Preamble Encryption Mechanism for Enhanced Privacy in Ethernet ...

Preamble Encryption Mechanism for Enhanced Privacy in Ethernet Passive Optical Networks Pedro R.M. Inácio1,2, Marek Hajduczenia1,3, Mário M. Freire2, Henrique J.A. da Silva3, and Paulo P. Monteiro1,4 1

Siemens S. A., Research and Development Department, Rua Irmãos Siemens, 1 2720-093 Amadora, Portugal 2 IT-Networks and Multimedia Group Department of Computer Science, University of Beira Interior Rua Marquês de Ávila e Bolama, P-6201-001 Covilhã, Portugal 3 Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Pólo II 3030-290 Coimbra, Portugal 4 Instituto de Telecomunicações – Pólo de Aveiro, Universidade de Aveiro 3810-193 Aveiro, Portugal {pedro.inacio, marek.hajduczenia, paulo.monteiro}@siemens.com, [email protected], [email protected]

Abstract. Ethernet Optical Passive Networks (EPONs), defined as low cost access networks, combine Ethernet technology with an optical fiber infrastructure to deliver voice, video and data services from a Central Office (CO) to end-users. Since all data in the downstream is broadcasted, it is susceptible to be eavesdropped by a malicious user, which can use it to try Theft of Service (ToS) through masquerading techniques. These threats remain present when encryption is applied to EPON frame payloads. In order to avoid user profile inference through data mining techniques, a method for encryption of the preamble of the data units is proposed in this paper and a short description of its operations is presented. This new encryption mechanism assures that any two EPON frames are always transmitted with different and uncorrelated preambles. Keywords: Security, Ethernet Passive Optical Networks, EPON, encryption mechanism, EPON frame preamble, privacy.

1 Introduction Ethernet Optical Passive Networks (EPON), defined as low cost access networks, combine Ethernet link-layer protocol with an optical fibre infrastructure to deliver voice, video and data services from a Central Office (CO) to end users. The device terminating the optical fibre in the CO is normally referred to as Optical Line Terminal (OLT) and, at the other end of the Passive Optical Network (PON), endusers are connected through Optical Network Terminals (ONUs), placed in the house or business premises. Data transmissions originated by the OLT are usually known as downstream. The term upstream is used for transmissions initiated by a given ONU [1]. R. Meersman, Z. Tari, P. Herrero et al. (Eds.): OTM Workshops 2006, LNCS 4277, pp. 404 – 414, 2006. © Springer-Verlag Berlin Heidelberg 2006

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In order to maintain the cost of the network deployment as low as possible, Passive Splitter Combiners (PSCs) are placed between the OLT and the ONUs. The purpose of these devices in the upstream transmission is to combine several optical signals coming from individual ONUs into a single, shared, fiber channel; or to split the single optical signal coming from the OLT to all connected ONUs. A PSC, as the name suggests, is a completely passive device that does not perform logical operations or amplifies the signal. For those reasons, the number and position of PSCs units has to be carefully considered prior to network deployment.

PSC Service Providers (SP)

Home or business premises

ONU 1 ONU 2

CO OLT

Users (residential)

ONU 3 ONU … ONU 32

Users (business)

up to 20 km Fig. 1. Tree-and-branch EPON system topology with a number of connected ONUs with various deployment scenarios

PSC CO OLT

ONU 1

Malicious user (eavesdropper)

ONU 2

Other user

ONU 3

Destination user

ONU …

Other user

ONU 32

Other user

LLID filter Fig. 2. Upstream (dashed arrows) and downstream (normal arrows) communications in an EPON system. Data originated by the OLT is broadcasted to all ONUs in the system, while data originated by individual ONUs is delivered only to the OLT (unicast link).

Fig. 1 depicts an EPON tree-and-branch deployment, where all the aforementioned elements of the system are logically schematized. The number and localization of ONUs in the figure are merely symbolic, however, depending on several variables,

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their number is never superior to 64, neither their distance to the OLT bigger than 20 km. Due to the physical properties of the PSCs, any signal originating from the OLT will unavoidably arrive to every ONU on the system. Downstream communications are, therefore, of broadcast type (Fig. 2, normal arrows). In the upstream direction, if no reflections occur on the PON system (mainly on the PSCs), the signal originating from an ONU will only be seen by the OLT (Fig. 2, dashed arrows) [1]. As some segments of the optical fiber are shared between more than one ONU, the upstream transmission requires medium access protocol, in the form of the Time Division Multiple Access (TDMA), which in EPON system is supported through application of the Multi Point Control Protocol (MPCP), providing a general framework for TDMA channel access. Fig. 3 exemplifies how the upstream bandwidth management is performed. MPCP Data Units (MPCP DUs), called GATEs [2], emitted by the OLT, inform every ONU about the start time and length of its transmission slots.

