . . . . . 6
3.1. Client Hello . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Server Hello . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Server Certificate Payload Content . . . . . . . . . . . . 7
3.4. Certificate Request . . . . . . . . . . . . . . . . . . . 8
3.5. Client Certificate Payload Content . . . . . . . . . . . . 8
3.6. Other Handshake Messages . . . . . . . . . . . . . . . . . 8
4. Certificate Payloads . . . . . . . . . . . . . . . . . . . . . 9
4.1. Raw Public Keys . . . . . . . . . . . . . . . . . . . . . 9
4.2. Fingerprint of Raw Public Key . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
6.1. New TLS Certificate Types . . . . . . . . . . . . . . . . 15
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
The IETF has two sets of standards for public key certificates, one
set for use of X.509 certificates [RFC3280] and one for OpenPGP
certificates [RFC4880]. Transport Layer Security (TLS) standards
were initially defined to use only X.509 certificates but were later
extended with RFC 6091 [RFC6091] to also allow clients and servers to
negotiate the use of OpenPGP certificates for a TLS session, and to
transport OpenPGP certificates via TLS.
Traditionally, TLS server public keys are obtained in PKIX containers
in-band using the TLS connection and validated using trust anchors
based on a [PKIX] certification authority (CA). This method can add
a complicated trust relationship that is difficult to validate.
Examples of such complexity can be seen in [Defeating-SSL].
Alternative methods are available that allow a TLS client to obtain
the TLS server public key:
o The TLS server public key is obtained from a [PKIX] certificate
chain from an [LDAP] server.
o The TLS server public key is obtained from a DNSSEC secured RRset
using DANE [DANE].
o The TLS server public key is provisioned by the operating system
and updated via software updates.
o A TLS client has connected to the TLS server before and has cached
the TLS server certificate chain or TLS server public key for re-
use.
RFC 5246 [RFC5246] does not provide a mechanism for a TLS client to
tell the TLS server it is already in possession of the authenticated
public key. Therefore, a TLS server must always send a list of
trusted CA keys and its EE certificate containing its public key,
even when the TLS client does not require or desire that data for
authentication.
RFC 6066 [RFC6066] allows suppression of the certificate trust anchor
chain, but not suppression of the PKIX EE certificate container.
These certificate chains are large opague blocks of data containing
much more then the public key of the TLS server. Since the TLS
client might only be able to validate the PKIX SubjectPublicKeyInfo
via an out-of-band method, it has to explicitly forget any additional
information received that was sent by the server that it could not
validate. Furthermore, information that comes in via these
certificate chains could contain contradicting or additional
Wouters, et al. Expires March 2, 2012 [Page 3]
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information that the TLS client cannot validate or trust, such as an
expiry date that conflicts with information obtained from DNS or
LDAP. This document specifies a method to suppress sending this
additional information.
This document specifies a way to negotiate the use of raw public keys
for a TLS (and DTLS) session, and specifies how to transport these
raw public keys (or their fingerprints) via TLS. The proposed
extensions are backward-compatible with the current TLS
specification, so that existing client and server implementations
that make use of X.509 certificates are not affected.
The main use case of the provided functionality is in the area of TLS
and Datagram TLS (DTLS) when used with devices in a constrained
environment, as it is the case with sensors and other embedded
devices that that are constrained by memory, computational, and
communication limitations.
Imagine a small embedded device that interacts with a Web server to
upload temperature sensor readings at a regular interval. The
software stack uses the Constrained Application Protocol (CoAP), a
specialized web transfer protocol for use with constrained networks
and nodes for machine-to-machine applications. CoAP is a UDP-based
protocol that can utilize DTLS for its commuication security. As
part of the provisioning procedure the embeded device is configured
with the address of a dedicated CoAP server to upload sensor data.
For security purposes the public keys are also pre-configured on both
devices and the CoAP server is likely to use the provisioned public
keys of the embedded devices for access control purposes.
For packet size reasons and because of the constrained nature of the
sensor networking deployment environment with the pre-configured
client-server setup the usage of X.509 or PGP certificates represents
an unnecessary burden for implementers as well as for those who
deploy.
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2. Terminology
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 RFC 2119 [RFC2119].
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3. Changes to the Handshake Message Contents
This section describes the changes to the TLS handshake message
contents when raw public keys are to be used for authentication.
3.1. Client Hello
In order to indicate the support of multiple certificate types,
clients MUST include an extension of type "cert_type" to the extended
client hello message. The "cert_type" TLS extension, which is
defined in RFC 6091 [RFC6091], is assigned the value of 9 from the
TLS ExtensionType registry. This value is used as the extension
number for the extensions in both the client hello message and the
server hello message. The hello extension mechanism is described in
[RFC5246].
The "cert_type" TLS extension carries a list of supported certificate
types the client can use, sorted by client preference. This
extension MUST be omitted if the client only supports X.509
certificates. The "extension_data" field of this extension contains
a CertificateTypeExtension structure. Note that the
CertificateTypeExtension structure is being used both by the client
and the server, even though the structure is only specified once in
this document. Reusing a single specification for both client and
server is common TLS protocol design practice.
