Implementasi Skema Tanda Tangan Practical Forward Secure Sequential Aggregate

Skema forward secure sequential aggregate (FssAgg) signature memungkinkan forward security, efisieni penyimpanan/komunikasi, serta proses pengecekan integritas dari pesan-pesan yang ditandatangani. Skema ini cocok diterapkan pada sistem aplikasi yang membutuhkan data yang banyak tetapi tidak panjang yang tidak dijaga, seperti sistem login atau sistem monitoring jarak jauh. Skema ini menggunakan sebuah kunci publik tunggal untuk memverifikasi n sekuensial tanda tangan agregat. Skema practical forward secure sequential aggregate signature harus bebas modifikasi pesan maupun tanda tangan. Operasi-operasi dalam skema ini dipecah menjadi beberapa interval, dimana setiap interval menggunakan sebuah kunci privat berbeda (namun berhubungan) untuk proses tanda tangan. Tujuan penelitian ini adalah membangun sebuah perangkat lunak untuk mengimplementasikanpractical forward secure sequential aggregate dengan fokus untuk mencegah adanya kecurangan (modifikasi terhadap pesan atau tandatangan) dan mendeteksi jika ada signer yang tidak memberikan tanda tangan. dalam proses verifikasi. Hasil pengujian menunjukkan bahwa kemampuan sistem dalammendeteksi dan mencegah berbagai kemungkinan kecurangan atau jumlah tanda tangan yang tidak sesuai

