Cryptographie à base de politiques : théorie et applications.

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Bagga, Wohammed Walid (2006) Cryptographie à base de politiques : théorie et applications. Doctorat Informatique et Réseaux, Institut Eurécom, ENST p.194.

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Résumé

‘Identity-based cryptography’ is definitely one of the most popular topics addressed by the

cryptographic research community in the last five years. As can be guessed from the name,

the notion of ‘identity’ is central to identity-based cryptographic primitives. As for identitybased

encryption, access to an encrypted message is only permitted to the entity whose identity

is the one according to which the message was encrypted, whereas for identity-based signature,

the validity of the signature on a message is defined with respect to the identity of the

entity that generated the signature. In general, identity is not sufficient for authorization and

trust establishment, especially in the context of large-scale open environments like the Internet,

where interactions often occur between parties with no pre-existing familiarity of one another.

An increasingly popular approach to determining the trustworthiness of the interacting entities

consists in using policies fulfilled by digital credentials.

In this thesis, we present a new concept in cryptography, called ‘policy-based cryptography’,

which allows to perform cryptographic operations with respect to policies fulfilled by digital

credentials. Intuitively, a policy-based encryption scheme allows to encrypt a message with respect

to a policy so that only an entity that is compliant with the policy can decrypt the message.

Similarly, a policy-based signature scheme allows to generate a signature on a message with respect

to a policy so that the signature is valid if and only if it was generated by an entity that is

compliant with the policy. We present three policy-based cryptographic primitives from bilinear

pairings over elliptic curves and prove their security under well-defined security models. We

further illustrate the usefulness of our concept of policy-based cryptography through the description

of application scenarios in the contexts of access control, privacy policy enforcement,

establishment of ad-hoc communities, automated trust negotiation and proxy certification.

Table des Matières

Résumé en Français i

Table of Contents xvii

Abstract xix

Introduction 1

Preliminaries 7

0.1 Standard Cryptography - 10

0.1.1 Symmetric-Key Encryption - 10

0.1.2 Public-Key Encryption - 10

0.1.3 Digital Signature - 11

0.1.4 Hash Function - 12

0.2 Provable Security - 13

0.2.1 Reductionist Security Proofs - 13

0.2.2 The Random Oracle Model - 15

0.2.3 Security Notions for Public-Key Encryption Schemes - 15

0.2.4 The Fujisaki-Okamoto Transformations - 18

0.2.5 Security Notions for Digital Signature Schemes - 21

0.2.6 The Oracle Replay Technique - 22

0.3 Bilinear Pairings - 23

0.3.1 Abstract Algebra - 24

0.3.2 Elliptic Curves - 25

0.3.3 Bilinear Pairings over Elliptic Curves - 27

0.3.4 Bilinear Diffie-Hellman Problems - 28

0.3.5 Pairing-Based Cryptographic Schemes - 30

0.4 Conclusion - 33

1 Policy-Based Encryption 35

1.1 Introduction - 35

1.2 Related Work - 38

1.3 Formal Definitions - 44

1.3.1 Policy Model - 44

1.3.2 Policy-Based Encryption - 47

1.3.3 Security Model - 48

1.4 A Pairing-Based Implementation - 49

1.4.1 Description - 49

1.4.2 Consistency and Efficiency - 51

1.4.3 Security - 52

1.5 Controlling Access to Released XML Documents - 58

1.5.1 The XML Data Model - 62

1.5.2 Policy Model - 65

1.5.3 Protection Model - 70

1.5.4 Protection Enforcement: Formal Description - 75

1.5.5 Protection Enforcement: XML Representation - 78

1.5.6 Summary - 82

1.6 The Sticky Privacy Policy Paradigm - 83

1.6.1 An Overview of EPAL - 84

1.6.2 Privacy Policy Refinement - 86

1.6.3 Sticky Policy through Policy-Based Cryptography - 87

1.7 Establishment of Ad-Hoc Communities - 90

1.7.1 Policy-Based Establishment of Ad-Hoc Communities - 90

1.7.2 Community Establishment using Policy-Based Encryption - 93

1.8 Conclusion - 95

2 Collusion-Free Policy-Based Encryption 97

2.1 Introduction - 97

2.2 Related Work - 100

2.3 Formal Definitions - 101

2.3.1 Policy Model - 101

2.3.2 Policy-Based Public-Key Encryption - 103

2.3.3 Security Model - 104

2.4 A Pairing-Based Implementation - 106

2.4.1 Description - 106

2.4.2 Consistency and Efficiency - 108

2.4.3 Security - 109

2.5 Automated Trust Negotiation - 117

2.5.1 Basic Negotiation Protocol - 118

2.5.2 Cryptography-Based Negotiation Protocol - 120

2.5.3 Concealing Sensitive Policies - 123

2.6 Conclusion - 124

3 Policy-Based Signature 125

3.1 Introduction - 125

3.2 Related Work - 128

3.3 Formal Definitions - 130

3.3.1 Policy Model - 130

3.3.2 Policy-Based Signature - 130

3.3.3 Security Model - 131

3.4 A Pairing-Based Implementation - 134

3.4.1 Description - 134

3.4.2 Consistency and Efficiency - 136

3.4.3 Security - 137

3.5 Proof-Carrying Proxy Certificates - 144

3.5.1 General Setting - 146

3.5.2 An Application Scenario - 148

3.5.3 Related Approaches - 149

3.6 Conclusion - 150

Conclusion 151

Bibliography 157

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