NTP Working Group D. Sibold
Internet-Draft PTB
Intended status: Standards Track S. Roettger
Expires: April 26, 2015 Google Inc
K. Teichel
PTB
R. Housley
Vigil Security
October 23, 2014
Protecting Network Time Security Messages with the Cryptographic Message
Syntax (CMS)
draft-ietf-ntp-cms-for-nts-message-00.txt
Abstract
This document describes a convention for using the Cryptographic
Message Syntax (CMS) to protect the messages in the Network Time
Security (NTS) protocol. NTS provides authentication of time servers
as well as integrity protection of time synchronization messages
using Network Time Protocol (NTP) or Precision Time Protocol (PTP).
Requirements Language
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].
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 April 26, 2015.
Sibold, et al. Expires April 26, 2015 [Page 1]
Internet-Draft October 2014
Copyright Notice
Copyright (c) 2014 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. 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. CMS Conventions for NTS Message Protection . . . . . . . . . 3
2.1. Fields of the employed CMS Content Types . . . . . . . . 5
2.1.1. ContentInfo . . . . . . . . . . . . . . . . . . . . . 5
2.1.2. SignedData . . . . . . . . . . . . . . . . . . . . . 6
2.1.3. EnvelopedData . . . . . . . . . . . . . . . . . . . . 8
3. Certificate Conventions . . . . . . . . . . . . . . . . . . . 9
4. Implementation Notes: ASN.1 Structures and Use of the CMS . . 9
4.1. Preliminaries . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Unicast Messages . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Association Messages . . . . . . . . . . . . . . . . 9
4.2.2. Cookie Messages . . . . . . . . . . . . . . . . . . . 10
4.2.3. Time Synchronization Messages . . . . . . . . . . . . 11
4.3. Broadcast Messages . . . . . . . . . . . . . . . . . . . 12
4.3.1. Broadcast Parameter Messages . . . . . . . . . . . . 12
4.3.2. Broadcast Time Synchronization Message . . . . . . . 12
4.3.3. Broadcast Keycheck . . . . . . . . . . . . . . . . . 13
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
This document provides detail on how to construct NTS messages in
practice. NTS provides secure time synchronization with time servers
using Network Time Protocol (NTP) [RFC5905] or Precision Time
Protocol (PTP) [IEEE1588]. Among other things, this document
Sibold, et al. Expires April 26, 2015 [Page 2]
Internet-Draft October 2014
describes a convention for using the Cryptographic Message Syntax
(CMS) [RFC5652] to protect messages in the Network Time Security
(NTS) protocol. Encryption is used to provide confidentiality of
secrets, and digital signatures are used to provide authentication
and integrity of content.
Sometimes CMS is used in an exclusively ASN.1 [ASN1] environment. In
this case, the NTS message may use any syntax that facilitates easy
implementation.
2. CMS Conventions for NTS Message Protection
Regarding the usage of CMS we differentiate between four archetypes
according to which the NTS message types can be structured:
NTS-Plain: This archetype is used for actual time synchronization
messages (explicitly, the message types: time_request,
time_response, server_broad, see
[I-D.ietf-ntp-network-time-security], section 6) as well as for
the very first messages of a unicast or a broadcast exchange
(client_assoc or client_bpar, respectively) and the broadcast
keycheck exchange (client_keycheck and server_keycheck). This
archetype does not make use of any CMS structures.
NTS-Signed-and-Encrypted: This archetype is used for secure
transmission of the cookie (only for the server_cook message type,
see [I-D.ietf-ntp-network-time-security], section 6). For this,
the following CMS structure is used:
First, the NTS message MUST be encrypted using the
EnvelopedData content type. EnvelopedData supports nearly any
form of key management. In the NTS protocol the client
provides a certificate in an unprotected message, and the
public key from this certificate, if it is valid, will be used
to establish a pairwise symmetric key for the encryption of the
protected NTS message.
Second, the EnvelopedData content MUST be digitally signed
using the SignedData content type. SignedData supports nearly
any form of digital signature, and in the NTS protocol the
server will include its certificate within the SignedData
content type.
Third, the SignedData content type MUST be encapsulated in a
ContentInfo content type.
Figure 1 illustrates this structure.