Advanced Cryptographic Techniques for Protecting Log Data

This thesis examines cryptographic techniques providing security for computer log files. It focuses on ensuring authenticity and integrity, i.e. the properties of having been created by a specific entity and being unmodified. Confidentiality, the property of being unknown to unauthorized entities, will be considered, too, but with less emphasis. Computer log files are recordings of actions performed and events encountered in computer systems. While the complexity of computer systems is steadily growing, it is increasingly difficult to predict how a given system will behave under certain conditions, or to retrospectively reconstruct and explain which events and conditions led to a specific behavior. Computer log files help to mitigate the problem of retracing a system’s behavior retrospectively by providing a (usually chronological) view of events and actions encountered in a system. Authenticity and integrity of computer log files are widely recognized security requirements, see e.g. [Latham, ed., "Department of Defense Trusted Computer System Evaluation Criteria", 1985, p. 10], [Kent and Souppaya, "Guide to Computer Security Log Management", NIST Special Publication 800-92, 2006, Section 2.3.2], [Guttman and Roback, "An Introduction to Computer Security: The NIST Handbook", superseded NIST Special Publication 800-12, 1995, Section 18.3.1], [Nieles et al., "An Introduction to Information Security" , NIST Special Publication 800-12, 2017, Section 9.3], [Common Criteria Editorial Board, ed., "Common Criteria for Information Technology Security Evaluation", Part 2, Section 8.6]. Two commonly cited ways to ensure integrity of log files are to store log data on so-called write-once-read-many-times (WORM) drives and to immediately print log records on a continuous-feed printer. This guarantees that log data cannot be retroactively modified by an attacker without physical access to the storage medium. However, such special-purpose hardware may not always be a viable option for the application at hand, for example because it may be too costly. In such cases, the integrity and authenticity of log records must be ensured via other means, e.g. with cryptographic techniques. Although these techniques cannot prevent the modification of log data, they can offer strong guarantees that modifications will be detectable, while being implementable in software. Furthermore, cryptography can be used to achieve public verifiability of log files, which may be needed in applications that have strong transparency requirements. Cryptographic techniques can even be used in addition to hardware solutions, providing protection against attackers who do have physical access to the logging hardware, such as insiders. Cryptographic schemes for protecting stored log data need to be resilient against attackers who obtain control over the computer storing the log data. If this computer operates in a standalone fashion, it is an absolute requirement for the cryptographic schemes to offer security even in the event of a key compromise. As this is impossible with standard cryptographic tools, cryptographic solutions for protecting log data typically make use of forward-secure schemes, guaranteeing that changes to log data recorded in the past can be detected. Such schemes use a sequence of authentication keys instead of a single one, where previous keys cannot be computed efficiently from latter ones. This thesis considers the following requirements for, and desirable features of, cryptographic logging schemes: 1) security, i.e. the ability to reliably detect violations of integrity and authenticity, including detection of log truncations, 2) efficiency regarding both computational and storage overhead, 3) robustness, i.e. the ability to verify unmodified log entries even if others have been illicitly changed, and 4) verifiability of excerpts, including checking an excerpt for omissions. The goals of this thesis are to devise new techniques for the construction of cryptographic schemes that provide security for computer log files, to give concrete constructions of such schemes, to develop new models that can accurately capture the security guarantees offered by the new schemes, as well as to examine the security of previously published schemes. This thesis demands that cryptographic schemes for securely storing log data must be able to detect if log entries have been deleted from a log file. A special case of deletion is log truncation, where a continuous subsequence of log records from the end of the log file is deleted. Obtaining truncation resistance, i.e. the ability to detect truncations, is one of the major difficulties when designing cryptographic logging schemes. This thesis alleviates this problem by introducing a novel technique to detect log truncations without the help of third parties or designated logging hardware. Moreover, this work presents new formal security notions capturing truncation resistance. The technique mentioned above is applied to obtain cryptographic logging schemes which can be shown to satisfy these notions under mild assumptions, making them the first schemes with formally proven truncation security. Furthermore, this thesis develops a cryptographic scheme for the protection of log files which can support the creation of excerpts. For this thesis, an excerpt is a (not necessarily contiguous) subsequence of records from a log file. Excerpts created with the scheme presented in this thesis can be publicly checked for integrity and authenticity (as explained above) as well as for completeness, i.e. the property that no relevant log entry has been omitted from the excerpt. Excerpts provide a natural way to preserve the confidentiality of information that is contained in a log file, but not of interest for a specific public analysis of the log file, enabling the owner of the log file to meet confidentiality and transparency requirements at the same time. The scheme demonstrates and exemplifies the technique for obtaining truncation security mentioned above. Since cryptographic techniques to safeguard log files usually require authenticating log entries individually, some researchers [Ma and Tsudik, "A New Approach to Secure Logging", LNCS 5094, 2008; Ma and Tsudik, "A New Approach to Secure Logging", ACM TOS 2009; Yavuz and Peng, "BAF: An Efficient Publicly Verifiable Secure Audit Logging Scheme for Distributed Systems", ACSAC 2009] have proposed using aggregatable signatures [Boneh et al., "Aggregate and Verifiably Encrypted Signatures from Bilinear Maps", EUROCRYPT 2003] in order to reduce the overhead in storage space incurred by using such a cryptographic scheme. Aggregation of signatures refers to some “combination” of any number of signatures (for distinct or equal messages, by distinct or identical signers) into an “aggregate” signature. The size of the aggregate signature should be less than the total of the sizes of the orginal signatures, ideally the size of one of the original signatures. Using aggregation of signatures in applications that require storing or transmitting a large number of signatures (such as the storage of log records) can lead to significant reductions in the use of storage space and bandwidth. However, aggregating the signatures for all log records into a single signature will cause some fragility: The modification of a single log entry will render the aggregate signature invalid, preventing the cryptographic verification of any part of the log file. However, being able to distinguish manipulated log entries from non-manipulated ones may be of importance for after-the-fact investigations. This thesis addresses this issue by presenting a new technique providing a trade-off between storage overhead and robustness, i.e. the ability to tolerate some modifications to the log file while preserving the cryptographic verifiability of unmodified log entries. This robustness is achieved by the use of a special kind of aggregate signatures (called fault-tolerant aggregate signatures), which contain some redundancy. The construction makes use of combinatorial methods guaranteeing that if the number of errors is below a certain threshold, then there will be enough redundancy to identify and verify the non-modified log entries. Finally, this thesis presents a total of four attacks on three different schemes intended for securely storing log files presented in the literature [Yavuz et al., "Efficient, Compromise Resilient and Append-Only Cryptographic Schemes for Secure Audit Logging", Financial Cryptography 2012; Ma, "Practical Forward Secure Sequential Aggregate Signatures", ASIACCS 2008]. The attacks allow for virtually arbitrary log file forgeries or even recovery of the secret key used for authenticating the log file, which could then be used for mostly arbitrary log file forgeries, too. All of these attacks exploit weaknesses of the specific schemes. Three of the attacks presented here contradict the security properties of the schemes claimed and supposedly proven by the respective authors. This thesis briefly discusses these proofs and points out their flaws. The fourth attack presented here is outside of the security model considered by the scheme’s authors, but nonetheless presents a realistic threat. In summary, this thesis advances the scientific state-of-the-art with regard to providing security for computer log files in a number of ways: by introducing a new technique for obtaining security against log truncations, by providing the first scheme where excerpts from log files can be verified for completeness, by describing the first scheme that can achieve some notion of robustness while being able to aggregate log record signatures, and by analyzing the security of previously proposed schemes

