PROTECTING ADAPTIVE MULTIMEDIA DELIVERY AND

ADAPTATION USING PROXY BASED APPROACH

Ahmed Reda Kaced

Graduate School of Telecommunications, Computer Sciences and Networks Department

37-39 Rue Dareau 75014, Paris - France

Jean-Claude Moissinac

Graduate School of Telecommunications, Computer Sciences and Networks Department

37-39 Rue Dareau 75014, Paris - France

Keywords:

Adaptive multimedia, encryption, digital signatures, content adaptation, multimedia security.

Abstract:

By breaking the end-to-end nature of the communication, proxies render the task of providing end-to-end

security much harder or even impossible in some cases. In this paper, we will address the questions of when

and how end-to-end security, like conﬁdentiality and authenticity can be preserved, in a multimedia content

delivery platform, when having one or more adaptation proxies in the data path.

We describe SEMAFOR, a platform for protecting adaptive multimedia content delivery in heterogeneous

environments. SEMAFOR aims to deliver an end-to-end authenticity of original content exchanged in a het-

erogeneous network while allowing content adaptation by intermediary proxies between the content trans-

mitter and the ﬁnal users. Adaptation and authentication management are done by the intermediary proxies,

transparently to connected hosts, which totally make abstraction of these processes.

SEMAFOR provides AMCA a new content authentication based on multi-hop signature scheme using a

Merkle Hash Tree, and XSST a secured transaction protocol that gives securely exchanges of transactions

in SEMAFOR.

1 INTRODUCTION

Today’s multimedia networks and multimedia com-

munication platforms are becoming increasingly het-

erogeneous, mostly due to the growth of mobile com-

puting. Users are accessing multimedia data with an

increasing number of different types of devices like

mobile phones, PDAs, Laptops and Desktop comput-

ers, with very different capabilities. this makes difﬁ-

cult to send the same content to all the end users.

Adaptation proxies can help to overcome the prob-

lem of heterogeneity. Generally, a proxy is an ac-

tive intermediary placed between two communication

endpoints, typically a client and a server. Such an

intermediary can be used to adapt a communication

session to the type of clients and networks involved

either by doing content adaptation or network proto-

col enhancement. Figure 1 sketches the basic archi-

tecture for a proxy-based adaptation approach.

The main problem of using proxies is that the end-

to-end nature of the communication is broken. This

leads to some severe security problems. One of the

main questions that arise is how content adaptation by

intermediaries can be done when end-to-end security

is required. For example, how can a proxy transcode

a data stream if it is encrypted? How can end-to-end

data integrity and authentication be provided if the

proxy needs to alter the data in transit?

In order to address the end-to-end security of adap-

tive multimedia content delivery challenges, we pro-

posed in (Kaced and Moissinac, 2006a) AMCA,

a new content authentication scheme which allows

proxy to adapt dynamically a multimedia document

by deleting or adding parts, preserving the client ca-

pacity to verify the original signature. So, in this pa-

per we give an overview of SEMAFOR, our multi-

media content delivery platform which uses a proxy-

based approach to perform adaptation operations for

the exchanged content and implements AMCA to sign

this later.

This paper is outlined as follows. In section 2

we give an overview of SEMAFOR’s architecture,

we give also a description of the two main contribu-

tions of SEMAFOR, AMCA, our content authentica-

tion scheme, and XSST the transaction security pro-

tocol. After that, section 3 presents brieﬂy some of

the related works to our present work, and we ﬁnish

by a conclusion in section 4.

Reda Kaced A. and Moissinac J. (2006).

PROTECTING ADAPTIVE MULTIMEDIA DELIVERY AND ADAPTATION USING PROXY BASED APPROACH.

In Proceedings of the International Conference on Security and Cryptography, pages 87-90

DOI: 10.5220/0002104500870090

 SciTePress

Adapted

content

Transmitter

(server)

Receivers

(clients)

Multimedia

content

Adaptation Proxy

Figure 1: Basic architecture of a proxy-side adaptation.