Sibold, et al. Expires April 26, 2015 [Page 3]
Internet-Draft October 2014
+---------------------------------------------------------+
| |
| ContentInfo |
| |
| +-----------------------------------------------------+ |
| | | |
| | SignedData | |
| | | |
| | +-------------------------------------------------+ | |
| | | | | |
| | | EnvelopedData | | |
| | | | | |
| | | +---------------------------------------------+ | | |
| | | | | | | |
| | | | NTS Message Object | | | |
| | | | | | | |
| | | | | | | |
| | | +---------------------------------------------+ | | |
| | | | | |
| | +-------------------------------------------------+ | |
| | | |
| +-----------------------------------------------------+ |
| |
+---------------------------------------------------------+
NTS-Signed: This archetype is used for server_assoc and server_bpar
message types. It uses the following CMS structure:
First, the NTS message object MUST be wrapped in a SignedData
content type. The messages MUST be digitally signed, and
certificates included. SignedData supports nearly any form of
digital signature, and in the NTS protocol the server will
include its certificate within the SignedData content type.
Second, the SignedData content type MUST be encapsulated in a
ContentInfo content type.
Figure 2 illustrates this structure.
Sibold, et al. Expires April 26, 2015 [Page 4]
Internet-Draft October 2014
+---------------------------------------------------------+
| |
| ContentInfo |
| |
| +-----------------------------------------------------+ |
| | | |
| | SignedData | |
| | | |
| | +-------------------------------------------------+ | |
| | | | | |
| | | NTS Message Object | | |
| | | | | |
| | | | | |
| | +-------------------------------------------------+ | |
| | | |
| +-----------------------------------------------------+ |
| |
+---------------------------------------------------------+
NTS-Certified: This archetype is used for the client_cook message
type. It uses a CMS structure much like the NTS-Signed archetype
(see Figure 2), with the only difference being that messages
SHOULD NOT be digitally signed. This archetype employs the CMS
structure merely in order to transport certificates.
Whichever archetype is used, the resulting structure is always
transported in an extension field of an NTP packet. In the case of
messages that also need to carry time synchronization data, this data
is written into the regular fields of the NTP packet.
2.1. Fields of the employed CMS Content Types
Overall, three CMS content types are used for NTS messages:
ContentInfo, SignedData and EnvelopedData. The following is a
description of how the fields of those content types are used in
detail.
2.1.1. ContentInfo
The ContentInfo content type is used in all four archetypes. The
fields of the SignedData content type are used as follows:
contentType -- indicates the type of the associated content. For
the archetype NTS-Plain, it MUST identify the NTS message object
that is included. For all other archetypes (NTS-Certified, NTS-
Signed and NTS-Signed-and-Encrypted), it MUST contain the object
identifier for the SignedData content type:
Sibold, et al. Expires April 26, 2015 [Page 5]
Internet-Draft October 2014
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
content is the associated content. For the NTS-Plain archetype,
it MUST contain the DER encoded NTS message object. For all other
archetypes, it MUST contain the DER encoded SignedData content
type.
2.1.2. SignedData
The SignedData content type is used in the NTS-Certified, NTS-Signed
and NTS-Signed-and-Encrypted archetypes but not in the NTS-Plain
archetype. The fields of the SignedData content type are used as
follows:
version -- the appropriate value depends on the optional items
that are included. In the NTS protocol, the signer certificate
MUST be included, and other items MAY be included. The
instructions in [RFC5652] section 5.1 MUST be followed to set the
correct value.
digestAlgorithms -- is a collection of message digest algorithm
identifiers. In the NTS protocol, there MUST be exactly one
algorithm identifier present. The instructions in Section 5.4 of
[RFC5652] MUST be followed.
encapContentInfo -- this structure is always present. In the NTS
protocol, it MUST follow these conventions:
eContentType -- is an object identifier. In the NTS protocol,
for the NTS-Certified and NTS-Signed archetypes, it MUST
identify the type of the NTS message that was encapsulated.
For the NTS-Signed-and-Encrypted archetype, it MUST contain the
object identifier for the EnvelopedData content type:
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }.
eContent is the content itself, carried as an octet string.
