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 Log Management, NIST Special Publication 800-92, 2006, Section 2.3.2], [Guttman and Roback, Introduction to Computer Security: The NIST Handbook, superseded NIST Special Publication 800-12, 1995, Section 18.3.1],[Nieles et al., Introduction to Information Security , NIST Special Publication 800-12, 2017, Section 9.3], [Common Criteria Editorial Board, ed., Common Criteria for Information Technology 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. 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., 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.