CoRE Working Group Namhi Kang
Internet Draft Duksung Women's University
Intended status: Standard Track October 15, 2013
Expires: April 15, 2014
Secure initial-key reconfiguration for resource constrained devices
draft-kang-core-secure-reconfiguration-00
Abstract
This document presents a secure method to configure a key for a node
when it initially joins to network that is currently in operation.
The method is suited for a scenario, where resource constrained
objects are interconnected with each other and thus form a network
called Internet of Things. It is assumed that communications for all
nodes are based on TCP/IP protocols and some of the nodes use the
constrained application protocol (CoAP). The method does not cover
all operations of secure bootstrapping, but it is intended to
securely support self-reconfiguration of the pre-installed temporary
key of new node.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
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This Internet-Draft will expire on January 14, 2014.
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Table of Contents
1. Introduction ................................................ 4
2. Terminology ................................................. 5
3. System Architecture ......................................... 6
4. Process Flow ................................................ 8
5. Security Considerations ..................................... 9
6. IANA Considerations ........................................ 10
7. Acknowledgments ............................................ 10
8. References ................................................. 10
8.1. Normative References .................................. 10
8.2. Informative References ................................ 11
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1. Introduction
A rapidly growing number and various types of devices including smart
small things such as sensors and actuators are trying to connect with
Internet as time goes by. This draft presents a simple but efficient
approach to reconfigure a secure key for resource constrained small
things that are often defined as network nodes having 8 bit
processing microcontrollers with limited amounts of memory. The
network is also constrained one (e.g. 6LoWPAN having high packet
error rates and a typical throughput of 10s of kbit/s) [CoAP].
Pre-shared key (PSK) based secure schemes are well known and
frequently used for various security services in Internet. All such
schemes strictly assume that the PSK is only known to two entities
involved in current security service. Consequently, the security of
the schemes are compromised if the assumption is broken.
However, it is still not clear how PSK of resource constrained node
can be initially configured in a secure manner in Internet of things
(IoT). Typically, things used for IoT might be manufactured and
installed by different subjects (simply person) [SecCons]. That is,
in general situation, a system administrator may make orders to
several different installers. After that, each of the installers
purchases one or more different set of things from one or more
different manufacturers. It is also unlikely that a single subject
installs all nodes used for a large application domain (e.g. all
nodes in huge building).
This draft considers a scenario, where nodes are initially configured
by an installer during bootstrapping phase. If a PSK is also required
to be configured in this phase, the trust between installer and
system administrator is extremely important. This is not easy process.
Even further, if the case is settled, there are several secure
threats and vulnerabilities to be handled.
The basic idea of the method specified in this document is motivated
from a lock of suitcase. Simple and default password such as '0000'
or '1234' is initially setup on a lock of suitcase in selling. Owner
can change the password after purchasing. In our method, similarly,
initial key of a node is configured by installer during bootstrapping
phase. When the node join to an existing network, the key (i.e. PSK)
can be securely reconfigured.
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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
[RFC2119] when they appear in ALL CAPS. These words may also appear
in this document in lower case as plain English words, absent their
normative meanings.
This draft uses notations and abbreviations as follows.
SBI(i)
Shorten abbreviation of a secure bootstrapping initiator i (i.e.
new node required to be reconfigured)
SBR(c)
Shorten abbreviation of a secure bootstrapping respondent c (i.e.
a type of controller)
SBS(s)
Shorten abbreviation of a secure bootstrapping server s (i.e. an
authenticated register or authentication server)
ID_A
Denoting 32bits identifier (ID) of entity A
NID_A
Denoting network ID used for access to communication entity A
(e.g. IPv4 or IPv6 address and port number).
RN_A
Denoting 128bits integer used for a secure random number
generated by entity A; for example, a random number generated by
SBI is referred to as RN_i.
IK_N
Denoting 128bits symmetric key pre-installed by installer or
manufacturer for node N; The key is used for a partial
transaction of mutual authentication and derivation of PSK (see
section 4 in detail).
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PSK
Shorten abbreviation of a 128bits pre-shared key derived from the
IK. The PSK is a shared key between a node and authenticated
register (or authentication server) in a specific service domain.
A PSK can be used to derive session keys for various security
protocols designed by service administrator (see [RFC4764] for
example).
TS
Denoting time stamp of operation; it enables sender (TS
generator) to inform timeliness and uniqueness to receiver.
SK_cs
Denoting a 128bits symmetric key shared between entity c and s.
||
Notation used to denote concatenation of data.
V
Notation used to denote a logical operator Exclusive OR.
E(M, SK)
Denoting a function to encrypt a plain text 'M' by using a
symmetric key SK.
D(C, SK)
Denoting a function to decrypt a cipher text 'C' by using a
symmetric key SK.
Other security related terminologies used in this document are based
on [RFC4949].
3. System Architecture
Secure bootstrapping is regarded as a difficult problem in Internet
of Things. This is mainly because lots of things connected to
Internet are resource constrained. Especially, user interface they
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have is not enough for doing configurations manually by person (i.e.
inadequate or even no in/out equipment such as display or keyboard).
As one of solutions, this document proposes a method which allows a
node to reconfigure a symmetric key automatically upon joining to
existing network.
The method of this document is based on a straightforward scenario,
where resource constrained things in IoT such as sensors or actuators
are generally designed and manufactured according to their own
specific tasks in advance. Also, a pre-defined controller covers and
communicates with his associated things according to his rolls
defined in a service domain. For example, a thermostat manages and
communicates several temperature sensors, humidity sensors, windows,
heating controller, air conditioner, and more. This document does not
assume that a system administrator trusts an installer even though he
makes orders for the installer. This is because trust and
responsibility of installer who buys and install devices is different
from those of system administrator.
