Martín A. Gagliotti Vigil

Assessing trust in the long-term protection of documents

Digital archives rely on trusted parties, such as certification authorities, to ensure authenticity, integrity and proof of existence protection for documents. In this paper, we analyse the trust assumptions that a verifier has to make in order to trust in the protection of a document. We show that trust fades out in the long term due to the ever-growing number of trusted parties. Despite such a dire prospect, current technologies such as X.509 PKI do not assess trust, thereby leaving verifiers in the dark. We present a certification scheme for documents that provides verifiers with a better assessment of trust than in X.509 PKI. In the proposed scheme, trusted parties are rated based on the correctness of their performance. From the ratings, verifiers can assess quantitatively the trust in the trusted parties for the short term, and in the protection of documents for the long term. The proposed scheme encourages trusted parties to work properly.

The Notary Based PKI

We propose a new Public Key Infrastructure model for long-term signatures. It is based on X.509 and the real world of handwritten signatures. In the model, notaries certify that a signers certificate is trustworthy to verify a particular signature at a specific time. An end user issues his own X.509 certificate, whose validity period is meaningless and whose trustworthiness is accepted only if the certificate was certified by a notary. After the certification, the certificate remains trustworthy even if later keys are compromised or notaries disappear. The benefits for signed document users are: i the maintenance of a document signature is simple and only necessary to prevent the obsolescence of cryptographic algorithms; ii the overhead to store and verify a document signature does not increase significantly in the long term; and iii there is only one trust decision when verifying a document signature.

Optimized Certificates --- A New Proposal for Efficient Electronic Document Signature Validation

Optimized certification is a new method for efficient certificate path verification and digital signing. The basic idea is to issue special certificates (called optimized certificates) for an electronic document to replace the signers certificate. Optimized certificates are issued to be only valid for a specific time, i.e., the fields notBeforeand notAfterof the certificate are the same. Therefore, certificate revocation are not a requirement as it is no longer necessary to request the status of certificates from a certification authority repository to validate signatures.

A Performance Analysis of Long-Term Archiving Techniques

A challenge for digital archives managing long lived data, such as medical records and land registers, is to guarantee long-term authenticity, integrity, and datedness. Although some techniques have been proposed that provide these security goals, most proposals lack a corresponding performance analysis. Thus, in this paper we provide an analytical analysis of the performance of several proposals. Following, to see the impact of different approaches on the runtime, we implemented the most promising schemes and compared them with respect to their required storage space and verification time. Furthermore, the identified computational bottlenecks of the techniques are presented and corresponding improvements are indicated. Finally, we show how to select a long-term archiving scheme and the appropriate key sizes based on trust assumptions and the number of documents to be archived.

An efficient time-stamping solution for long-term digital archiving

Long-term archiving of digital data is necessary to meet many legal requirements. For example, hospitals in many countries must keep health records of patients for decades. Archiving usually relies on digital signatures and time-stamps to prove the security properties of archived data, such as integrity and proof of existence. Moreover, archived data often needs to be updated, e.g. a new prescription is added to a patients record, but without compromising integrity and proof of existence. To date, a solution that guarantees integrity and proof of existence indefinitely and allows for updates on archived data is Content Integrity Service (CIS). In this paper, we introduce an improved version of CIS named Content Integrity Service with Skip Lists (CISS) that changes the time-stamping process of CIS by using two different types of time-stamps together with skip lists. We demonstrate that CISS outperforms CIS by analyzing the algorithms and running experiments in realistic scenarios.

How to Avoid the Breakdown of Public Key Infrastructures

Recent attacks and publications have shown the vulnerability of hierarchical Public Key Infrastructures PKIs and the fatal impact of revoked Certification Authority CA certificates in the PKIX validity model. Alternative validity models, such as the extended shell and the chain model, improve the situation but rely on independent proofs of existence, which are usually provided using time-stamps. As time-stamps are validated using certificates, they suffer from the same problems as the PKI they are supposed to protect. Our solution to this problem is abandoning time-stamps and providing proof of existence using Forward Secure Signatures FSS. In particular, we present different possibilities to use the chain model together with FSS, resulting in schemes that include the necessary proofs of existence into the certificates themselves.

MoPS: A Modular Protection Scheme for Long-Term Storage

Current trends in technology, such as cloud computing, allow outsourcing the storage, backup, and archiving of data. This provides efficiency and flexibility, but also poses new risks for data security. It in particular became crucial to develop protection schemes that ensure security even in the long-term, i.e. beyond the lifetime of keys, certificates, and cryptographic primitives. However, all current solutions fail to provide optimal performance for different application scenarios. Thus, in this work, we present MoPS, a modular protection scheme to ensure authenticity and integrity for data stored over long periods of time. MoPS does not come with any requirements regarding the storage architecture and can therefore be used together with existing archiving or storage systems. It supports a set of techniques which can be plugged together, combined, and migrated in order to create customized solutions that fulfill the requirements of different application scenarios in the best possible way. As a proof of concept we implemented MoPS and provide performance measurements. Furthermore, our implementation provides additional features, such as guidance for non-expert users and export functionalities for external verifiers.

A User-Centric Digital Signature Scheme

We observe that current mainstream digital signature schemes are complex and inconvenient for end users. We group the main problems related to these schemes and propose a new approach, centered on the needs of the end user. The new model is a redesign of the overall signature process, discarding certificates and the X509 PKI in favor of simple structures and natural trust relationships modeled on conventional handwritten signatures.