Sanitizable signatures with strong transparency in the standard model

Sanitizable signatures provide several security features which are useful in many scenarios including military and medical applications. Sanitizable signatures allow a semi-trusted party to update some part of the digitally signed document without interacting with the original signer. Such schemes, where the verifer cannot identify whether the message has been sanitized, are said to possess strong transparency. In this paper, we have described the first efficient and provably secure sanitizable signature scheme having strong transparency under the standard model

Stronger Security for Sanitizable Signatures

Sanitizable signature schemes (SSS) enable a designated party (called the sanitizer ) to alter admissible blocks of a signed message. This primitive can be used to remove or alter sensitive data from already signed messages without involvement of the original signer. Current state-of-the-art security definitions of SSSs only dene a \weak form of security. Namely, the unforgeability, accountability and transparency definitions are not strong enough to be meaningful in certain use-cases. We identify some of these use-cases, close this gap by introducing stronger definitions, and show how to alter an existing construction to meet our desired security level. Moreover, we clarify a small yet important detail in the state-of-the-art privacy definition. Our work allows to deploy this primitive in more and different scenarios

Rethinking Privacy for Extended Sanitizable Signatures and a Black-Box Construction of Strongly Private Schemes

Sanitizable signatures, introduced by Ateniese et al. at ESORICS\u2705, allow to issue a signature on a message where certain predefined message blocks may later be changed (sanitized) by some dedicated party (the sanitizer) without invalidating the original signature. With sanitizable signatures, replacements for modifiable (admissible) message blocks can be chosen arbitrarily by the sanitizer. However, in various scenarios this makes sanitizers too powerful. To reduce the sanitizers power, Klonowski and Lauks at ICISC\u2706 proposed (among others) an extension that enables the signer to limit the allowed modifications per admissible block to a well defined set each. At CT-RSA\u2710 Canard and Jambert then extended the formal model of Brzuska et al. from PKC\u2709 to additionally include the aforementioned and other extensions. We, however, observe that the privacy guarantees of their model do not capture privacy in the sense of the original definition of sanitizable signatures. That is, if a scheme is private in this model it is not guaranteed that the sets of allowed modifications remain concealed. To this end, we review a stronger notion of privacy, i.e., (strong) unlinkability (defined by Brzuska et al. at EuroPKI\u2713), in this context. While unlinkability fixes this problem, no efficient unlinkable scheme supporting the aforementioned extensions exists and it seems to be hard to construct such schemes. As a remedy, in this paper, we propose a notion stronger than privacy, but weaker than unlinkability, which captures privacy in the original sense. Moreover, it allows to easily construct efficient schemes satisfying our notion from secure existing schemes in a black-box fashion

Practical Strongly Invisible and Strongly Accountable Sanitizable Signatures

Sanitizable signatures are a variant of digital signatures where a designated party (the sanitizer) can update admissible parts of a signed message. At PKC’17, Camenisch et al. introduced the notion of invisible sanitizable signatures that hides from an outsider which parts of a message are admissible. Their security definition of invisibility, however, does not consider dishonest signers. Along the same lines, their signer-accountability definition does not prevent the signer from falsely accusing the sanitizer of having issued a signature on a sanitized message by exploiting the malleability of the signature itself. Both issues may limit the usefulness of their scheme in certain applications. We revise their definitional framework, and present a new construction eliminating these shortcomings. In contrast to Camenisch et al.’s construction, ours requires only standard building blocks instead of chameleon hashes with ephemeral trapdoors. This makes this, now even stronger, primitive more attractive for practical use. We underpin the practical efficiency of our scheme by concrete benchmarks of a prototype implementation

Protean Signature Schemes

We introduce the notion of Protean Signature schemes. This novel type of signature scheme allows to remove and edit signer-chosen parts of signed messages by a semi-trusted third party simultaneously. In existing work, one is either allowed to remove or edit parts of signed messages, but not both at the same time. Which and how parts of the signed messages can be modified is chosen by the signer. Thus, our new primitive generalizes both redactable (Steinfeld et al., ICISC \u2701, Johnson et al., CT-RSA \u2702 & Brzuska et al., ACNS\u2710) and sanitizable signatures schemes (Ateniese et al., ESORICS \u2705 & Brzuska et al., PKC\u2709). We showcase a scenario where either primitive alone is not sufficient. Our provably secure construction (offering both strong notions of transparency and invisibility) makes only black-box access to sanitizable and redactable signature schemes, which can be considered standard tools nowadays. Finally, we have implemented our scheme; Our evaluation shows that the performance is reasonable