2 BASIC FRAMEWORK

In the light of the above problems, we propose a

SEMAFOR framework with a content authentication

scheme called AMCA. Figure 2 shows the architec-

ture of SEMAFOR, and describes the organization of

the content delivery system.

The basic framework for our adaptive content de-

livery system consists of three main parts: client,

proxy and server, each part consists of different mod-

ules.

AMCA

Security

policy

Adaptation

engine

Encryption

XML-ENC

Encryption

XML-ENC

profils repository

Signature

XML-DSIG

Signature

XML-DSIG

Multimedia documents

meta-data

adaptation

meta-data

adaptation

Media

adaptation

Media

adaptation

Encryption

XML-ENC

Encryption

XML-ENC

Signature

XML-DSIG

Signature

XML-DSIG

Terminal user

authentication

Service

Key

management

Key

management

Key

management

Verification - Decryption

XML-DSIG XML-DEC

Verification - Decryption

XML-DSIG XML-DEC

Web plate

forme API

ClientProxy Server

Player

User profil

XSST server

XSST client

XSST proxy

Figure 2: SEMAFOR architecture.

The Server is the sender of the multimedia content.

It consists of two main modules, AMCA to sign the

ﬂow and XSST Server to send it. In a content deliv-

ery process, it’s main task is to sign the ﬂow using

AMCA, generating a data structure which is sent to

the client using the XSST-Server module.

The Client represents the end-user which receives

the adapted document. It have the necessary tools

needed to verify the authenticity of the received

content.

The Proxy is an active intermediary placed between

the server and the client. It is used to adapt the ex-

changed content to the type of clients and to secure

the communication session by doing content adap-

tation and encryption. Modules implemented at

the proxy-side are adaptation engine, that synchro-

nizes adaptation operations, and XSST-Proxy mod-

ule which secures communications with the other

sides.

When the client requests a document from the

server, the proxy makes request to the server on be-

half of the client. The server signs the document to

send using AMCA, encrypts it using XSST-Server,

and sends it to the client(saying that it allows some

proxies to adapt its content). The proxy, intercepts

the reply from the server, decides on and performs the

adaptation after having decrypted the document (or

synchronizes adaptation on several proxies). Then it

sends the transformed content on to the client. Finally,

the client decrypts the received data using XSST-

Client, veriﬁes the signature then plays the multime-

dia document.

In the following, we describe AMCA and XSST

modules that we use in SEMAFOR framework.

2.1 AMCA

Giving a secure delivery process of an adaptive mul-

timedia starts by securing the content itself. So,

to ensure an end-to-end authentication of the ex-

changed ﬂow, we proposed AMCA (Adaptive Mul-

timedia Content Authentication), a signature scheme

based on a binary tree representation of the multime-

dia ﬂow. Where each media object of the content

is represented by a leaf of a binary tree, while new

leaves (which we call Freeleaves) are added to ensure

the insertion of adapted media.

The binary tree resulted is associated to a Merkle

Hash Tree (MHT) (Merkle, 1990), for the purpose of

allowing adaptation operation in the signed content.

A Merkle Hash Tree is a complete binary tree

equipped with a candidate one-way function hash

and an assignment ψ , which maps the set of

nodes to the set of media objects: n → ψ{n}∈

,...,m

}. And for each two child nodes,

left

and n

right

, of any interior node, n

parent

, the

assignment ψ is required to satisfy

ψ (n

parent

)=hash (ψ (n

left

)  ψ (n

right

))

We describe in the following the signature and ver-

iﬁcation schemes used in AMCA, and we talk after

about the freeleaves that allow the dynamic insertion

of media elements in the binary tree.

