Cryptographic protocols are used to encrypt data during their transmission over a network or to store it on a data carrier. This thesis is about the method of identity-based encryption. In this form of encryption, the name or identity of the target subject is used to encrypt the data. This property makes it a perfect tool for modern electronic communication, because all involved identities and endpoint addresses (e.g. IP addresses) have to be unique worldwide and must be known in order to establish a communication. The identity-based key agreement protocol that has been invented in this thesis has several advantages compared to existing schemes. One important property is its complete independence of key generators. This independence allows each participating security domain to set up and maintain its own key generator. They are not forced to agree on a common setup or a common secret anymore. Due to the properties of the protocol, the security domains are still compatible to each other. Users from one security domain can communicate with users from another security domain using encryption. This new property of independence is also carried over to a signature protocol. It allows users from different security domains to sign a certain object. Additionally, the act of signing is independent and the signers do not need to communicate with each other. Apart from the protocol and its security proofs with respect to standard definitions from the literature, the thesis contains an analysis of existing schemes. Attacks on known protocols and assumptions are presented, and it is shown under which circumstances these become insecure. On the one hand, a completely new approach that is based on defined or rather undefined objects in discrete structures is used. On the other hand, the method of lattice based reduction is successfully applied to the new area of secret sharing schemes. Finally, application scenarios for the protocol are presented. These scenarios are chosen such that the advantages of the protocol become apparent. Cryptographic protocols are used to encrypt data during their transmission over a network or to store it on a data carrier. This thesis is about the method of identity-based encryption. In this form of encryption, the name or identity of the target subject is used to encrypt the data. This property makes it a perfect tool for modern electronic communication, because all involved identities and endpoint addresses (e.g. IP addresses) have to be unique worldwide and must be known in order to establish a communication. The identity-based key agreement protocol that has been invented in this thesis has several advantages compared to existing schemes. One important property is its complete independence of key generators. This independence allows each participating security domain to set up and maintain its own key generator. They are not forced to agree on a common setup or a common secret anymore. Due to the properties of the protocol, the security domains are still compatible to each other. Users from one security domain can communicate with users from another security domain using encryption. This new property of independence is also carried over to a signature protocol. It allows users from different security domains to sign a certain object. Additionally, the act of signing is independent and the signers do not need to communicate with each other. Apart from the protocol and its security proofs with respect to standard definitions from the literature, the thesis contains an analysis of existing schemes. Attacks on known protocols and assumptions are presented, and it is shown under which circumstances these become insecure. On the one hand, a completely new approach that is based on defined or rather undefined objects in discrete structures is used. On the other hand, the method of lattice based reduction is successfully applied to the new area of secret sharing schemes. Finally, application scenarios for the protocol are presented. These scenarios are chosen such that the advantages of the protocol become apparent. The first application is telephony, GSM as well as Voice over IP (VoIP). In this case, the telephone number of the callee is used as the encryption key. Implementations on a modern mobile phone as well as within existing Voice over IP software are presented. The second application is IP networks. Here, the IP address of a communication unit is used as the encryption key. However, in this case, there are more problems than in the GSM/VoIP case, e.g., dynamic IP addresses or network address translation (NAT) where an IP address is substituted by another one. These are only two problems out of several for which solutions are presented.