A Genuine Random Sequential Multi-signature Scheme

The usual sequential multi-signature scheme allows the multi-signers to sign the document with their own information and sequence, and the signature is not real random and secure. The paper analyzes the reasons for the insecurity of the previous multi-signature scheme, and puts forward a Genuine Random Sequential Multi-signature Scheme based on The Waters signature scheme, and the experiment proves that this scheme is a good scheme suitable for the practical application with high computing efficiency

Fault-Tolerance and Deaggregation Security of Aggregate Signatures

Ein zentrales Problem der digitalen Kommunikation ist die Absicherung der Authentizität und Integrität digitaler Dokumente, wie etwa Webseiten, E-Mails oder Programmen. So soll beispielsweise für den Empfänger einer E-Mail nachvollziehbar sein, dass die empfangene E-Mail tatsächlich vom angegebenen Absender stammt (Authentizität) und nicht durch Dritte verändert wurde (Integrität). Digitale Signaturen sind ein Hauptwerkzeug der Kryptographie und IT-Sicherheit, um diese Eigenschaften zu gewährleisten. Hierzu wird vom Absender ein geheimer Schlüssel verwendet, um für das zu sichernde Dokument eine Signatur zu erstellen, die mithilfe eines öffentlich bekannten Verifikationsschlüssels jederzeit überprüft werden kann. Die Sicherheitseigenschaften solcher digitaler Signaturverfahren garantieren sowohl, dass jede Änderung am Dokument dazu führt, dass diese Überprüfung fehlschlägt, als auch dass eine Fälschung einer Signatur praktisch unmöglich ist, d.h. ohne den geheimen Schlüssel kann keine gültige Signatur berechnet werden. Somit kann bei einer erfolgreichen Verifikation davon ausgegangen werden, dass das Dokument tatsächlich vom angegebenen Absender erstellt und seit der Berechnung der Signatur nicht verändert wurde, da nur der Absender über den geheimen Schlüssel verfügt. Aggregierbare Signaturen bieten zusätzlich die Möglichkeit Signaturen mehrerer Dokumente zu einer einzigen Signatur zusammenzuführen bzw. zu aggregieren. Diese Aggregation ist dabei jederzeit möglich. Eine aggregierte Signatur bezeugt weiterhin sicher die Integrität und Authentizität aller ursprünglichen Dokumente, benötigt dabei aber nur so viel Speicherplatz wie eine einzelne Signatur. Außerdem ist die Verifikation einer solchen aggregierten Signatur üblichrweise schneller möglich als die sukzessive Überprüfung aller Einzelsignaturen. Somit kann die Verwendung eines aggregierbaren Signaturverfahrens anstelle eines gewöhnlichen Verfahrens zu erheblichen Verbesserungen der Performanz und des Speicherverbrauchs bei Anwendungen von Signaturen führen. In dieser Dissertation werden zwei zusätzliche Eigenschaften von aggregierbaren Signaturverfahren namens Fehlertoleranz und Deaggregationssicherheit untersucht. Fehlertoleranz bietet eine Absicherung des Verfahrens gegen fehlerhafte Signier- und Aggregationsvorgänge und Deaggregationssicherheit schützt vor ungewollten Löschungen. Beide Eigenschaften werden im Folgenden erläutert. Fehlertoleranz: Durch System- und Programmfehler, sowie inkorrektes oder auch bösartiges Nutzerverhalten ist es möglich, dass fehlerhafte Einzelsignaturen zu einer bestehenden aggregierten Signatur hinzugefügt werden. Alle bisherige aggregierbaren Signaturverfahren haben jedoch den Nachteil, dass bereits das Aggregieren einer einzigen fehlerhaften Einzelsignatur dazu führt, dass auch die aggregierte Signatur fehlerhaft und somit unbrauchbar wird. Die aggregierte Signatur kann danach nicht mehr korrekt verifiziert werden. Insbesondere kann aus ihr nun keinerlei Aussage mehr über die Integrität und Authentizität der Dokumente abgeleitet werden, die vor dem Hinzufügen der fehlerhaften Einzelsignatur korrekt signiert wurden. Dies hat zur