For the NTS-Certified and NTS-Signed archetypes, it MUST
contain the DER encoded encapsulated NTS message object. The
instructions in Section 6.3 of [RFC5652] MUST be followed. For
the NTS-Signed-and-Encrypted archetype, it MUST contain the DER
encoded EnvelopedData content type.
certificates -- is a collection of certificates. In the NTS
protocol, it MUST contain the DER encoded certificate [RFC5280] of
the sender. It is intended that the collection of certificates be
Sibold, et al. Expires April 26, 2015 [Page 6]
Internet-Draft October 2014
sufficient for the recipient to construct a certification path
from a recognized "root" or "top-level certification authority" to
the certificate used by the sender.
crls -- is a collection of revocation status information. In the
NTS protocol, it MAY contain one or more DER encoded CRLs
[RFC5280]. It is intended that the collection contain information
sufficient to determine whether the certificates in the
certificates field are valid.
signerInfos -- is a collection of per-signer information. In the
NTS protocol, for the NTS-Certified archetype, this SHOULD be left
out. For both the NTS-Signed and the NTS-Signed-and-Encrypted
archetypes, there MUST be exactly one SignerInfo structure
present. The details of the SignerInfo type are discussed in
Section 5.3 of [RFC5652]. In the NTS protocol, it MUST follow
these conventions:
version -- is the syntax version number. In the NTS protocol,
the SignerIdentifier is subjectKeyIdentifier, therefore the
version MUST be 3.
sid -- identifies the signer's certificate. In the NTS
protocol, the sid field contains the subjectKeyIdentifier from
the signer's certificate.
digestAlgorithm -- identifies the message digest algorithm, and
any associated parameters, used by the signer. In the NTS
protocol, the identifier MUST match the single algorithm
identifier present in the digestAlgorithms.
signedAttrs -- is a collection of attributes that are signed.
In the NTS protocol, it MUST be present, and it MUST contain
the following attributes:
Content Type -- see Section 11.1 of [RFC5652].
Message Digest -- see Section 11.2 of [RFC5652].
In addition, it MAY contain the following attributes:
Signing Time -- see Section 11.3 of [RFC5652].
Binary Signing Time -- see Section 3 of [RFC5652].
signatureAlgorithm -- identifies the signature algorithm, and
any associated parameters, used by the signer to generate the
digital signature.
Sibold, et al. Expires April 26, 2015 [Page 7]
Internet-Draft October 2014
signature is the result of digital signature generation, using
the message digest and the signer's private key. The
instructions in Section 5.5 of [RFC5652] MUST be followed.
unsignedAttrs -- is an optional collection of attributes that
are not signed. In the NTS protocol, the it MUST be absent.
2.1.3. EnvelopedData
The EnvelopedData content type is used only in the NTS-Signed-and-
Encrypted archetype. The fields of the EnvelopedData content type
are used as follows:
version -- the appropriate value depends on the type of key
management that is used. The instructions in [RFC5652] section
6.1 MUST be followed to set the correct value.
originatorInfo -- this structure is present only if required by
the key management algorithm. In the NTS protocol, it MUST be
present when a key agreement algorithm is used, and it MUST absent
when a key transport algorithm is used. The instructions in
Section 6.1 of [RFC5652] MUST be followed.
recipientInfos -- this structure is always present. In the NTS
protocol, it MUST contain exactly one entry that allows the client
to determine the key used to encrypt the NTS message. The
instructions in Section 6.2 of [RFC5652] MUST be followed.
encryptedContentInfo -- this structure is always present. In the
NTS protocol, it MUST follow these conventions:
contentType -- indicates the type of content. In the NTS
protocol, it MUST identify the type of the NTS message that was
encrypted.
contentEncryptionAlgorithm -- identifies the content-encryption
algorithm, and any associated parameters, used to encrypt the
content.
encryptedContent -- is the encrypted the content. In the NTS
protocol, it MUST contain the encrypted NTS message. The
instructions in Section 6.3 of [RFC5652] MUST be followed.
unprotectedAttrs -- this structure is optional. In the NTS
protocol, it MUST be absent.
Sibold, et al. Expires April 26, 2015 [Page 8]
Internet-Draft October 2014
3. Certificate Conventions
The syntax and processing rules for certificates are specified in
[RFC5652]. In the NTS protocol, the server certificate MUST contain
the following extensions:
Subject Key Identifier -- see Section 4.2.1.2 of [RFC5652].
Key Usage -- see Section 4.2.1.3 of [RFC5652].
Extended Key Usage -- see Section 4.2.1.22 of [RFC5652].
The Extended Key Usage extension MUST include the id-kp-NTSserver
object identifier. When a certificate issuer includes this object
identifier in the extended key usage extension, it provides an
attestation that the certificate subject is a time server that
supports the NTS protocol.