OLT

ONU 1 ONU 2

Other ONUs

Time slot granted To ONU 1 Time slot granted to ONU 2

time

Grants emitted from OLT to a given ONU ONU 1 start of transmission ONU 2 start of transmission ONU 1 end of transmission ONU 2 end of transmission OLT finishes receiving all the data from ONU 2

Upstream bandwidth granted to other ONUs

Fig. 3. Upstream bandwidth management using assignment of transmission time slots

The remaining part of this paper is organized as follows. Section 2 describes EPON security threats according to their severity. Section 3 presents the most common security mechanisms used in EPONs. Section 4 proposes a new mechanism preventing data mining and profiling based on the Logical Link Identifier (LLID) values and section 5 presents main conclusions.

2 EPON Security Threats 2.1 High Severity Threats

High severity threats derive directly from the fact that downstream communications are of broadcast type [3], [4]. In such medium, point-by-point connections are only possible through link-layer emulation, where every ONU connected to the system has one or more than one assigned LLID that univocally identifies it in the network. When transmitting, an ONU stamps every frame with one of its LLIDs. Frames sent in the upstream direction carry, consequently, the address of the source of the communication. In the downstream direction, all frames carry the LLID address of the

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destination entity, with the exception of the broadcast data units, which are delivered with the so-called broadcast LLID. Fig. 4 emphasizes the differences between Ethernet (upper section of the figure) and EPON frames (lower section of the figure). While on EPONs the preamble of the frame carries addressing information (LLID and its respective Cyclic Redundancy Check 8 (CRC8)), the very same preamble in Ethernet frames is only used to assure proper clock recovery process and data stream alignment. The reserved fields in the EPON frame preamble are not used within the PON context. When an ONU receives an EPON frame, it applies a filtering policy based on the set of LLIDs assigned to it. If the LLID on the frame coincides with one of the ONU’s LLIDs, the frame is to be accepted and forwarded for further processing; if not, it should be discarded. A malicious EPON system user, aware of the operation of the LLID filtering rules, can deactivate them on a given ONU, enabling it to work in a promiscuous mode. This procedure will give him the capability to listen to all downstream communications, in a completely unnoticeable manner. Once active, a promiscuous mode allows an attacker to learn Medium Access Control (MAC) and LLID addresses of other ONUs on the EPON system; perform user profiling (quantity and type of traffic) by monitoring LLID, MAC or content information; and infer characteristics about the upstream traffic by observing downstream MPCP DU exchange, especially the GATE MPCP DUs, carrying bandwidth allocation to particular ONUs and LLIDs. Privacy is said to be assured when it is not possible to infer confidential information through passive attacks. For this reason, traffic analysis can be seen as an attempt against privacy [5].

Ethernet frame preamble

Start of Frame Delimiter

S Destiny Preamble F MAC D address

Frame Check Sequence

Source MAC Size/ type address payload

EPON frame preamble S Reserved L Reserved LLID D

Data

FCS

C R C 8

Fig. 4. Ethernet/EPON frame scheme. EPON and Ethernet frames differ only in the preamble format, which in EPON case contains additional Point-to-Point (P2P) emulation data.

2.2 Medium Severity Threats

In the upstream direction, EPON is a Multi-Point-to-Point (MP2P) network. Upstream transmissions are controlled by the OLT through application of the MPCP protocol and of a Bandwidth Allocation Algorithm (BAA). An attacker can masquerade himself as another ONU (by stamping all the emitted frames with an LLID that is assigned to other ONU) and try to theft service from a legal user. He can also obtain access to confidential information or restricted resources or even take advantage of

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the operation of the BAA mechanism through spoofed MPCP messages, attempting to reduce the upstream channel bandwidth assigned to other, legitimate users [4]. Another medium severity threat, normally classified as a physical layer threat, is the possibility that an attacker has to superpose the upstream signal from others ONUs with a high power signal, aiming to compromise the optical detection capabilities of the OLT sensor [3], leading the system to a downtime. 2.3 Low Severity Threats

If there are significant reflections on the medium, or if the attacker has technologically advanced devices (with high detection capacities), he can try to eavesdrop frames sent by another ONU [3]. After analyzing them, he can send a modified version of the data units upstream, or simply discard them. Although theoretically possible, occurrence of optical reflections is highly improbable since their value, caused by the PSCs and fibre channel (more specifically: splices between fibre sections) is extremely low.