Cryptographic protocols are used to encrypt data during their transmission over a network or to store it on a data carrier. This thesis is about the method of identity-based encryption. In this form of encryption, the name or identity of the target subject is used to encrypt the data. This property makes it a perfect tool for modern electronic communication, because all involved identities and endpoint addresses (e.g. IP addresses) have to be unique worldwide and must be known in order to establish a communication. The identity-based key agreement protocol that has been invented in this thesis has several advantages compared to existing schemes. One important property is its complete independence of key generators. This independence allows each participating security domain to set up and maintain its own key generator. They are not forced to agree on a common setup or a common secret anymore. Due to the properties of the protocol, the security domains are still compatible to each other. Users from one security domain can communicate with users from another security domain using encryption. This new property of independence is also carried over to a signature protocol. It allows users from different security domains to sign a certain object. Additionally, the act of signing is independent and the signers do not need to communicate with each other. Apart from the protocol and its security proofs with respect to standard definitions from the literature, the thesis contains an analysis of existing schemes. Attacks on known protocols and assumptions are presented, and it is shown under which circumstances these become insecure. On the one hand, a completely new approach that is based on defined or rather undefined objects in discrete structures is used. On the other hand, the method of lattice based reduction is successfully applied to the new area of secret sharing schemes. Finally, application scenarios for the protocol are presented. These scenarios are chosen such that the advantages of the protocol become apparent. Cryptographic protocols are used to encrypt data during their transmission over a network or to store it on a data carrier. This thesis is about the method of identity-based encryption. In this form of encryption, the name or identity of the target subject is used to encrypt the data. This property makes it a perfect tool for modern electronic communication, because all involved identities and endpoint addresses (e.g. IP addresses) have to be unique worldwide and must be known in order to establish a communication. The identity-based key agreement protocol that has been invented in this thesis has several advantages compared to existing schemes. One important property is its complete independence of key generators. This independence allows each participating security domain to set up and maintain its own key generator. They are not forced to agree on a common setup or a common secret anymore. Due to the properties of the protocol, the security domains are still compatible to each other. Users from one security domain can communicate with users from another security domain using encryption. This new property of independence is also carried over to a signature protocol. It allows users from different security domains to sign a certain object. Additionally, the act of signing is independent and the signers do not need to communicate with each other. Apart from the protocol and its security proofs with respect to standard definitions from the literature, the thesis contains an analysis of existing schemes. Attacks on known protocols and assumptions are presented, and it is shown under which circumstances these become insecure. On the one hand, a completely new approach that is based on defined or rather undefined objects in discrete structures is used. On the other hand, the method of lattice based reduction is successfully applied to the new area of secret sharing schemes. Finally, application scenarios for the protocol are presented. These scenarios are chosen such that the advantages of the protocol become apparent. The first application is telephony, GSM as well as Voice over IP (VoIP). In this case, the telephone number of the callee is used as the encryption key. Implementations on a modern mobile phone as well as within existing Voice over IP software are presented. The second application is IP networks. Here, the IP address of a communication unit is used as the encryption key. However, in this case, there are more problems than in the GSM/VoIP case, e.g., dynamic IP addresses or network address translation (NAT) where an IP address is substituted by another one. These are only two problems out of several for which solutions are presented.
Key agreement and digital signature are two significant and most useful contributions of modern cryptography. Such protocols and schemes allow two or more parties to establish a common session key securely in the presence of a malicious adversary and provide means of ensuring data origin authentication, data integrity and non-repudiation. Thus, secure key agreement protocols and signature schemes are fundamental building blocks for constructing complex higher-level protocols.
In this thesis, we deal with security analysis of existing key agreement protocols and digital signature schemes. We examine ten authenticated key agreement protocols without key confirmation, out of which half are two-party and the other half are three-party, and one deterministic identity-based aggregate signature scheme. By giving concrete attacks, we show that these protocols and the scheme do not possess the desirable security attributes or security attributes claimed by their authors. We also present a secure deterministic identity-based signature scheme that supports partial aggregation.
First, we show that Okamoto's and Chen et al.’s protocol cannot withstand the key-compromise impersonation attack and that Tan et al.’s, Lim et al.’s and two protocols of Holbl et al. are insecure against the impersonation attack and the man-in-the-middle attack. Next, we prove that one protocol of Holbl et al. is vulnerable to the insider attack, one to the unknown key-share attack, one protocol allows an adversary to compute the private key of any user and one protocol allows her to compute the shared session key. We also show that Selvi et. al.'s deterministic identity-based aggregate signature scheme is universally forgeable, i.e., anyone is able to easily generate valid signatures on any messages of his choice having on average twelve genuine digital signatures.