The id-kp-NTSserver object identifier is:
id-kp-NTSserver OBJECT IDENTIFIER ::= { TBD }
4. Implementation Notes: ASN.1 Structures and Use of the CMS
This section gives some hints on the structures of the NTS message
objects for the different message types when one wishes to implement
the protocol.
4.1. Preliminaries
The following ASN.1 coded data type "NTSNonce" is needed for other
types used below for NTS messages. It specifies a 128 bit nonce as
required in several message types:
NTSNonce ::= OCTET STRING (SIZE(16))
4.2. Unicast Messages
4.2.1. Association Messages
4.2.1.1. Message Type: "client_assoc"
This message is structured according to the NTS-Plain archetype. It
is realized as an NTP packet with an extension field which holds all
the data relevant for NTS. Explicitly, the extension field contains
an ASN.1 object of type "ClientAssocData", which is structured as
follows:
Sibold, et al. Expires April 26, 2015 [Page 9]
Internet-Draft October 2014
ClientAssocData ::= SEQUENCE {
clientId SubjectKeyIdentifier,
digestAlgos DigestAlgorithmIdentifiers,
keyEncAlgos KeyEncryptionAlgorithms,
contentEncAlgos ContentEncryptionAlgorithms
}
4.2.1.2. Message Type: "server_assoc"
This message is structured according to the NTS-Signed archetype.
The NTS message object in this case is an ASN.1 object of type
"ServerAssocData", which is structured as follows:
ServerAssocData ::= SEQUENCE {
clientId SubjectKeyIdentifier,
choiceDigestAlgo DigestAlgorithmIdentifier,
choiceKeyEncAlgo KeyEncryptionAlgorithmIdentifier,
choiceContentEncAlgo ContentEncryptionAlgorithmIdentifier
}
4.2.2. Cookie Messages
4.2.2.1. Message Type: "client_cook"
This message is structured according to the NTS-Certified archetype.
The NTS message object is a "ClientCookieData" type ASN.1 object,
structured as follows:
ClientCookieData ::= SEQUENCE {
nonce NTSNonce,
signAlgo SignatureAlgorithmIdentifier,
digestAlgo DigestAlgorithmIdentifier,
encAlgo ContentEncryptionAlgorithmIdentifier,
keyEncAlgo KeyEncryptionAlgorithmIdentifier
}
It is identified by the following object identifier (fictional
values):
id-clientCookieData OBJECT IDENTIFIER ::=
{nts(??) cookie(3) clientcookiedata(1)}
4.2.2.2. Message Type: "server_cook"
This message is structured according to the "NTS-Signed-and-
Encrypted" archetype. The NTS message object is a "ServerCookieData"
object, specified as:
Sibold, et al. Expires April 26, 2015 [Page 10]
Internet-Draft October 2014
ServerCookieData ::= SEQUENCE {
nonce NTSNonce,
cookie OCTET STRING (SIZE(16))
}
It is identified by the following object identifier (fictional
values):
id-serverCookieData OBJECT IDENTIFIER ::=
{nts(??) cookie(3) servercookiedata(2)}
4.2.3. Time Synchronization Messages
4.2.3.1. Message Type: "time_request"
This message is structured according to the "NTS-Plain" archetype.
It is realized as an NTP packet which actually contains regular NTP
time synchronization data, as an unsecured NTP packet from a client
to a server would. Furthermore, the packet has an extension field
which contains an ASN.1 object of type "TimeRequestSecurityData",
whose structure is as follows:
TimeRequestSecurityData ::=
SEQUENCE {
nonce_t NTSNonce,
digestAlgo DigestAlgorithmIdentifier,
hashOfClientCert BIT STRING
}
4.2.3.2. Message Type: "time_response"
This message is also structured according to "NTS-Plain". It is
realized as an NTP packet which, like "time_request", contains
regular NTP time synchronization data, as an unsecured NTP packet
from a server back to a client would. The packet also has an
extension field which contains an ASN.1 object of type
"TimeResponseSecurityData", with the following structure:
TimeResponseSecurityData ::=
SEQUENCE {
nonce_t NTSNonce,
}
Finally, this NTP packet has a MAC field which contains a Message
Authentication Code generated over the whole packet (including the
extension field).
