Xtea Comparison Essay

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1. Introduction

Wireless sensor networks (WSNs) consist of multiple distributed sensors that are deployed in an area of interest to sense physical phenomena or environmental conditions, such as temperature, vibrations, sounds, etc. The sensor motes act autonomously in a distributed fashion without a central coordination entity, so that the network topology can be created on a peer-to-peer basis on demand. Data are transmitted cooperatively through the network by forwarding the data from mote to mote, resulting in a multiple-hop transmission.

Recently, WSNs have become an increasingly important research topic due to the growing demand for monitoring applications in various contexts, in the civilian, as well as the military sector. Moreover, technological progress enabled the mass production of sensors, resulting in decreased manufacturing costs and, thus, less expensive sensors for customers. While sensor motes are getting smaller and less expensive, their functionality is continuously improved.

One of the main characteristics of sensor motes is that they are usually limited by energy constraints due to the fact that most sensors are powered by battery. For that reason, the motes are often equipped with energy-preserving components, which results in low computational, as well as low transmission power. Furthermore, the available memory is rather limited.

Due to the fact that WSNs are often deployed in unattended or even hostile areas, security-related issues have to be considered. Particularly in medical and military applications, data confidentiality, i.e., keeping the data transmitted by sensor motes confidential, is of high relevance. Furthermore, data integrity often needs to be guaranteed so that modifications of data packets, caused intentionally by an attacker or unintentionally by interferences, can be easily identified. Other concerns, such as the injection of falsified data, replaying of old packets and related issues, also need to be taken into account.

A good counter measure to mitigate those security issues is the application of cryptographic measures. However, in the special case of WSNs, the existing security mechanisms cannot be applied directly to the sensor motes due to their resource constraints in terms of computational power and memory limitations. Therefore, often, only light-weight symmetric cryptography algorithms have been applied on sensor networks [1]. Although, for a long time, researchers neglected public key cryptography on sensor motes, recently, its aptitude was shown in a few papers [2,3]. One of the most promising approaches in this area seems to be elliptic curve cryptography (ECC), which offers a high level of security with small key sizes at an acceptable level of performance.

In this paper, a hybrid cryptography approach is employed to build a simple security scheme, which combines the security benefits of a public key infrastructure (PKI) during the handshake, with the power efficiency of symmetric cryptography for the bulk of communication. This security scheme is implemented for TinyOS [4], an open source operating system for sensor motes. The implementation was simulated in TOSSIM [5], the TinyOS simulator, and, subsequently, tested in our test bed of sensor motes. The motes we used are IRIS motes produced by Crossbow, based on an 8 bit microcontroller with 8 KB of RAM and 128 KB of program memory [6].

During the course of developing this security framework, we have considered and analyzed a variety of cryptographic primitives for their suitability. Therefore, a modular architecture has been chosen for the developed framework, which allows the easy replacement of one component with another, so it can be adapted to various circumstances with different requirements.

Specifically, this paper will give a detailed analysis of the performance characteristics of certain block ciphers (Skipjack [7], RC5 [8], AES128 [9], eXtended Tiny Encryption Algorithm (XTEA) [10]), the Spritz [11] stream cipher, block cipher modes and authenticated encryption modes (counter (CTR) [12], Counter with Cipher block chaining Message authentication code (CCM) [13], Synthetic Initialization Vector (SIV) [14], Offset Codebook mode (OCB) [15]) and MAC functions (hash-based message authentication code (HMAC)-SHA1 [16,17], Cipher-based Message Authentication Code (CMAC) [18], SipHash [19], keyed BLAKE2s [20]) on IRIS motes. The results should be applicable to other platforms based on Atmel ATmega128 microcontrollers, such as the popular MICAz and MICA2 motes.

The rest of the paper is organized as follows: In Section 2, related works of the paper are discussed. Afterwards, in Section 3, the security scheme is presented and its foundations are explained in depth. Then, in Section 4, the conducted simulations, as well as the experiments in our test bed are discussed and evaluated. In Section 5, we describe our methodology for comparing various cryptographic primitives, which can be used for the bulk data communication part of the previously described scheme. Next, in Section 6, the results for the comparison are presented. Finally, in Section 7, conclusions are drawn and future work is discussed.