In this scenario, the following transactions MUST be done prior to
the key reconfiguration.
1. System administrator makes orders including setup information
required to be initially configured. These are ID and NID of
controller for each of nodes, temporary key used as an IK. In
particular, all nodes handled by a single installer can share
the same IK similarly to the default password for all suitcases
manufactured by a single company.
2. System administrator also stores the same initial information
for each of nodes in authentication server (or authenticated
register). Note that a controller can also perform operations
of an authentication server in case of a small network.
3. Installer purchases devices and then configures the information
requested by the administrator in doing installation. Some of
the information for a node may be pre-configured by
manufacturer.
4. When a node joins to network, it knows NID of his associated
controller with which he can communicate. Also, authentication
server has lists of IDs for new nodes.
5. PSK reconfiguration phase is then started.
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In order to make a practical and reasonable method, the proposed
method requires only a single cryptographic primitive that is AES
with 128bits length of key [AES]. All cryptographic primitives cannot
be installed on resource restricted devices, mainly because of
limited size of flash or RAM. For this reason, CoAP also do not
consider all modes of cryptographic operations in DTLS which is a
regarded secure protocol for CoAP applications. In case of
establishing a CoAP session using a pre-shared key mod of DTLS,
implementation of cipher suite TLS_PSK_WITH_AES_128_CCM_8 specified
in [RFC6655] is mandatory.
4. Process Flow
There are three message exchanges between new node SBI(i) and network
(SBR(c) and SBS(s)). A controller SBR(c) MAY include functions of
both SBR(c) and SBS(s) depending on the size of application domain.
Mutual authentication and PSK reconfiguration procedures are shown in
Figure 1.
------- ------ ------
SBI(i) SBR(c) SBS(s)
------- ------ ------
| | |
| ID_i, RN_i | |
| -------------------------->| |
| |ID_i, ID_c, RN_i, RN_c, TS, TID |
| |------------------------------->|
| | |
| | |
| | E(IK_i || TID,SK_cs) |
| |<-------------------------------|
| | |
| | |
| ID_c, E(RN_i || RN_c, IK_i)| |
|<---------------------------| |
| | |
| | |
| E(RN_c, PSK) | |
|--------------------------->| |
| | |
| | |
Figure 1 Message Exchange for PSK Reconfiguration
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When a new node SBI(i) joins an existing network, he generates a
random number RN_i and sends it with his identifier ID_i to SBR(s).
Upon receiving the message, SBR(c) generates a random number RN_c and
a serial number used as a transaction ID (i.e. TID). Then he sends
the two numbers with his ID_c, time stamp (TS) and the message
received from SBI(i) to the authentication server SBS(s). TS allows
SBR(s) to derive an expiration of key and verify the freshness of the
arrived message. Specific period of the expiration of key (i.e. PSK)
does not covered in this document.
The authentication server SBS(s) now can derive a new PSK for the
node SBI(i) and replace the IK_i, which he initially stored, to the
PSK, where the PSK for SBI(i) is derived as follows.
PSK_i = E(RN_i V RN_c, IK_i)
At this moment, IK_i does not known to SBS(c). After reconfiguration
of key for node SBI(i), SBS(s) encrypts the concatenation value of
IK_i and ID_i with the symmetric key SK_cs which is a shared key
between SBS(s) and SBR(c).
On receiving the encrypted value from SBS(s), SBR(c) can know the key
IK_i thereby calculating PSK. SBR(c) encrypts the concatenation value
of RN_i and RN_c with key IK_i. Then it sends the encryption value
and his ID_r to SBI(i).
Finally, SBI(i) can reconfigure his PSK thereafter sending the
encryption value of RN_c with the new key PSK to SBR(c) to verify
himself.
5. Security Considerations
The method of this draft uses a single cryptographic primitive AES
[AES]. Single cryptographic primitive implementation is rationally
suited for the scenario where applications or services require a
secure session (confidentiality of data) in IoT. Because small
devices with low computing power and little storage are major
entities. In this draft, a single primitive AES is used for secure
bootstrapping (exactly PSK reconfiguration phase). Further, the PSK
can be used for session key derivation and entity authentication.
As discussed in ESP-PSK [RFC4764], it goes without saying that a
single cryptographic primitive may not support extensible security
services such as identity protection, perfect forward secrecy and
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others. However, small devices consisting of Internet of Things might
not support all of security services inherently. Service developer
should therefore define a scope of his service strictly and consider
trade-off between capability and security.
Security analysis and evaluation of various aspects of the method
remain to be done.
6. IANA Considerations
This memo includes no request to IANA
7. Acknowledgments
(TBD)
8. References
8.1. Normative References
[RFC4764] F. Bersani, H. Tschofenig, "The EAP-PSK Protocol: A Pre-
Shared Key Extensible Authentication Protocol (EAP) Method",
RFC 4764, January 2007.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655, July 2012.
[CoAP] Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)", draft-ietf-core-
coap-18 (work in progress), June 2013.
[SecCons] O. Garcia-Morchon, S. Kumar, S. Keoh, R. Hummen, R. Struik,
"Security Considerations in the IP-based Internet of
Things", Internet draft (draft-garcia-core-security-06),
September 2013.
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[AES] National Institute of Standards and Technology, "Specification
for the Advanced Encryption Standard (AES)", Federal
Information Processing Standards (FIPS) 197, November 2001.
8.2. Informative References
[RFC2119] S. Brander, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Addresses
Namhi Kang
Duksung Women's University
Seoul Korea
Email: kang@duksung.ac.kr
URI: http://www.duksung.ac.kr
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