3 Currently Available Mechanisms EPON security has been subject of concern for quite a while [1], [3]-[11], and it is commonly accepted that encryption of the payload of the Ethernet frames solves the problem of confidentiality [3], [4] and of data origin authentication [9]. Symmetric encryption standards, as the Advanced Encryption Standard (AES) or Data Encryption Standard (DES), are commonly used to encrypt private data contents on EPON communications. At the EPON level, content encryption corresponds, in the best case scenario, to the encryption of the Data, FCS, Size/Type and MAC fields. Preamble encryption is not currently performed in any existing EPON implementation [12], [13]. Fig. 5 shows how content encryption can be seen from the data link layer point-of-view. S Reserved L Reserved D

LLID

C R C 8

Encrypted payload

Non encrypted EPON preamble Fig. 5. EPON frame scheme: content encrypted

In the downstream direction, encryption of the Ethernet frames payload prevents a malicious user to access confidential data within the frames. Without the correct decryption keys, using state of the art computers, message reconstruction is virtually impossible; and depending on the size of the encryption keys, their regeneration rate and their secrecy level, the data transported within the EPON data units, can be considered to be more or less secure. In the upstream direction, the error detection code included in the encrypted payload (a 32 bit wide Cyclic Redundancy Check (CRC32) code transported in the Ethernet frame FCS field) can be used to validate the origin of the data. Decryption of the payload followed by a successful match between the calculated and the decrypted

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CRC32 assures, at least, that the party that sent the message had the correct encryption key. If the key, used for encryption and decryption of the content of the frames, was traded using reliable means, and its secrecy is guaranteed, the ONU from which the frame was originated is not faking its identity. In the downstream direction, unless the hypothesis of having a fake OLT is accounted for, it makes no sense to talk about data origin authentication, since in that direction the data can only be originated by the OLT. The low severity problem of potential upstream reflections has also been addressed by the solution described in [14], which comprises a set of reflecting devices, that should be strategically placed on the PON system to intentionally generate noise. Any unwanted reflection is merged with its non-correlated reflections on the disturbing devices, turning the echoed signal into non decodable. Disturbing device

ONU 1 ONU 2

CO OLT

ONU 3 ONU … PSC ONU 32

Fig. 6. State of the art solution to protect the EPON system against threats that explore possible upstream reflections

Fig. 6 depicts this physical layer security mechanism. In the example, a fraction of the signal emitted from ONU 32 is intentionally reflected downstream to all ONUs by more than one disturbing device. In the particular case, the signal emitted by ONU 32 is only reflected by the two disturbing devices of the upstream path. The problem of possible upstream reflections can also be solved by encryption of the upstream communications. On switched networks, destiny information is crucial to correctly forward the data units, from the source to their destiny. Encryption of fields containing such information requires switching devices along the way, to be capable of decrypting the necessary address information before forwarding it. As such process normally decreases the overall networks speed and increases their deployment complexity, letting some of the address information in plain text (destiny address) is typically considered a solution of a choice. In case of EPONs, the downstream Ethernet frames (even the ones with the contents encrypted) include the LLID information in the preamble of the frames. As the downstream data can be passively observed, a malicious user can still sort the incoming data based on the LLID or CRC8 information. Some implicit/explicit information can be obtained through the observation of such data. For instance, the malicious user will know which are the currently active LLIDs, their activity rate or the downstream bandwidth assigned to them. From the downstream MPCP messages, he can extrapolate the upstream assigned bandwidth for a given LLID. Additionally,

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since two different frames with the same LLID are supposedly encrypted with the same key, the collected messages can be also used to feed encryption key searching algorithms or data mining techniques.

