pay-TV Systems Research Articles

Tree based symmetric key broadcast encryption

The most influential broadcast encryption (BE) scheme till date was introduced in 2001 by Naor, Naor and Lotspiech (NNL) and is based on binary trees. This paper generalizes the ideas of NNL to obtain BE schemes based on k-ary trees for any k≥2. The treatment is uniform across all k and essentially provides a single scheme which is parameterized by the arity of the underlying tree. We perform an extensive analysis of the header length and user storage of the scheme. It is shown that for a k-ary tree with n users out of which r are revoked, the maximum header length is min⁡(2r−1,n−r,⌈n/k⌉). An expression for the expected header length is obtained and it is shown that the expression can be evaluated in O(rlog⁡n) time. Experimental results indicate that for values of r one would expect in applications such as pay TV systems, the average header length decreases as k increases. The number of keys to be stored by any user is shown to be at most (χk−2)ℓ0(ℓ0+1)/2, where ℓ0=⌈logk⁡n⌉ and χk is the number of cyclotomic cosets modulo 2k−1. In particular, when the number of users is more than 1024, we prove that the user storage required for k=3 is less than that of k=2. For higher values of k, the user storage is greater than that for binary trees. The option of choosing the value of k provides a designer of a BE system with a wider range of trade-offs between average header length and user storage. The effect of layering on the k-ary tree SD scheme is also explored.

A provably secure and efficient authentication scheme for access control in mobile pay-TV systems

To guarantee the secure access by authorized subscribers in mobile pay-TV systems, user authentication is required. User authentication is a security mechanism used to verify the identity of a legal subscriber. In 2012, Yeh and Tsaur proposed an authentication scheme for access control in mobile pay-TV systems to improve the Sun and Leu's scheme. Yeh and Tsaur claimed that their scheme meets all security requirements for mobile pay-TV systems. However, this paper indicates that Yeh and Tsaur's scheme suffers from some critical weaknesses. An attacker without knowing secret information can successfully impersonate both mobile sets and head-end system. As a remedy, we propose an improved authentication scheme for mobile pay-TV systems using bilinear pairings. The proposed scheme maintains the merits and covers the demerits of the previous schemes, and provides a higher level of the efficiency for mobile pay-TV systems.

On the Security of an Anonymous Multi-Receiver Encryption Protocol

Anonymous multi-receiver encryption (AME) achieves the security properties of confidentiality and anonymity. Thus, it can be used in VoIP (Voice over Internet Protocol), pay-TV systems, et al. In 2012, Zhang et al. proposed an efficient anonymous multi-receiver encryption protocol. They claimed that their protocol can protect the receiver identity privacy and keep message confidential. In this paper, we point out the weaknesses of Zhang et al.'s protocol. Their protocol cannot satisfy the anonymity against the inner attackers. On the other hand, their AME protocol cannot satisfy the indistinguishability of encryptions under selective multi-identity, chosen ciphertext attacks (IND-AME-CCA). Thus, Zhang et al.'s protocol is insecure. The analysis is important for the engineer to mitigate the security risks.

A novel key refreshment scheme increasing the security of conditional access systems in digital satellite pay-TV

This paper proposes a new scheme that faces one of the basic deficiencies of Conditional Access Systems (CASs) employed in satellite Pay-TV systems, Master Private Key's (MPK) infinite crypto period. The novelty of the proposed scheme is that it offers the ability to the administrator of the CAS to refresh users' MPKs at desired times. Thereby, the security of the CAS is significantly increased. The refreshment of the MPKs is accomplished through Lagrange Interpolation Method (LIM). In addition, the bandwidth consumption of several contemporary CASs of the Pay-TV market is presented. Based on those measurements, the bandwidth overhead that the new scheme imposes is proven to be negligible. The efficiency of the LIM for the refreshment of the MPK is verified through its implementation on a 65 nm Field Programmable Gate Array (FPGA). Finally, the required user's MPK refreshment time was achieved to be less than 0.15 ms.