Folge, dass alle gegebenen Sicherheitsgarantien verloren gehen und es wird ein aufwändiges Neusignieren aller Dokumente notwendig, welches unter Umständen und je nach Anwendung nur schwer bis überhaupt nicht möglich ist. In dieser Dissertation wird das erste fehlertolerante aggregierbare Signaturverfahren vorgestellt, bei dem das Hinzufügen einzelner falscher Signaturen bis zu einer gewissen Grenze keine schädlichen Auswirkungen hat. Eine aggregierte Signatur wird erst dann ungültig und unbrauchbar, sobald die Anzahl hinzugefügter fehlerhafter Signaturen diese Grenze überschreitet und behält davor weiterhin seine Gültigkeit für die korrekt signierten Dokumente. Dazu wird ein Verfahren vorgestellt, mit dem jedes beliebige aggregierbare Signaturverfahren in ein fehlertolerantes Verfahren transformiert werden kann. Das zugrundeliegende Verfahren wird dabei nur als Black-Box verwendet und der Schutz gegen Fälschungsangriffe übertragt sich beweisbar und ohne Einschränkung auf das neue fehlertolerante Verfahren. Des Weiteren wird als Anwendung von fehlertoleranten Verfahren gezeigt, wie aus ihnen ein sicheres Log-Verfahren konstruiert werden kann. Deaggregationssicherheit: Erlangt ein Angreifer Zugriff auf eine aggregierte Signatur für einen bestimmten Datensatz, so sollte es ihm nicht möglich sein aus diesem Aggregat eine gültige Signatur für einen Teil der geschützten Dokumente abzuleiten, indem er einzelne Signaturen entfernt oder deaggregiert. Solche Angriffe können für viele Anwendungsfälle problematisch sein, da so Signaturen für Mengen von Dokumenten berechnet werden könnten, die nicht von den eigentlichen Erstellern beabsichtigt waren und nie von ihnen selbst signiert wurden. Wird ein aggregierbares Signaturverfahren etwa verwendet um eine Datenbank abzusichern, so sollte es Angreifern nicht möglich sein einzelne Einträge daraus zu entfernen. In dieser Dissertation werden mehrere Deaggregationssicherheitsbegriffe entwickelt, vorgestellt und untersucht. Dazu wird eine Hierarchie von verschieden starken Sicherheitsbegriffen entwickelt und die Zusammenhänge zwischen den einzelnen Begriffen werden formal untersucht. Dabei wird auch gezeigt, dass der von aggregierbaren Signaturverfahren garantierte Schutz gegen Fälschungen keinerlei Sicherheit gegen Deaggregationsangriffe gewährleistet. Des Weiteren wird die Deaggregationssicherheit einer Reihe von bekannten und wichtigen aggregierbaren Signaturverfahren näher betrachtet. Die von diesen Verfahren gebotene Sicherheit wird exakt klassifiziert, indem entweder Angriffsmöglichkeiten demonstriert werden oder formal bewiesen wird, welcher Sicherheitsbegriff der Hierarchie vom Verfahren erfüllt wird. Außerdem wird die Verbindung von Fehlertoleranz und Deaggregationssicherheit untersucht. Dabei stellt sich heraus, dass beide Begriffe nicht zueinander kompatibel sind, indem bewiesen wird, dass fehlertolerante aggregierbare Signaturverfahren keinerlei Sicherheit gegen Deaggregationsangriffe bieten können. Somit muss bei Anwendungen von aggregierbaren Verfahren genau abgewogen werden, welche der beiden Eigenschaften notwendig ist und ob zusätzliche Sicherheitsmaßnahmen angewendet werden müssen, um dieses Problem für die konkrete Anwendung zu beheben

A Genuine Random Sequential Multi-signature Scheme

The usual sequential multi-signature scheme allows the multi-signers to sign the document with their own information and sequence, and the signature is not real random and secure. The paper analyzes the reasons for the insecurity of the previous multi-signature scheme, and puts forward a Genuine Random Sequential Multi-signature Scheme based on The Waters signature scheme, and the experiment proves that this scheme is a good scheme suitable for the practical application with high computing efficiency