2.1.1 Signature

Let T denote the server and let R denote the client

who veriﬁes the signature. The data pass through a

proxy P. We assume the existence of an open or

closed public-key infrastructure where T has key pair

). Here K

is T ’s private key for comput-

ing a traditional signature on a message, and K

the public veriﬁcation key. Sign(K

,M) denotes an

algorithm that outputs a signature ϕ on message

SECRYPT 2006 - INTERNATIONAL CONFERENCE ON SECURITY AND CRYPTOGRAPHY

under signing key K

and Verif(K

,M,ϕ) denotes

the veriﬁcation algorithm. P

need not know K

. T and R may also use a symmetric key (which

proxies need not know).

Let

denote the initial content that is composed

by a meta-data description (m

) and n − 1 media

objects(m

,...,m

). Where convenient, we as-

sume

is a power of 2. In our scheme, adapting a me-

dia object means remove the corresponding leaf in the

binary tree representation of the content and replace

it by a new leaf.The intermediary may also choose to

add a new media object or remove one.

Let

h()

denote a collision resistant hash function

that takes as input a leaf

and a (ﬁxed and publicly

known) v-bit initialization vector (IV), and produces

a v-bit output.

To process ﬂow authentication, the framework con-

sists of three main steps: document preparation and

signature by transmitter, document adaptation by

proxies and document veriﬁcation by end users. We

describe them as follows:

Preparation The meta-data document is ﬁrst

parsed to generate an adaptive format according to

the server policy. To build an MHT from the origi-

nal content, each media is associated to a leaf, and for

each adaptive media object we join a Freeleaf as sib-

ling. The Merkle tree associated with M is a balanced

binary tree in which each node v is assigned a value

(v)

To sign

, the transmitter computes the root value

Ω of the MHT associated with

. The signature is

ϕ=Sign(K

,Ω).

Adaptation On receiving the protected document,

a proxy can apply adaptation operations to ﬁt the doc-

ument into some different format. as we said above,

the adaptation operation for a media in our scheme is

performed by deleting this media in the MHT and in-

serting the adapted format as new leaf. Deletion and

insertion by the intermediaries are ensured like fol-

lows:

Deletion is supported by supplying some extra ver-

iﬁcation data so that the veriﬁer can still compute the

root of the MHT, as we now describe. First, let Mi de-

note the transformed data after the removal of blocks.

Insertion of an adapted media by the intermediary

is done using freeleaves. We use conventional public-

key signatures, S places P’s public key, or instruc-

tions on where to retrieve it, in the freeleaf. S then

creates a MHT digest and signs as described above.

P, in turn, attaches its content and signs it separately.

R checks the validity of both signatures.

Veriﬁcation The veriﬁcation procedure is actually

reversing the above process. Suppose a receiver re-

trieves a content as well as its authentication data from

some proxy. It then analyze it. With recovered au-

thentication data, the receiver can verify the stream

integrity and signatures with proper aggregate signa-

ture veriﬁcation scheme.

The veriﬁcation process includes unpacking, de-

coding and verifying. At least k out of

initial me-

dia objects should be received in order to recover

the authentication data. Suppose

media objects



,...,m



are received successfully.

1. For m



(1≤ j ≤ k), computes ψ(m



h(IV,



)

2. Let {



,



,...,



} the set of results. If any pair

correspond to siblings, replace the pair with their

hash (which corresponds to their MHT parent). Re-

peat step 2 until only one value remains. call it Ω



3. Finally run Verif(K

, Ω



,ϕ).

We can see that Ω = Ω



, from which it is easy to

see why the above algorithm works. If one has all

the initial media objects, then the above procedure is

the standard algorithm for computing the MHT root.

Now, observe that whenever R receives some hashes



,

...,

these come from P running the same

algorithm on the subset of missing frames. There-

fore, Pand R have together run the algorithm on all n

blocks which yields the Merkle root value.

2.2 XSST

In order to ensure a fully secured service for adaptive

multimedia content delivery in SEMAFOR, we have

to ensure secure transactions between all parts of the

platform. In other words, all communications have to

be authenticated and encrypted.