In the end, we introduce a new deterministic identity-based signature scheme and formally prove its security in the random oracle model. The scheme is a major improvement of the signature scheme proposed by Selvi et. al. and offers the same tight security reduction to the underlying gap Diffie-Hellman problem. Because the newly proposed scheme is deterministic it allows partial aggregation, i.e., all the signatures from the same signer can be aggregated into one single short signature.
With the advancement in Internet-based technologies, network applications are no longer just limited to one-to-one communication; instead, it has become a publicly accessible resource. Regardless of this growth, such applications bring specific challenges concerning authentication, integrity, and privacy. Cryptography stands as an ideal solution to overcome these issues through an interactive method of secure communication called Key Exchange. Key exchange protocols allow two or more entities to agree upon a shared secret transmitted over a public channel without revealing any information to an eavesdropper.
This thesis contributes to the design and analysis of the variants of key exchange protocols along with their applications. As per the security requirements, the proposed schemes are either designed in an interactive model or a non-interactive model. Each scheme undergoes a series of formal and informal security analysis. The formal analysis provides simulation result using cryptographic models. Whereas, an informal analysis showcases the theoretical proves on known attacks where an adversary controls the network. The performance analysis maps the protocol’s efficiency based on the computation cost and communication size.
The first contribution introduces an interactive two-party authenticated key exchange protocol. We begin by investigating the security flaws in an existing system such as, lack of user anonymity, trace attack, clogging attack, and failure to resist replay attack. Then proposed an authenticated self-certified key exchange protocol that overcomes the addressed issues. Also, it eliminates the requirements of a trusted certificate authority to authorize the entities. The scheme achieves anonymity during session execution, mutual authentication and enables session key agreement.Based on the ideology, an application is designed, forsecure communication between ad-hoc nodes in a mobile ad-hoc network.
The second contribution contributes to the design of an Identity-based authenticated three-party key exchange protocol that overcomes the incapability of managing huge
peer-to-peer communication and high computation cost faced by two-party authenticated key exchange protocols. The proposed scheme preserves user privacy by enabling the property of anonymity and ensures non-repudiation. Based on the ideology, an application is designed to establish a secure session key between two users with the help of a trusted server for an electronic payment system.
The third contribution proposes an interactive pairing-free certificateless authenticated group key agreement scheme suitable for a resource constrained mobile network. The scheme allows any user to join or leave the group dynamically. The key agreement protocol devised in this scheme ensures un-deniability property by incorporating the signcryption, and unsigncryption algorithm.
The final contribution presents a non-interactive Identity-based deniable authentication protocol that allows an intended receiver to identify the origin of a given message without being able to prove sender’s identity to a third party.
The contributions of this thesis fall into three areas of certificateless cryptography. The first area is encryption, where we propose new constructions for both identity-based and certificateless cryptography. We construct an n-out-of- n group encryption scheme for identity-based cryptography that does not require any special means to generate the keys of the trusted authorities that are participating. We also introduce a new security definition for chosen ciphertext secure multi-key encryption. We prove that our construction is secure as long as at least one authority is uncompromised, and show that the existing constructions for chosen ciphertext security from identity-based encryption also hold in the group encryption case. We then consider certificateless encryption as the special case of 2-out-of-2 group encryption and give constructions for highly efficient certificateless schemes in the standard model. Among these is the first construction of a lattice-based certificateless encryption scheme. Our next contribution is a highly efficient certificateless key encapsulation mechanism (KEM), that we prove secure in the standard model. We introduce a new way of proving the security of certificateless schemes based that are based on identity-based schemes. We leave the identity-based part of the proof intact, and just extend it to cover the part that is introduced by the certificateless scheme. We show that our construction is more efficient than any instanciation of generic constructions for certificateless key encapsulation in the standard model. The third area where the thesis contributes to the advancement of certificateless cryptography is key agreement. Swanson showed that many certificateless key agreement schemes are insecure if considered in a reasonable security model. We propose the first provably secure certificateless key agreement schemes in the strongest model for certificateless key agreement. We extend Swanson's definition for certificateless key agreement and give more power to the adversary. Our new schemes are secure as long as each party has at least one uncompromised secret. Our first construction is in the random oracle model and gives the adversary slightly more capabilities than our second construction in the standard model. Interestingly, our standard model construction is as efficient as the random oracle model construction.