The RFC 6091 [RFC6091] defined CertificateTypeExtension is extended
as follows:
enum { client, server } ClientOrServerExtension;
enum { X.509(0), OpenPGP(1),
RawPublicKey(2), Fingerprint_RawPublicKey(3),
(255) } CertificateType;
struct {
select(ClientOrServerExtension)
case client:
CertificateType certificate_types<1..2^8-1>;
case server:
CertificateType certificate_type;
}
} CertificateTypeExtension;
No new cipher suites are required to use raw public keys or
fingerprints of raw public keys. All existing cipher suites that
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support a key exchange method compatible with the defined extension
can be used.
3.2. Server Hello
If the server receives a client hello that contains the "cert_type"
extension and chooses a cipher suite then two outcomes are possible.
The server MUST either select a certificate type from the
certificate_types field in the extended client hello or terminate the
session with a fatal alert of type "unsupported_certificate".
The certificate type selected by the server is encoded in a
CertificateTypeExtension structure, which is included in the extended
server hello message using an extension of type "cert_type". Servers
that only support X.509 certificates MAY omit including the
"cert_type" extension in the extended server hello.
3.3. Server Certificate Payload Content
In TLS the contents of the certificate message sent from server to
client and vice versa are determined by the negotiated certificate
type and the selected cipher suite's key exchange algorithm.
If the Raw Public Key certificate type (or the fingerprinted version
of it) is negotiated, then it is required to present the raw public
key (or of the fingerprint of it) in the certificate message. The
public key MUST match the selected key exchange algorithm, as shown
below. This document defines new certificate types that carry raw
public keys and the fingerprinted version.
Key Exchange Algorithm | Type of Public Key
------------------------+--------------------
RSA, DHE_RSA, DH_RSA, | RSA public key
ECDH_RSA, ECDHE_RSA) |
------------------------+--------------------
DHE_DSS, DH_DSS | DSA public key
------------------------+--------------------
ECDH_ECDSA, ECDHE_ECDSA | ECC public key
------------------------+--------------------
The content of the certificate payload sent by the server depends on
the chosen type of public key crypto system. Details can be found in
Section 4.
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3.4. Certificate Request
The semantics of this message remain the same as in the TLS
specification. However, if this message is sent, and the negotiated
certificate type is RawPublicKey (or Fingerprint_RawPublicKey), the
"certificate_authorities" list MUST be empty.
3.5. Client Certificate Payload Content
This message is only sent in response to the certificate request
message. The client certificate message is sent using the same
formatting as the server certificate message, and it is also required
to present a certificate extension payload that matches the
negotiated certificate type. If the RawPublicKey (or the
Fingerprint_RawPublicKey) certificate type has been selected and no
public key from the client is available, then the server SHOULD
respond with a "handshake_failure" fatal alert if the public key
cannot be obtained out-of-band.
The content of the certificate payload sent by the client depends on
the chosen type of public key crypto system. Details can be found in
Section 4.
3.6. Other Handshake Messages
All the other handshake messages are identical to the TLS
specification.
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4. Certificate Payloads
The authors of this document suggest to convey raw public keys (and
fingerprints of raw public keys) in Certificate payloads and this
section defines the structure of these payloads.
4.1. Raw Public Keys
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enum {
SHA-1(0)
SHA-224(1),
SHA-256(2),
SHA-384(3),
SHA-512(4),
MD5(5)
} FingerprintHashFunction;
enum {
empty_key(1),
raw_key(2),
raw_key_fingerprint(3),
(255)
} DescriptorType;
struct {
opaque KeyID<8..255>;
} EmptyKey_Structure;
struct {
opaque KeyID<8..255>;
opaque RawKey<0..2^24-1>;
} RawPublicKey_Structure;
struct {
opaque KeyID<8..255>;
opaque RawKeyFingerprint<20..255>;
FingerprintHashFunction fingerprintHashFunction;
} RawPublicKey_Fingerprint_Structure;
struct {
DescriptorType descriptorType;
select (descriptorType) {
case empty_key: EmptyKey;
case raw_key: RawPublicKey_Structure;
case raw_key_fingerprint:
RawPublicKey_Fingerprint_Structure;
}
} Certificate;
Figure 1: Content of the Certificate Payload
The fields are defined as follows:
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KeyID:
The KeyID structure contains an identifier for the key structure.
No specific syntax or content is mandated by this document.
RawKeyFingerprint:
The RawKeyFingerprint structure carries the hash of the raw public
key.
FingerprintHashFunction:
The FingerprintHashFunction structure explicitly states the hash
function used for computing the fingerprint of the raw public key.
More information about the computation can be found in
Section 4.2.
RawKey:
The RawKey structure contains the raw public key as described
below for the different types of public key cryptosystems.