A conditional access system with efficient key distribution and revocation for mobile pay-TV systems

Current mobile pay-TV systems have two types of Conditional Access Systems (CAS): group-key-based and public-key systems. The best feature of group-key-based systems is the ability to enjoy the broadcast nature in delivery multimedia contents, while the major advantage of public-key systems is consolidating the security foundation to withstand various attacks, such as collusion attacks. However, the problems of group-key-based systems include collusion attacks, lack of nonrepudiation, and troublesome key distribution. Even worse, the benefit of broadcast efficiency is confined to a group size of no more than 512 subscribers. For public-key systems, the poor delivery scalability is the major shortcoming because the unique private key feature is only suitable for one-to-one delivery. In this article, we introduce a scalable access control scheme to integrate the merits of broadcasting regardless of group size and sound security assurance, including fine-grained access control and collusion attack resistance. For subscriber revocation, a single message is broadcast to the other subscribers to get the updated key, thus significantly boosting subscriber revocation scalability. Due to mobile subscribers' dynamic movements, this article also analyzes the benefit of retransmission cases in our system. Through the performance evaluation and functionality comparison, the proposed scheme should be a decent candidate to enhance the security strength and transmission efficiency in a mobile pay-TV system.

Improved one-to-many authentication scheme for access control in pay-TV systems

Mutual authentication is important in a mobile pay-TV system. Traditional authentication schemes make use of one-to-one delivery, that is, one authentication message per request is delivered from a head-end system to subscriber. This delivery occupies too much bandwidth and therefore is inefficient and costly. One-to-many authentication scheme for access control in mobile pay-TV systems was proposed by Sun et al. in 2009. In one-to-many authentication scheme, only one authentication message for multiple requests is broadcasted from the head-end system (HES) to subscribers. Sun et al. claimed that their scheme is secure and provides anonymous authentication for protecting user privacy. However, the authors demonstrate that their scheme has a critical weakness. An attacker without any secret information can not only successfully impersonate mobile set (MS) to cheat the HES but also impersonate HES to cheat MS. The authors result is important for security engineers who design and develop user authentication systems. Afterwards, the authors design a novel one-to-many authentication scheme from bilinear pairings. They give the formal security proof in the random oracle model. In addition, they present the performance analysis of our scheme. The analysis results showed that their novel authentication scheme has shorter transmission message and can be applied in the environment which has limited bandwidth. At the same time, their scheme is also the first secure one-to-many authentication scheme for access control in pay-TV systems.

A Secure and Efficient Authentication Scheme for Access Control in Mobile Pay-TV Systems

Recently, Sun and Leu proposed an efficient authentication scheme for access control in mobile pay-TV systems based on pairing and elliptic curve cryptography. In Sun and Leu's scheme, there exist two fatal security flaws, including 1) failure in subscriber authentication and 2) vulnerable to unauthorized access, which contradict to the security claims of Sun and Leu's scheme. To meet the claimed security requirements, we strengthen Sun and Leu's scheme with only lightweight modifications.

A New Type of ID-based Encryption System and Its Application to Pay-TV Systems

We proposed a new type of ID-based encryption scheme. Our scheme is different from other schemes on that we use tamper resistant smart card to store the private key and do the decryption job for the users. The user knows nothing about the private key. Our scheme is identical to ElGamal encryption scheme, but using RSA framework to avoid ID replacement attack. In a way, our scheme can be regarded as new application for the widely deployed RSA system. By employing the skill introduced by Dodis and Fazio for converting symmetric broadcast encryption schemes into public key ones, we extend our scheme into an efficient broadcast encryption scheme and apply it to the pay-TV system. We show that our broadcast encryption scheme is efficient in some aspects. For example, public keys of our system are of the size O(1) and decryption cost for each receiver is one modular exponentiation.

Erratum to: Dynamic subtree tracing and its application in pay-TV systems

Due to a production error, the following sentences were rendered incorrectly: the second sentence in Lemma 2 should have read: “Further, let m = ∑ logk m i=0 bi ki be the representation of m in base k, and let wk(m) denote the integer sum of all base k digits of m”; the last sentence of the second paragraph of Sect. 4.2.4 should have read: “If the available bandwidth in the transport stream is less than 2 Mbps, then the video streams in the transport stream can be compressed more, causing a slight reduction in their quality.” Further, the first sentence after Corollary 1 should have read: “Measured in the total number ofmessages (with amessage being either: a content segment variant, a tracing message or a revocation message), the computational cost for the head-end system and the bandwidth usage of Step 2 are O(p2 log n).”

