ACE Working Group Namhi Kang Internet Draft Duksung Women's University Intended status: Informational Jaeduck Choi Expires: April 22, 2015 NSRI Seungwook Jung Souhwan Jung Younghan Kim Soongsil University October 23, 2014 Security Key configuration for resource constrained devices draft-kang-ace-secure-configuration-00 Abstract This document presents a secure method to configure/reconfigure a key for a resource constrained node when it initially joins to network that is currently in operation. The method is suited for a scenario, where resource constrained nodes 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 the nodes use the constrained application protocol (CoAP). The presented method does not cover all operations of secure bootstrapping for IoT networks, but it is intended to securely support self-reconfiguration of the pre-installed temporary key of joined node. Status of this Memo This Internet-Draft is submitted to IETF 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." N. Kang. Expires April 22, 2015 [Page 1] Internet-Draft Secure configuration February 2014 This Internet-Draft will expire on April 22, 2015. Copyright Notice Copyright (c) 2013 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. N. Kang. Expires April 22, 2015 [Page 2] Internet-Draft Secure configuration February 2014 Table of Contents 1. Introduction ................................................ 4 2. Terminology ................................................. 5 3. System Architecture ......................................... 7 4. Process Flow ................................................ 9 5. Security Considerations..................................... 11 6. IANA Considerations ........................................ 11 7. Acknowledgments ............................................ 11 8. References ................................................. 12 8.1. Normative References................................... 12 8.2. Informative References................................. 12 N. Kang. Expires April 22, 2015 [Page 3] Internet-Draft Secure configuration February 2014 1. Introduction A rapidly growing number and various types of devices including smart small things such as sensors and actuators are trying to connect to the Internet as time goes by. This draft presents a simple but efficient approach to reconfigure a security 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) [RFC7252]. Pre-shared key (PSK) based secure schemes are well known and widely used for various security services in Internet. All such schemes strictly assume that the PSK is only known to the two communication 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 (or a manufacturer in some cases) during enrolment phase (or manufacturing/factory configuration phase). If secure credential including PSK is required to be configured in this phase, the trust between installer (or manufacturer) and system administrator is extremely important. However, this is not easy process because manufacturer, installer and service provider do not share a tight and trust relationships in general cases. Even if the case is properly settled, there might be several secure threats and vulnerabilities to be handled. As a conceptual solution, this draft presents an initial setup method that might be a part of secure bootstrapping scheme. The basic idea of the method specified in this document is motivated from a lock of N. Kang. Expires April 22, 2015 [Page 4] Internet-Draft Secure configuration February 2014 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. 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); it is a constrained device having poor input/output interfaces. SBR(c) Shorten abbreviation of a secure bootstrapping respondent c; it is generally regarded as a controller (not highly constrained) of a service domain. SBS(s) Shorten abbreviation of a secure bootstrapping server s; it can be 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; it can be a socket ID (i.e. IPv4 or IPv6 address and port number). RN_A N. Kang. Expires April 22, 2015 [Page 5] Internet-Draft Secure configuration February 2014 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). 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. SK_i Shorten abbreviation of a 128bits session key for i^th session. A PSK can be used to derive session keys for various security protocols designed by service administrator (see [RFC4764] for example). AK_N Denoting 128bits symmetric key generated by authentication server (i.e. SBS(s) in this draft) or system administrator to protect the PSK stored in node N. 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. N. Kang. Expires April 22, 2015 [Page 6] Internet-Draft Secure configuration February 2014 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-device interfaces they have are not enough for doing configurations manually by person (i.e. inadequate or even no input/output equipment such as display or keyboard). As one of solutions, this document proposes a method which allows a node to reconfigure a symmetric key (i.e. pre-installed key in enrolment phase) automatically upon joining to existing network. After the secure configuration phase, an installer (or manufacturer) cannot read/modify/insert any communication data even though he did initial pre-setup of secure credential of communicating nodes. The following figure illustrates simplified lifecycle of a constrained nodes. |Re-Ownership| |Re-Bootstrap| | | V V |Manufacture| --> |Install| --> |Bootstrapping| --> |Operation| <--- Enrolment Phase ----> <-- Handled by System Admin -> Figure 1. Simplified lifecycle of a constrained nodes N. Kang. Expires April 22, 2015 [Page 7] Internet-Draft Secure configuration February 2014 The method of this document is based on a straightforward scenario, where resource constrained things 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 roles (or policy) defined in a service domain. For example, a thermostat, which can be a controller, manages and communicates several temperature sensors, humidity sensors, window handle devices, heating controller, air conditioner, and more. This document does not assume that a system administrator trusts an installer (or manufacturer) even though he makes orders for the installer. This is because trust and responsibility of installer, who buys and install devices, are different from those of system administrator. In this scenario, the following transactions MUST be done prior to the secure key reconfiguration (i.e. procedures in enrolment Phase). 