2. Related Works

Several cryptographic methods for wireless sensor networks have been studied and evaluated, due to their constrained resources in terms of energy consumption and computational power. Sensor motes are limited in their computational and memory capabilities, so that the well-known traditional cryptographic techniques cannot be simply transferred to WSNs. TinySec [1], introduced as a security framework for WSNs, addresses security in motes where energy and computational power impose significant limitations on the approaches available for securing such networks. It supports the use of symmetric cryptography, namely block ciphers in cipher block chaining (CBC) mode, to secure sensor mote communications, but it was never ported to TinyOS 2.x, the latest TinyOS version. Recently, there has been a change in the research community from symmetric cryptography to public key cryptography, which has been traditionally considered as too expensive on sensor motes. One main area of interest in the area of public key cryptography is elliptic curve cryptography (ECC) [21]. The advantage of ECC in comparison to other public key approaches, such as RSA, is its mathematical foundation: while the best algorithm that solves the integer factorization is a sub-exponential problem, the best algorithm that solves the ECC discrete logarithm problem is exponential. For that reason, ECC is faster, but at the same time, it can reach equivalent security with smaller keys. The benefit of smaller keys is that they need less processing time, less storage, less bandwidth and, therefore, less energy, which is ideal for energy-constrained sensor motes. As a conclusion, ECC offers an alternative to both symmetric cryptographic systems and more heavy-weight algorithms, such as RSA. Wander et al. [2] have previously shown that the use of ECC on wireless sensor motes can be viable. As a result, ECC is likely to play an important role for public key cryptography in WSNs in the future.

As far as implementation goes, TinyECC [22] is a high-speed implementation with real-world security. According to a recent study, if reasonable implementation security is required, it is still the fastest available ECC implementation for AVR-based platforms, such as IRIS and MICAz motes [23].

Since then, various proposals for both pure public key cryptography-based communication systems and hybrid schemes, using both symmetric and asymmetric methods, have been introduced. For example: Pugliese, M. and Santucci, F. [24] discuss in their paper a novel hybrid cryptographic scheme for the generation of pairwise network topology authenticated keys (TAK) in WSNs, based on vector algebra in GF(q). For the ciphering and authentication model, symmetric cryptography is used, while the key generation model is drawn on asymmetric cryptography [25].

Investigations of the efficiency of different cryptographic primitives for their use on wireless sensor networks have been conducted before. A survey of such investigations has been conducted by Roman et al. [26]. Most of the surveyed works make use of specialized hardware and asymmetric cryptography, but software implementations of symmetric ciphers are also briefly discussed.

In the aforementioned TinySec framework, which provided a cryptographic communication layer for TinyOS 1.0 using symmetric cryptography, only a single key, and thus, cipher instance, was used, which means that initialization costs and key schedule storage costs were not evaluated in detail [1].

A survey on block ciphers for their use in wireless sensor networks was performed by Law et al. [27]. This work concentrated on the use of Smart Dust, EYESnode and Intel mote sensor motes, all of which have rather different performance characteristics than IRIS and similar motes. No analysis of the message authentication code algorithms was performed.

Ganesan et al. [28] have also done an analysis on the performance of different cryptographic algorithms on different CPUs used in wireless sensor motes. However, neither the memory used by an instantiated cipher is considered, nor are MAC algorithms or block cipher modes analyzed.

However, at this time, no detailed analysis of OCB [15] and SIV [14] performance on WSN motes has been performed. Additionally, the new SipHash [19] and BLAKE2s [20] functions are similarly unexplored territory.

3. Scheme Design

An inherent problem of security schemes in WSNs relying solely on symmetric cryptography is their vulnerability against attackers that gain access to the keys stored on one of the sensor motes. Besides, due to memory constraints and other efficiency reasons, only a limited number of keys can be pre-distributed in WSN scenarios. As a consequence, already a low number of compromised motes allows an attacker to gain access to big parts of transmitted information. Tamper-resistant hardware would provide a way to mitigate this problem, but this is generally too expensive.