Dynamic subtree tracing and its application in pay-TV systems

A traitor tracing algorithm can be used to identify content decoders that are used in the process of committing piracy. Fiat and Tassa introduced the concept of dynamic traitor tracing. A dynamic traitor tracing algorithm is based on examining content, either rendered illegally by a decoder or re-broadcast by the pirate. In addition, real-time feedback from the pirate’s network is used to adapt the state of the algorithm. This results in very efficient schemes, measured by the number of iterations required to identify all traitors. Drawbacks of currently available dynamic traitor tracing algorithms are that the best-known, real-time computational cost and bandwidth usage depend linearly on the number of content decoders. In particular, for large populations of content decoders, for example, common in pay-TV applications, these schemes can become impractical as system requirements can no longer be fulfilled. This paper presents a new dynamic traitor tracing algorithm, referred to as dynamic subtree tracing. The new algorithm inherits the advantages of the previously presented dynamic schemes and eliminates their disadvantages. In particular, it is shown that the number of iterations, the computational cost, the bandwidth usage, and the amount of storage in a decoder are all logarithmic in the number of content decoders. This makes the new scheme attractive for application in pay-TV systems, as illustrated with numerical examples.

An efficient anonymous authentication protocol for mobile pay-TV

In terms of convenience requirements, mobility has been one of the most important services for pay-TV systems. In 2009, Yang and Chang proposed an authentication protocol for mobile devices using elliptic curves cryptography (ECC) and claimed that their mechanism is secure and efficient using in mobile pay-TV systems. In this paper, we demonstrate that their protocol still is insecure for authentication without password protection and performs inefficiently. Therefore, we offer an anonymous authentication protocol (AAP) to solve the performance issue and insecure risks. In addition, we present an analysis of our protocol to show that our protocol suits better for applications with higher security requirements and low power-consuming devices.

A Cooperative Cellular and Broadcast Conditional Access System for Pay-TV Systems

The lack of interoperability between Pay-TV service providers and a horizontally integrated business transaction model have compromised the competition in the Pay-TV market. In addition, the lack of interactivity with customers has resulted in high churn rate and improper security measures have contributed into considerable business loss. These issues are the main cause of high operational costs and subscription fees in the Pay-TV systems. As a result, this paper presents the Mobile Conditional Access System (MICAS) as an end-to-end access control solution for Pay-TV systems. It incorporates the mobile and broadcasting systems and provides a platform whereby service providers can effectively interact with their customers, personalize their services and adopt appropriate security measurements. This would result in the decrease of operating expenses and increase of customers' satisfaction in the system. The paper provides an overview of state-of-the-art conditional access solutions followed by detailed description of design, reference model implementation and analysis of possible MICAS security architectures.

An Efficient Authentication Scheme for Access Control in Mobile Pay-TV Systems

In a mobile pay-TV system, a large number of messages are exchanged for mutual authentication purposes. In traditional authentication schemes, with one-to-one delivery, one authentication message per request is delivered from a head end system to subscribers. This results in the delivery of a large quantity of messages and therefore is inefficient and costly. Moreover, since most traditional schemes use an RSA-based signature for identity validation and nonrepudiation of communication, they suffer from high communication costs. Due to its wireless nature, mobile pay-TV is vulnerable to attacks during hand-off. As traditional schemes do not support hand-off authentication, they are insecure during hand-off. With these shortcomings, they are not suitable for mobile pay-TV. In this paper, we propose an innovative authentication scheme, in which, by providing one-to-many facility, only one authentication message for multiple requests is broadcasted from the head end system to subscribers. By employing bilinear property of pairing and elliptic curve cryptography, our scheme provides one-to-many facility in the case of multiple requests for the same service in a short period of time. This new scheme achieves better broadcast efficiency and performance on communication costs than traditional ones. Additionally, this scheme provides a hand-off authentication mechanism to protect the access of services while preventing attacks during hand-off; therefore, the scheme is more secure to support access control. Moreover, to provide anonymous authentication for protecting identity privacy, the scheme adopts an identity-based scheme while traditional schemes do not apply. The scheme inherits advantages of the identity-based scheme that a public key does not need to be certificated, the certification authority mechanism will not be needed and the key exchange overhead can be reduced. With these advantages of our scheme, it is well suited for mobile pay-TV system.