1. System administrator makes orders and requests initial setup of devices to an installer. Pre-setup information is a set of values that include ID and NID of controller for each of the devices, and a temporary key used as an initial key (i.e. IK_N). Note that, all devices handled by a single installer may share the same IK_N. This concept is similar 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). The administrator may utilize procedures (e.g. web based registration) managed by manufacturer to get the information. 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 phase (a part of enrolment phase). Some of the information for a node may be pre-configured by manufacturer. 4. When a node joins to network, it knows NID of its associated controller with which he can communicate. Also, authentication server has lists including node ID and pre-installed key for new nodes. 5. PSK reconfiguration phase can be then started. N. Kang. Expires April 22, 2015 [Page 8] Internet-Draft Secure configuration February 2014 In order to make a practical and efficient 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 does not consider all modes of cryptographic operations in DTLS which is a recommended 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 a new node SBI(i) and network node(s) (i.e. 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 or the ability of SBR (i.e. computing power and memory). Mutual authentication and PSK reconfiguration procedures are shown in Figure 2. ------- ------ ------ SBI(i) SBR(c) SBS(s) ------- ------ ------ | | | | ID_i, RN_i | | | --------------------------->| | | |ID_i, ID_c, RN_i, RN_c, TS, TID | | |------------------------------->| | | | | | | | | E(IK_i||ID_i||TID||AK_i, SK_cs)| | |<-------------------------------| | | | | | | |ID_c,E(RN_i||RN_c||AK_i,IK_i)| | |<--------------------------- | | | | | | | | | E(RN_c, PSK) | | | --------------------------->| | | | | | | | Figure 2. Message Exchange for PSK Reconfiguration N. Kang. Expires April 22, 2015 [Page 9] Internet-Draft Secure configuration February 2014 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 his controller SBR(c). The NID_of SBR(c) (i.e. IP address and port number) has been pre-configured by installer of the SBI(i) in the enrolment phase as specified in section 3 of this draft. Upon receiving the message, SBR(c) generates a random number RN_c and a sequence number used as a transaction ID (i.e. TID). Then he sends the two values 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 the valid time of key and verify the freshness of the arrived message. Specific period of the expiration of key (i.e. PSK) is out of scope of this document. The authentication server SBS(s) first discovers the IK_i for node ID_i in his secure repository. SBS(s) now can derive a new PSK for the node SBI(i) and replace the IK_i with the PSK, where the PSK for SBI(i) is derived as follows. PSK_i = E(RN_i V RN_c, IK_i) After the reconfiguration of PSK for node SBI(i), SBS(s) generates a AK_i which is a secret key (or password). The AK_i is used for protecting PSK_i to be stored in constrained Node i. All nodes covered by SBS(s) can share a single AK_i or SBS(s) can generate a key for each of the nodes depending on service or security policy. Finally, SBS(s) encrypts the concatenation value of IK_i, ID_i, TID and AK_i with the symmetric key SK_cs which is a shared key between SBS(s) and SBR(c). This is because SBR(c) does not have the key IK_i at this moment. SBS(s) then sends the encrypted value to 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, RN_c and AK_i with the key IK_i. Then it sends both the encrypted value and his ID_c to SBI(i). Note that, SBR(c) MUST not transmit the derived PSK over the public network. SBI(i) can verify the authenticity of SBR(c) by using the decrypted RN_i value from the received message. Finally, SBI(i) can configure his PSK thereafter sending the encryption value of RN_c with the new key PSK to SBR(c) for the authenticity validation. SBI(i) derives a session key SK_i from the PSK and then reconfigures his secure credential as follows. IK_i <-- E(PSK, AK_i) N. Kang. Expires April 22, 2015 [Page 10] Internet-Draft Secure configuration February 2014 After that, SBI(i) deletes AK_i which is only stored in SBS(s). This is because small device is generally more vulnerable to various physical attacks such as theft and forgery than SBS(s). When a node needs to reconfigure such secure parameters, SBS(s) must send the encrypted AK_i. 5. Security Considerations The method of this draft uses a single cryptographic primitive AES [AES] which is used for secure bootstrapping (exactly in the PSK reconfiguration phase). Single cryptographic primitive implementation is rationally suited for the scenario where applications or services require a secure session (confidentiality and integrity of data) in IoT. Because small devices with low computing power and little storage are major entities in IoT. According to a full bootstrapping policy, the PSK can be used for mechanisms of session key derivation and/or 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 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) N. Kang. Expires April 22, 2015 [Page 11] Internet-Draft Secure configuration February 2014 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. [RFC7252] Shelby, Z., Hartke, K., and Bormann, C., "The Constrained Application Protocol (CoAP)", RFC 7252, June 2014. [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. [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. N. Kang. Expires April 22, 2015 [Page 12] Internet-Draft Secure configuration February 2014 Author's Addresses Namhi Kang Duksung Women's University Seoul Korea Email: kang@duksung.ac.kr URI: http://www.duksung.ac.kr Jaeduck Choi NSRI (National Security Research Institute) Daejeon, Korea Email: cjduck@ensec.re.kr Seungwook Jung Soongsil University Seoul Korea Email: seungwookj@ssu.ac.kr Souhwan Jung Soongsil University Seoul Korea Email: souhwanj@ssu.ac.kr Younghan Kim Soongsil University Seoul Korea Email: younghak@ssu.ac.kr N. Kang. Expires April 22, 2015 [Page 13]