3.2. Scheme Description

After start-up, the sensor motes begin broadcasting their certificates. Each broadcast certificate includes the mote’s address, its public ECC key and a signature provided by the CA. To avoid too many concurrent broadcasts, which could not be handled at the same time by receiving nodes, a small random delay is added. To provide resilience in the case of packet loss, during the handshake, packets are presented a predefined number of times if no response has been received.

To save some bandwidth, the certificate broadcast frequency is gradually lowered over time. This can be justified by the assumption that more motes will try to establish a connection at the beginning. Other motes are still able to join the network later, but not as quickly as in the initial setup phase.

Due to the memory limitations of the sensor motes, the number of stored certificates and public keys, as well as the number of current connections and handshakes are limited. Using multiple hops, it should generally still be possible to find a path from any mote within the network to any other. To counteract packet loss, packets sent during handshakes are retransmitted a number of times. Timeouts are used to mitigate incomplete handshakes that cannot be finalized.

When a certificate is received, a mote checks the obtained signature, and if it is valid, the public key and the mote’s address from the certificate are added to its list of known public keys. From this point on, a secured connection can be established between both nodes. Depending on the desired behavior, a handshake can be triggered automatically or on demand, i.e., when the application tries to send data to the mote. The handshake is inspired by the transport layer security (TLS) handshake, but highly simplified [31].

To establish a connection, a mote can send a key exchange offer to another mote from which it has received a public key. The setup of a connection is implemented as follows: at the beginning, 128 bits of random data are generated and stored. These random data are then encrypted using ECIES towards the target mote’s public key. The resulting message will be signed using ECDSA, using the sending mote’s private key. Though ECIES already provides an HMAC over the message, it is not really useful, since this HMAC does not provide sender authentication. Instead ECDSA is used to provide sender authentication, while the HMAC is neither calculated nor sent.

Upon receiving a key exchange offer, the receiving mote will first check that it still has a free connection and handshake slots. If this is not the case, an error message is sent, so that the mote that initiated the key exchange offer can immediately drop the connection attempt and try it again with a different mote. The message sequence chart in Figure 1 shows an example of the handshakes between three motes.

Error packets contain an eight-bit error code and include information about the packet they were sent in response to, as well as an ECDSA signature of the sending mote. If the error packet was sent in response to a user data packet within the context of an established connection, an HMAC can be substituted for the ECDSA signature.

After ensuring that there are still enough resources to accept a connection, the mote that received the key exchange offer will look up the sender’s public key. If it is unavailable, it can either wait for it to be broadcast or it can send an error packet with an error code indicating that the sender’s public key is unknown. On receiving the error packet, the sender will start broadcasting its public key again. If the public key is available, the signature can be checked. If the signature is valid, the received ECIES message will be decrypted and a further 128 bits of random data will be generated and stored. This new chunk of random data will be encrypted using ECIES with the public key of the mote that sent the key exchange offer.

The received 128 bits of random data and the newly generated 128 bits will be XORed and used as input for the ANSI-X9.63 key derivation function [21] to produce two 128-bit keys, one for use in the symmetric encryption within the established connection and the other for use in the HMAC calculation.

Kxtea||Khmac = KDF(randkeo ⊕ randaccept)

After deriving the keys, the HMAC of the new ECIES message will be calculated, and a packet containing the ECIES message and the HMAC will be sent to the original mote to announce the acceptance of the key exchange offer.

Upon receipt of the accept packet, the ECIES message will be decrypted, and the contained 128 bits of random data will be used with the original 128 bits to derive keys as specified before. Using the newly derived key, the HMAC of the ECIES message will be verified. If it is valid, a final packet will be sent to the other mote and the connection is marked as “established”. The final packet contains the HMAC of the string “connection key is OK”. This is sufficient to demonstrate to the other mote that no replay attack occurred and the handshake was successfully completed.

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