IP기반의 Pay-TV 시스템을 위한 안전하고 효율적인 그룹 키 분배 프로토콜

ABSTRACT Recently,IP-based broadcasting systems,such as Mobile-TV and IP-TV, have been widely deployed. These systems require a security system to allow only authorized subscribers access to broadcasting services. We analyzed the Conditional Access System, which is a security system used in the IP-based Pay-TV systems. A weakness of the system is that it does not scale well when the system experiences frequent membership changes. In this paper, we propose a group key distribution protocol which overcomes the scalability problem by reducing communication and computation overheads without loss of security strength. Our experimental results show that computation delay of the proposed protocol is smaller than one of the Conditional Access System. This is attributed to the fact that the proposed protocol replaces expensive encryption and decryption with relatively inexpensive arithmetic operations. In addition, the proposed protocol can help to set up a secure channel between a server and a client with the minimum additional overhead.Keywords:Key Distribution, Group Key, Pay-TV, Conditional Access System

Sharing conditional access modules through the home network for pay TV access

In this article we present a system for sharing a Conditional Access Module among different visualization devices. Current pay-TV systems require having conditional access modules and smart cards replicated at every visualization device willing to access Pay-TV contents. In this article, we show how a home gateway, capable of securely distribute decryption keys to home visualization device, an inexpensive descrambler, and a home network, enable a flexible solution to access Pay-TV.

Pay per view – a multimedia multicast application with effective key management

Multicast communications seem particularly well adapted for large scale commercial distribution applications, such as pay per view systems. Successful implementation of pay per view systems is underway in many countries, and as a result, many cable companies have to replace their distribution networks. Conditional access system is the essential basis of a pay-TV system to charge the subscriber for the subscribing fee, and key management is the essential part of a CAS. The problem of controlling access to multimedia multicasts requires the distribution and maintenance of keying information. The conventional, approach employs the use of a media-independent channel. We propose an approach that involves the use of a media-dependent channel, and is achieved by using data embedding techniques. In this paper, four levels of key hierarchy for CAS are discussed which reduce the complexity of key distributions. Finally, we introduce multilayer applications where group members may subscribe or cancel membership to some layers while maintaining membership to other layers.

Flexible-Pay-Per-Channel: A New Model for Content Access Control in Pay-TV Broadcasting Systems

In the past, two models, pay-per-channel (PPC) and pay-per-view (PPV), for content access control in pay-TV broadcasting systems have been proposed. In PPC, a subscriber subscribes to one or multiple group(s) of channels, prearranged by the service provider, for a fixed period of time. Thus, it is unfair for the subscriber because the subscriber can not subscribe an arbitrary combination of channels according to his/her preference, and can not dynamically unsubscribe/change his/her subscription during the subscription period. On the other hand, PPV specifies that a subscriber can pay for each single program; thus, it provides a fair service. However, compared with PPC, PPV makes the subscriber inconvenient because of the high subscription frequency and low flexibility of channel selection. Moreover, all the existing PPV systems are only suitable for a small-scale Pay-TV system with limited users because of the overhead for transmission and storage. In this paper, we define a new model, flexible-PPC (F-PPC), which combines the properties and the advantages of PPC and PPV. F-PPC is an improved model of PPC for its fairness. It allows a subscriber to subscribe freely to his/her favorite channels, and unsubscribe/change his/her subscription at any time. Furthermore, it eliminates the inconvenience in PPV, and is suitable for a large-scale Pay-TV system with numerous subscribers and channels. In order to fulfill the requirements of F-PPC, we propose a new conditional access system (CAS) with a four-level key hierarchy. The proposed CAS utilizes a structure of a binary tree to update keys rapidly; therefore, the service provider can promptly update necessary keys when the subscribers change their subscriptions or leave the system. Compared with the existing CAS systems, our CAS is more efficient and flexible to support F-PPC.