For this purpose, we proposed XSST (Xml Se-

cure SEMAFOR Transaction) providing the user with

guarantee on the conﬁdentiality, integrity, authenti-

cation and non-repudiation. XSST consists of three

major components: XSST Client, XSST Proxy and

XSST Server. It will ﬁrstly establish a secure tun-

nel for a real transaction before this transaction takes

place. This secure tunnel is established by using a

public-key based key exchange between the XSST

proxy and XSST server in order to derive a unique se-

cret key that can then be used to ensure data integrity

and conﬁdentiality throughout the session. XSST al-

lows both Proxy and Server sign the messages, this

signature can be generated directly by the proxy and

server, or by other applications which the XSST in-

teracts with. More informations about XSST can be

found in (Kaced and Moissinac, 2006b).

Figure 3 illustrates the operations between the mul-

timedia server S and the proxy P, and the operations

between the proxy P and the client C for requesting

and adapting the multimedia data.

PROTECTING ADAPTIVE MULTIMEDIA DELIVERY AND ADAPTATION USING PROXY BASED APPROACH

SPC

(

)

(

)

ck(

{

}

{

}

(

{

}

Authentication

and key

generation

Data

encryption

Data

re-encryption

Data

decryption

Retrieve d

request

Retrieve e

{

}

{

}

{

-1

}

{

}

Figure 3: Operations between the source server S and the

proxy P , and the client C.

3 RELATED WORKS

Various research works in proxy-based multime-

dia adaptation approach have been made in this

area. Solutions proposed by (Johanson, 2001),

Layaida (Layaida et al., 2004), use a proxy based

adaptation for continuous data ﬂow transcoding, other

projects use a multi-proxy adaptation, Appat platform

proposed by Lapeyre (Lapayre and Renard, 2005),

ubiQoS presented by Bellavista (Bellavista et al.,

2004). Unfortunately, these existing adaptation plat-

forms do not address security and authentication of

exchanged ﬂows.

However, there are a small set of papers emphasiz-

ing security issues in a proxy-based approach. Re-

cently, Gentry proposed in (Gentry et al., 2005)

two new provably secure schemes that ensure secure

streaming media authentication with adaptive proxies.

(Li et al., 2004) proposed a secure MPEG-4 stream

authentication scheme that allows a proxy ﬂexibly

manipulate streaming packets while they can still be

veriﬁed by the ﬁnal receiver. Suzuki in (Suzuki et al.,

2004) proposed a multimedia content delivery system

that protects the end-to-end authenticity of adaptive

multimedia. However, they had the same problems;

they only considered generic multimedia content sim-

ply as message blocks instead of real tree representa-

tion format like MPEG-21 or SMIL. Moreover most

of them did not address the multi-proxy adaptation

authentication problem.

4 CONCLUSION

A framework for securing an adaptive content deliv-

ery in heterogeneous network environments has been

presented. To achieve this goal, we used the conven-

tional extension of the client-server model to a client-

proxy-server model.

The different aspects of system technologies in-

clude content authentication modules for signing the

exchanged ﬂow and a transaction encryption protocol

for securing communication in the platform.

The content authentication scheme uses a multi-

time signature scheme which is based on MHT tech-

nique. The concept of FreeLeaves was introduced to

allow adaptation operations by the intermediary prox-

ies. Our experiences suggest that a MHT-based signa-

ture scheme is both a feasible and practical solution to

this problem.

While we have only touched upon some of the is-

sues necessary for deploying Merkle hash tree based

authentication schemes for adaptive multimedia con-

tent in this short paper, an extended article is available

(Kaced and Moissinac, 2006b) that provides more de-

tails, including: a deeper look at Merkle tree main-

tenance, algorithms for the veriﬁcation procedures,

and experiments and experiences with an actual im-

plementation.

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SECRYPT 2006 - INTERNATIONAL CONFERENCE ON SECURITY AND CRYPTOGRAPHY