RSA Public Key:
The raw RSA public key contains a PKCS #1 encoded RSA key, that
is, a DER-encoded RSAPublicKey structure (see [RSA] and
[RFC3447]).
Diffie-Hellman Public Key:
This payload defines a raw Diffie-Hellman (DH) public key and
parameters.
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struct {
opaque dh_p<1..2^16-1>;
opaque dh_g<1..2^16-1>;
opaque dh_Ys<1..2^16-1>;
} DHParams; /* DH parameters */
dh_p
The prime modulus used for the Diffie-Hellman operation.
dh_g
The generator used for the Diffie-Hellman operation.
dh_Ys
The server's Diffie-Hellman public value (g^X mod p).
Raw ECC Public Key:
This payload defines a raw Elliptic Curve Cryptography (ECC)
public key. The encoding and the parameters of the ECC public key
and parameters are defined in Section 5.4 of RFC 4492 [RFC4492]
with 'curve_params' specifying the elliptic curve domain
parameters associated with the public key and with 'public'
representing the public key itself.
struct {
ECParameters curve_params;
ECPoint public;
} ECCParam;
DSS Public Key:
This payload defines a raw Digital Signature Standard (DSS) public
key. The encoding and the parameters of the DSS public key and
parameters are defined in FIPS 186-3 [DSS]. The
domain_parameter_seed and counter values that were used to
generate p and q are not carried in the structure.
struct {
opaque p<1..2^16-1>;
opaque q<1..2^16-1>;
opaque g<1..2^16-1>;
} DSSParam; /* DSS parameters */
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4.2. Fingerprint of Raw Public Key
The fingerprint value for the RawKeyFingerprint structure is a hash
computed as follows:
RawKeyFingerprint = func(RawKey)
whereby
- the func() is the hash function indicated in
FingerprintHashFunction (e.g., SHA-256)
- RawKey is the value of the public key
with the encoding defined in this document.
This document defines the following hash functions:
o SHA-1: NIST FIPS PUB 180-3 [SHA]
o SHA-224: RFC 3874 [RFC3874]
o SHA-256: NIST FIPS PUB 180-3 [SHA]
o SHA-384: NIST FIPS PUB 180-3 [SHA]
o SHA-512: NIST FIPS PUB 180-3 [SHA]
o MD5: RFC 1321 [RFC1321]
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5. Security Considerations
This document defines an extension to TLS and therefore inherits the
security considerations from TLS as described in [RFC5246] and
[RFC6066].
The protocol for certificate type negotiation is identical in
operation to cipher suite negotiation as described in the TLS
specification [RFC5246], with the addition of default values when the
extension is omitted. Since those omissions have a unique meaning
and the same protection is applied to the values as with cipher
suites, it is believed that the security properties of this
negotiation are the same as with cipher suite negotiation.
Raw public keys and fingerprints of them are, unlike certificates,
not secured by their own cryptographic mechanism against
modification. As such, this extension assumes that there was an out-
of-band procedure to ensure that the public keys (or their
fingerprints) have been securely provisioned. This is not only
important from an authentication point of view but also with respect
to the authorization procedure.
The information that is available to participating parties and
eavesdroppers (when confidentiality is not available through a
previous handshake) is the number and the types of certificates they
hold, plus the contents of certificates.
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6. IANA Considerations
With this document IANA is kindly asked to add values to one existing
registry.
6.1. New TLS Certificate Types
The "TLS Certificate Types" registry was created with RFC 6091
[RFC6091] and this document defines two new values, namely
RawPublicKey([TBD: Suggested value is 2]), and
Fingerprint_RawPublicKey([TBD: Suggested value is 3]).
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7. Acknowledgements
We would like to thank the participants of the CORE and TLS working
group session at IETF#81 for their discussions around raw public keys
in TLS/DTLS. Additionally we would like to thank Pasi Eronen for his
review comments and the authors of RFC 6091 since this documents
builds on their work.
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8. References
8.1. Normative References
[DSS] "FIPS PUB 186-3, "Digital Signature Standard", National
Institute of Standards and Technology, U.S. Department of
Commerce", June 2009.
[PKIX] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3874] Housley, R., "A 224-bit One-way Hash Function: SHA-224",
RFC 3874, September 2004.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6091] Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
for Transport Layer Security (TLS) Authentication",
RFC 6091, February 2011.
[RSA] Rivest, R., Shamir, A., and L. Adleman, "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems", February 1978.
[SHA] "Federal Information Processing Standards Publication
(FIPS PUB) 180-3, Secure Hash Standard (SHS)",
October 2008.
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8.2. Informative References
[DANE] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names For TLS",
draft-ietf-dane-protocol-08 (work in progress), July 2011.
[Defeating-SSL]
Marlinspike, M., "New Tricks for Defeating SSL in
Practice", February 2009, <http://www.blackhat.com/
presentations/bh-dc-09/Marlinspike/
BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf>.
[LDAP] Sermersheim, J., "Lightweight Directory Access Protocol
(LDAP): The Protocol", RFC 4511, June 2006.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.