An Efficient Time-Bound Hierarchical Key Management Scheme for Secure Broadcasting

In electronic subscription and pay TV systems, data can be organized and encrypted using symmetric key algorithms according to predefined time periods and user privileges and then broadcast to users. This requires an efficient way of managing the encryption keys. In this scenario, time-bound key management schemes for a hierarchy were proposed by Tzeng and Chien in 2002 and 2005, respectively. Both schemes are insecure against collusion attacks. In this paper, we propose a new key assignment scheme for access control, which is both efficient and secure. Elliptic-curve cryptography is deployed in this scheme. We also provide the analysis of the scheme with respect to security and efficiency issues.

Efficient Key Distribution for Access Control in Pay-TV Systems

The conditional access system (CAS) is an essential part of digital pay-TV systems for controlling access to the program services. Conventionally, due to the restrictions of bandwidth and computational capability, a CAS only supports period subscription services that are charged on a monthly basis. In this paper, based on the concept of hierarchical key assignment, we propose three key distribution schemes for the access control of pay-TV systems. With these schemes, a CAS can support more charging strategies for service providers, such as adopting a smaller charging unit and allowing a subscription of any subset of channels with little communication and computational overhead. In addition, the piracy management problem can also be dealt with easily.

전방향 안전성을 보장하는 공개키 브로드캐스트 암호 기법

본 논문에서는 전방향 안전성(forward-secrecy)을 보장하는 공개키 브로드캐스트 암호 기법을 제안한다. 공개키 브로드캐스트 암호는 공개키를 이용하여 구성원 누구나 메시지를 전송할 수 있고, 탈퇴자 그룹을 효율적으로 배제(revocation)할 수 있는 기법이다. 여기에 전방향 안전성을 보장하려는데, 전방향 안전성은 사용자의 비밀키가 노출되더라도 그 노출된 시점 이전의 암호문을 쉽게 복호화 할 수 없도록 하는 것이다. 이러한 기능이 없다면 권한 없는 수신자가 과거의 방송을 수집하고 이후 정당한 비밀키를 받아서 과거의 방송을 복호화할 수 있는 문제가 발생한다. 전방향 안전성은 특히 유료 방송 등의 환경에서 요구된다. 본 논문에서 제안되는 기법은 2005년 Boneh-Boyen-Goh가 제시한 계층구조의 신원 기반 암호기법을 변형하여 설계된다. 먼저 BBG기법을 사용하여 새로운 공개키 브로드캐스트 암호기법을 설계하고, 다시 BBG 기법에서 사용된 하위레벨 비밀키 생성 알고리즘을 사용하여 전방향 안전성을 부여한다. 제안되는 기법은 타원곡선 위의 페어링(pairing)을 이용하여 설계되며, 전체 사용자 n에 대하여 <TEX>$O(\sqrt{n})$</TEX> 사이즈의 통신량과 비밀키 저장량을 가진다. 특히 비밀키 저장량은 탈퇴자 수가 증가할수록 줄어드는 장점을 가진다. 통신량이 중요한 환경에서는 이전에 제시된 기법보다 본 논문에서 제안된 기법을 사용하는 것이 더 바람직한데, 이는 통신량은 동일하지만 비밀키 저장량이 더 적기 때문이다. 제안된 기법은 Bilinear Diffie-Hellman Exponent 가정 하에서 선택 암호문 공격에 안전하도록 설계되며, 그 증명은 랜덤 오라클을 사용하지 않는다. Public Key Broadcast Encryption (PKBE) allows a sender to distribute a message to a changing set of users over an insecure channel. PKBE schemes should be able to dynamically exclude (i.e., revoke) a certain subset of users from decrypting a ciphertext, so that only remaining users can decrypt the ciphertext. Another important requirement is for the scheme to be forward-secrecy. A forward-secure PKBE (fs-PKBE) enables each user to update his private key periodically. This updated private key prevents an adversary from obtain the private key for certain past period, which property is particularly needed for pay-TV systems. In this paper, we present a fs-PKBE scheme where both ciphertexts and private keys are of <TEX>$O(\sqrt{n})$</TEX> size. Our PKBE construction is based on Boneh-Boyen-Goh's hierarchical identity-based encryption scheme. To provide the forward-secrecy with our PKBE scheme, we again use the delegation mechanism for lower level identities, introduced in the BBG scheme. We prove chosen ciphertext security of the proposed scheme under the Bilinear Diffie-Hellman Exponent assumption without random oracles.