Over the last few years, researchers have introduced many group key agreement protocols, aiming to construct new efficient schemes in terms of both computation and communication. Group key agreement protocols aim to derive a common session key among a group of users, which will be used later in order to achieve secure communication between them. In this thesis, we provide a complete analysis of the protocols that have already been introduced in the literature from 2010 until now and require only one round. The main goal of this analysis is to evaluate these protocols in both communication and computation cost, in order to provide a comprehensive comparative analysis. The second part of the thesis is to analyze and evaluate the multi key encapsulation mechanisms that exist in the literature. A multi key encapsulation mechanism is the encryption technique by which a symmetric cryptographic algorithm is combined with an asymmetric one. To be more specific, such a mechanism uses an asymmetric cryptographic algorithm, as a method to encrypt the session key, while the key is derived by a symmetric cryptographic algorithm. Finally, we propose two new group key agreement protocols which use the multi key encapsulation mechanisms, and compare them with existing group key agreement protocols. These constructions are based on a generic methodology for creating a group key exchange protocol by using multi key encapsulation mechanisms. For this reason, emphasis has been given to both one round group key agreement protocols and multi key encapsulation mechanisms, in order to examine the possibility of combining them
In order to protect some important information communicated through an insecure network, a common hidden key must be used.One can produce the common hidden key using key agreement protocols; and this helps to have high security in modern data networks.Today, the designers of public key cryptography protocols try to set the public identity of a system's users (like their email addresses) as their public key.This not only makes a cryptographic protocol more efficient but also decreases its cost.These protocols are called "identitybased ".In this article, an identitybased multiple key agreement scheme will be presented; this scheme uses the challengeresponse method to do the verification.While the number of random values produced in our scheme is the same as other schemes, the number of keys generated in this scheme is much more than what many other key agreement schemes produce,.Therefore, we will have less computational complexities campered with other schems.In this paper, we consider the security of our scheme and consequently,we will show that it satisfies many security conditions such as strong security.
Cryptographic protocols, though used in a wide range of everyday tasks, are typically built for certain unrealistic ideal conditions in which the user is assumed to work. For example, one such condition that is implicitly assumed is that the user has perfect randomness that she can use as her cryptographic secret key. Another commonly made assumption is that the user runs her cryptographic tasks (such as encryption) on a device that is perfectly secure; that is, its internals are not tampered with or even observed by an adversary.
In this thesis, we focus on designing cryptographic techniques that are secure under less-than-ideal conditions. In order to remove the assumption that users share perfectly random secret keys, we study the problem of privacy amplification: key agreement between two parties who both know a weak secret w, such as a password. The goal of the protocol is to convert this non-uniform secret w into a uniformly distributed string R that is fully secret from the adversary. R may then be used as a key for running symmetric cryptographic protocols (such as encryption or authentication). Because we make no computational assumptions, the entropy in R can come only from w. Thus such a protocol must minimize the entropy loss during its execution, so that R is as long as possible. In this thesis, we present the first protocol for information-theoretic key agreement that has entropy loss linear in the security parameter. The result is optimal up to constant factors. We also extend our results to the case where the users share correlated secrets w and w' . This setting occurs, for example, when the secrets are biometric readings taken at different points of time and is more generally known as information-reconciliation.