4 Anti Data-Monitor Mechanism As indicated previously, most of the existing security solutions for EPONs are focused on the confidentiality problems. These are, in fact, considered the most severe problems on this kind of networks. However, there is useful information that can be directly deduced from the unencrypted fields within the preamble of the PON frames, or inferred from their analysis. Once a malicious user has gathered and examined sufficient profiling information about an LLID (or from a user associated to it), he can devise better the next step. This constitutes a medium severity problem within the context of EPONs, since private information can be easily obtained by a malicious person. In this paper, we propose a mechanism for encryption of the EPON frame preambles. This encryption scheme can be applied on EPON systems because the origin and destiny of the information are always known and no switching or routing is performed along the path. The signals are simply separated and sent to all ONUs or combined and sent to the OLT. ID information is only important to the filtering module in the receiving devices and not to data forwarding along the path. As every downstream frame is submitted to the filtering module in all the ONUs on the system, the proposed mechanism will only adds one additional decryption step before filtering. In the upstream direction, the encryption mechanism provides data origin authentication at the bottom of the data link layer, providing that the shared keys are unique and their secrecy is assured. In cases where the previous conditions are met, the BAA supports the capability of the OLT to validate the identity of an ONU. The proposed method offers the best data protection policy to EPON users. If the EPON frames are fully encrypted (preamble and payload), sensitive information is no longer accessible and, consequently, the purpose to eavesdrop the downstream traffic ceases to exist. Users are protected against privacy attempts while the confidentiality of the data is assured by the payload encryption. Since the computational cost of the solution is to maintain as low as possible, the encryption/decryption functions are based on simple xOR operations. Table 1. Logical table for xOR operation

⊕ 1 0

1 0 1

0 1 0

⊕ : (0,1) → (0,1) ⊕ ( A, B ) = A ⊕ B = ( A∩ ~ B ) ∪ (~ A ∩ B )

(1)

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Table 1 and equation (1) define mathematically the aforementioned function. In the equation, the symbol ~ stands for “negation of”, while ∪ represents the bitwise operator OR and ∩ the bitwise operator AND. The xOR operator is represented by the ⊕ symbol. In the description below, the expression “encryption of sequence A with sequence B” refers to the result of the bit-by-bit application of the bitwise operator xOR to the two inputs. Sequence A must be of the same length of sequence B. 4.1 Encryption

Encryption of the sensitive Ethernet frame preamble fields (along with the encryption of the Ethernet frame payload) would significantly reduce the amount of exposed system’s information. In order to exhibit anti-monitor capabilities, the mechanism must meet some criteria. For instance, it has to be suitable for operating at the data link layer; and the cipher texts, resulting from the encryption of two consecutive frames (with the same LLID), have to have an infinitesimal probability of being equal. By other words, once encrypted, two consecutive frames will have always different preambles, even when the respective plain text ones are equal. Once met, this particular property mitigates any type of data mining techniques based on the values of the LLID or CRC8 fields. EPON frame preamble encryption is depicted in Fig. 7. As the Start of LLID Delimiter (SLD) field does not contain any valuable system information, its encryption is useless and it was not further considered. The method proposed herein assumes that a pair of Secret Keys (SK) (one for the upstream encryption/decryption, one for the downstream encryption/decryption) is shared between each ONU in the PON system and the OLT. The key exchange must be carried out previously, using a secure Key Agreement Protocol (KAP). When a frame is to be sent from the OLT, or from an ONU, the transmitting party should generate two different keys: a Random Key for the LLID (RKLLID) and a Random Key for the CRC8 (RKCRC8). Assuring that, for each time the keys are generated they are completely random, the bitwise operation xOR of each one of them with a static sequence of bits will produce a random sequence as well. Full EPON frame encrypted Encrypted preamble

Signaling byte

RKCRC8 ⊕ SKCRC8

SLD (not encrypted)

Encrypted payload RKLLID ⊕ SKLLID

LLID ⊕ F(RKLLID)

CRC8 ⊕ F(RKCRC8)

Fig. 7. EPON frame scheme: completely encrypted frame

The LLID field is then encrypted with the bit sequence that results from the application of a non invertible Function (F) to the RKLLID. A reciprocal procedure applies itself for the CRC8 field. In order to prove its worth against data sorting techniques, the F function should produce an output apparently as random as its input. In this case, hash functions fit perfectly the aforementioned requirements.