Sibold, et al. Expires April 26, 2015 [Page 11]
Internet-Draft October 2014
4.3. Broadcast Messages
4.3.1. Broadcast Parameter Messages
4.3.1.1. Message Type: "client_bpar"
This first broadcast message is structured according to the NTS-Plain
archetype. It is realized as an NTP packet which is empty except for
an extension field which contains ans ASN.1 object of type
"BroadcastParameterRequest", which is structured as follows:
BroadcastParameterRequest ::=
SEQUENCE {
clientId SubjectKeyIdentifier
}
4.3.1.2. Message Type: "server_bpar"
This message is structured according to "NTS-Signed". It is realized
as an NTP packet whose extension field carries the necessary CMS
structure. The NTS message object in this case is an ASN.1 object of
type "BroadcastParameterResponse", with the following structure:
BroadcastParameterRequest ::=
SEQUENCE {
oneWayAlgo1 DigestAlgorithmIdentifier,
oneWayAlgo2 DigestAlgorithmIdentifier,
lastKey OCTET STRING (SIZE (16)),
intervalDuration BIT STRING,
disclosureDelay INTEGER,
nextIntervalTime BIT STRING,
nextIntervalIndex INTEGER
}
4.3.2. Broadcast Time Synchronization Message
4.3.2.1. Message Type: "server_broad"
This message is structured according to the "NTS-Plain" archetype.
Its realization works via an NTP packet which carries regular NTP
broadcast time data as well as an extension field, which contains an
ASN.1 object of type "BroadcastTime". It has the following
structure:
Sibold, et al. Expires April 26, 2015 [Page 12]
Internet-Draft October 2014
BroadcastTime ::=
SEQUENCE {
thisIntervalIndex INTEGER,
disclosedKey OCTET STRING (SIZE (16)),
}
In addition, this packet has a MAC field which contains a Message
Authentication Code generated over the whole packet (including the
extension field).
4.3.3. Broadcast Keycheck
4.3.3.1. Message Type: "client_keycheck"
This message is structured according to the "NTS-Plain" archetype.
It is realized as an NTP packet with an extension field, which
contains an ASN.1 object of type "ClientKeyCheckSecurityData", whose
structure is as follows:
ClientKeyCheckSecurityData ::=
SEQUENCE {
nonce_k NTSNonce,
interval_number INTEGER,
digestAlgo DigestAlgorithmIdentifier,
hashOfClientCert BIT STRING
}
4.3.3.2. Message Type: "server_keycheck"
This message is also structured according to "NTS-Plain". It is also
realized as an NTP packet with an extension field, which contains an
ASN.1 object of type "ServerKeyCheckSecurityData", with the following
structure:
ServerKeyCheckSecurityData ::=
SEQUENCE {
nonce_t NTSNonce,
interval_number INTEGER
}
Additionally, this NTP packet has a MAC field which contains a
Message Authentication Code generated over the whole packet
(including the extension field).
Sibold, et al. Expires April 26, 2015 [Page 13]
Internet-Draft October 2014
5. IANA Considerations
IANA needs to assign an object identifier for id-kp-NTSserver key
purpose and another one for the ASN.1 module in the appendix.
6. Security Considerations
To be written.
7. References
7.1. Normative References
[ASN1] International Telecommunication Union, "Abstract Syntax
Notation One (ASN.1): Specification of basic notation",
ITU-T Recommendation X.680, November 2008.
[IEEE1588]
IEEE Instrumentation and Measurement Society. TC-9 Sensor
Technology, "IEEE standard for a precision clock
synchronization protocol for networked measurement and
control systems", 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
7.2. Informative References
[I-D.ietf-ntp-network-time-security]
Sibold, D., Roettger, S., and K. Teichel, "Network Time
Security", draft-ietf-ntp-network-time-security-04 (work
in progress), July 2014.
Appendix A. ASN.1 Module
The ASN.1 module contained in this appendix defines the id-kp-
NTSserver object identifier.
Sibold, et al. Expires April 26, 2015 [Page 14]
Internet-Draft October 2014
NTSserverKeyPurpose
{ TBD }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
id-kp-NTSserver OBJECT IDENTIFIER ::= { TBD }
END
Authors' Addresses
Dieter Sibold
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de
Stephen Roettger
Google Inc
Email: stephen.roettger@googlemail.com
Kristof Teichel
Physikalisch-Technische Bundesanstalt
Bundesallee 100
Braunschweig D-38116
Germany
Phone: +49-(0)531-592-8421
Email: kristof.teichel@ptb.de
Russ Housley
Vigil Security
Sibold, et al. Expires April 26, 2015 [Page 15]