A unified framework for trapdoor-permutation-based sequential aggregate signatures

We give a framework for trapdoor-permutation-based sequential aggregate signatures (SAS) that unifies and simplifies prior work and leads to new results. The framework is based on ideal ciphers over large domains, which have recently been shown to be realizable in the random oracle model. The basic idea is to replace the random oracle in the full-domain-hash signature scheme with an ideal cipher. Each signer in sequence applies the ideal cipher, keyed by the message, to the output of the previous signer, and then inverts the trapdoor permutation on the result. We obtain different variants of the scheme by varying additional keying material in the ideal cipher and making different assumptions on the trapdoor permutation. In particular, we obtain the first scheme with lazy verification and signature size independent of the number of signers that does not rely on bilinear pairings. Since existing proofs that ideal ciphers over large domains can be realized in the random oracle model are lossy, our schemes do not currently permit practical instantiation parameters at a reasonable security level, and thus we view our contribution as mainly conceptual. However, we are optimistic tighter proofs will be found, at least in our specific application.https://eprint.iacr.org/2018/070.pdfAccepted manuscrip

多人数署名の証明可能安全性に関する研究

筑波大学 (University of Tsukuba)201

Performance Evaluation of Distributed Security Protocols Using Discrete Event Simulation

The Border Gateway Protocol (BGP) that manages inter-domain routing on the Internet lacks security. Protective measures using public key cryptography introduce complexities and costs. To support authentication and other security functionality in large networks, we need public key infrastructures (PKIs). Protocols that distribute and validate certificates introduce additional complexities and costs. The certification path building algorithm that helps users establish trust on certificates in the distributed network environment is particularly complicated. Neither routing security nor PKI come for free. Prior to this work, the research study on performance issues of these large-scale distributed security systems was minimal. In this thesis, we evaluate the performance of BGP security protocols and PKI systems. We answer the questions about how the performance affects protocol behaviors and how we can improve the efficiency of these distributed protocols to bring them one step closer to reality. The complexity of the Internet makes an analytical approach difficult; and the scale of Internet makes empirical approaches also unworkable. Consequently, we take the approach of simulation. We have built the simulation frameworks to model a number of BGP security protocols and the PKI system. We have identified performance problems of Secure BGP (S-BGP), a primary BGP security protocol, and proposed and evaluated Signature Amortization (S-A) and Aggregated Path Authentication (APA) schemes that significantly improve efficiency of S-BGP without compromising security. We have also built a simulation framework for general PKI systems and evaluated certification path building algorithms, a critical part of establishing trust in Internet-scale PKI, and used this framework to improve algorithm performance

A Touch of Evil: High-Assurance Cryptographic Hardware from Untrusted Components

The semiconductor industry is fully globalized and integrated circuits (ICs) are commonly defined, designed and fabricated in different premises across the world. This reduces production costs, but also exposes ICs to supply chain attacks, where insiders introduce malicious circuitry into the final products. Additionally, despite extensive post-fabrication testing, it is not uncommon for ICs with subtle fabrication errors to make it into production systems. While many systems may be able to tolerate a few byzantine components, this is not the case for cryptographic hardware, storing and computing on confidential data. For this reason, many error and backdoor detection techniques have been proposed over the years. So far all attempts have been either quickly circumvented, or come with unrealistically high manufacturing costs and complexity. This paper proposes Myst, a practical high-assurance architecture, that uses commercial off-the-shelf (COTS) hardware, and provides strong security guarantees, even in the presence of multiple malicious or faulty components. The key idea is to combine protective-redundancy with modern threshold cryptographic techniques to build a system tolerant to hardware trojans and errors. To evaluate our design, we build a Hardware Security Module that provides the highest level of assurance possible with COTS components. Specifically, we employ more than a hundred COTS secure crypto-coprocessors, verified to FIPS140-2 Level 4 tamper-resistance standards, and use them to realize high-confidentiality random number generation, key derivation, public key decryption and signing. Our experiments show a reasonable computational overhead (less than 1% for both Decryption and Signing) and an exponential increase in backdoor-tolerance as more ICs are added