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After encrypted with the SK, the RKLLID and RKCRC8 values are conveyed in the non used fields of the preamble (see Fig. 7). The initial byte of the preamble is used to indicate the EPON frame preamble (and even the payload) is encrypted or not (signaling byte in Fig. 7). By these means, the receiving end will know which frames must be decrypted before being filtered. Fig. 7 schematizes a completely encrypted EPON frame where system sensitive fields are directly accessible without previous decryption process, as described in Section 4.2. The encrypted preamble is depicted in detail to emphasize its internal structure. 4.2 Decryption

When a party receives a frame, it should first check the preamble signaling byte. If it indicates that the preamble of the frame is encrypted, the decryption mechanism should be applied to the cipher. An SK should be used to decrypt the RKLLID and RKCRC8, which are fed into the F function. With the resulting sequences, decryption of the LLID and CRC8 fields of the preamble is straightforward. The OLT must use the SK corresponding to the ONU currently transmitting. The key can be easily retrieved by the BAA that controls the upstream transmissions and, therefore, has the information of what ONU is transmitting at the moment. If, after decryption, the LLID or the CRC8 do not match the expected values, the frame has either an error or it comes from an invalid ONU (perhaps a promiscuous ONU) and should be discarded. Each ONU uses its respective SK to decrypt downstream encrypted frames. If after decryption, the LLID or CRC8 do not match, the frame was not intended to it and must be discarded. 4.3 Simulation Study

The proposed encryption mechanism was implemented in a distributed, event driven EPON system simulator, which reproduces all the aspects of data transmission in the said networks. All active EPON system components (OLT and ONUs) are represented by individual processes (software programs), operating on individual machines, communicating through sockets and emulating physical level in the EPON system. The encryption mechanism was implemented on a computer program that simulates an EPON system. The active EPON (OLT and ONUs) components are represented by individual threats that communicate via sockets. Before transmission, the Ethernet frame preamble is encrypted as described herein. At the receiving point, decryption also follows the same specifications: after decryption, the frames are sent to the filtering module which decides about the legitimacy (OLT case) or destiny (ONU case) of the message. The following lines were taken from one of the sockets log. They are related to the encryption and upstream transmission of EPON frames originated by the same ONU. Each line contains the encrypted EPON frame preamble (written in the hexadecimal notation) and the generated RK used to encrypt it. The plaintext preamble is [55-55d5-5555-7ffd-2b] and the SK for upstream transmissions for the given ONU is 57-bc91.

Preamble Encryption Mechanism for Enhanced Privacy

#1 [05-c8-d5-eb22-d68d-22]

RK:bc-9e59

#2 [05-67-d5-6466-3413-cf]

RK:33-daf6

#3 [05-df-d5-430e-1f73-43]

RK:14-b24e

#4 [05-bf-d5-4bab-56b2-7a]

RK:1c-172e

#5 [05-e2-d5-1d43-1da5-7a]

RK:4a-ff73

#6 [05-79-d5-d664-e7ee-b7]

RK:81-d8e8

#7 [05-e4-d5-043f-2115-6c]

RK:53-8375

#8 [05-2d-d5-8aeb-d610-5b]

RK:dd-57bc

413

It may be concluded that all the preambles of EPON frames originating from the very same LLID are different and statistically uncorrelated. The only fields that are equal in all messages are the SLD, containing the predefined value d5 [2] and the initial field used for signaling purposes and containing, in this case, the value 05. In the simulation, the value 55 is used to indicate that the frame is in the plain text format; a 05 signals preamble encryption; a 50 payload encryption; and 55 stands for complete frame encryption. Based on the above presented frame preambles it is impossible to say whether if they come from the same LLID or not. This constitutes the perfect example of how the proposed encryption mechanism works. Notice that all the messages were successfully delivered, decrypted and accepted by the filtering module in the OLT.

5 Conclusions Currently available security mechanisms do not take into account all inherent EPON system threats. The most severe security issues stem from the broadcast type of the downstream communications and, as encryption of the downstream frame payload does not cover the LLID and CRC8 information, it is possible to apply data mining techniques to this data and profile individual network users. Aiming for the solution of that problem, an anti data-monitoring mechanism was introduced and discussed in this paper. The proposed method not only turns unfeasible any attempt to sort the downstream data, but also assures data origin authentication in the upstream channel (providing that a secure KAP was used to exchange the encryption keys). The cryptographic properties of the algorithm counterbalance its implementation complexity. As the algorithm is very modest, in terms of processor operations and memory requirements, it is suitable for a data link layer implementation, which is the layer on which EPON systems operate. The decryption mechanism is straightforward and does not impose significant delay on data reception. As defined herein, preamble encryption is faster than payload encryption (simple xOR against AES encryption) and, some of the parts of the proposed encryption procedure can be pre-processed or processed in parallel. Acknowledgements. The authors would like to thank Fundação para a Ciência e Tecnologia (FCT), Portugal.

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