Chameleon-Hashes with Dual Long-Term Trapdoors and Their Applications

A chameleon-hash behaves likes a standard collision-resistant hash function for outsiders. If, however, a trapdoor is known, arbitrary collisions can be found. Chameleon-hashes with ephemeral trapdoors (CHET; Camenisch et al., PKC ’17) allow prohibiting that the holder of the long-term trapdoor can find collisions by introducing a second, ephemeral, trapdoor. However, this ephemeral trapdoor is required to be chosen freshly for each hash. We extend these ideas and introduce the notion of chameleon-hashes with dual long-term trapdoors (CHDLTT). Here, the second trapdoor is not chosen freshly for each new hash; Rather, the hashing party can decide if it wants to generate a fresh second trapdoor or use an existing one. This primitive generalizes CHETs, extends their applicability and enables some appealing new use-cases, including three-party sanitizable signatures, group-level selectively revocable signatures and break-the-glass signatures. We present two provably secure constructions and an implementation which demonstrates that this extended primitive is efficient enough for use in practice

Chameleon-Hashes with Ephemeral Trapdoors And Applications to Invisible Sanitizable Signatures

A chameleon-hash function is a hash function that involves a trapdoor the knowledge of which allows one to find arbitrary collisions in the domain of the function. In this paper, we introduce the notion of chameleon-hash functions with ephemeral trapdoors. Such hash functions feature additional, i.e., ephemeral, trapdoors which are chosen by the party computing a hash value. The holder of the main trapdoor is then unable to find a second pre-image of a hash value unless also provided with the ephemeral trapdoor used to compute the hash value. We present a formal security model for this new primitive as well as provably secure instantiations. The first instantiation is a generic black-box construction from any secure chameleon-hash function. We further provide three direct constructions based on standard assumptions. Our new primitive has some appealing use-cases, including a solution to the long-standing open problem of invisible sanitizable signatures, which we also present

On Structural Signatures for Tree Data Structures

Abstract. In this paper, we present new attacks on the redactable signature scheme introduced by Kundu and Bertino at VLDB '08. This extends the work done by Brzuska et al. at ACNS '10 and Samelin et al. at ISPEC '12. The attacks address unforgeability, transparency and privacy. Based on the ideas of Kundu and Bertino, we introduce a new provably secure construction. The corresponding security model is more flexible than the one introduced by Brzuska et al. Moreover, we have implemented schemes introduced by Brzuska et al. and Kundu and Bertino. The evaluation shows that schemes with a quadratic complexity become unuseable very fast

Fully Invisible Protean Signatures Schemes

Protean Signatures (PS), recently introduced by Krenn et al. (CANS \u2718), allow a semi-trusted third party, named the sanitizer, to modify a signed message in a controlled way. The sanitizer can edit signer-chosen parts to arbitrary bitstrings, while the sanitizer can also redact admissible parts, which are also chosen by the signer. Thus, PSs generalize both redactable signature (RSS) and sanitizable signature (SSS) into a single notion. However, the current definition of invisibility does not prohibit that an outsider can decide which parts of a message are redactable - only which parts can be edited are hidden. This negatively impacts on the privacy guarantees provided by the state-of-the-art definition. We extend PSs to be fully invisible. This strengthened notion guarantees that an outsider can neither decide which parts of a message can be edited nor which parts can be redacted. To achieve our goal, we introduce the new notions of Invisible RSSs and Invisible Non-Accountable SSSs (SSS\u27), along with a consolidated framework for aggregate signatures. Using those building blocks, our resulting construction is significantly more efficient than the original scheme by Krenn et al., which we demonstrate in a prototypical implementation

Contributions to Lifelogging Protection In Streaming Environments

Tots els dies, més de cinc mil milions de persones generen algun tipus de dada a través d'Internet. Per accedir a aquesta informació, necessitem utilitzar serveis de recerca, ja siguin motors de cerca web o assistents personals. A cada interacció amb ells, el nostre registre d'accions, logs, s'utilitza per oferir una millor experiència. Per a les empreses, també són molt valuosos, ja que ofereixen una forma de monetitzar el servei. La monetització s'aconsegueix venent dades a tercers, però, els logs de consultes podrien exposar informació confidencial de l'usuari (identificadors, malalties, tendències sexuals, creences religioses) o usar-se per al que es diu "life-logging ": Un registre continu de les activitats diàries. La normativa obliga a protegir aquesta informació. S'han proposat prèviament sistemes de protecció per a conjunts de dades tancats, la majoria d'ells treballant amb arxius atòmics o dades estructurades. Desafortunadament, aquests sistemes no s'adapten quan es fan servir en el creixent entorn de dades no estructurades en temps real que representen els serveis d'Internet. Aquesta tesi té com objectiu dissenyar tècniques per protegir la informació confidencial de l'usuari en un entorn no estructurat d’streaming en temps real, garantint un equilibri entre la utilitat i la protecció de dades. S'han fet tres propostes per a una protecció eficaç dels logs. La primera és un nou mètode per anonimitzar logs de consultes, basat en k-anonimat probabilística i algunes eines de desanonimització per determinar fuites de dades. El segon mètode, s'ha millorat afegint un equilibri configurable entre privacitat i usabilitat, aconseguint una gran millora en termes d'utilitat de dades. La contribució final es refereix als assistents personals basats en Internet. La informació generada per aquests dispositius es pot considerar "life-logging" i pot augmentar els riscos de privacitat de l'usuari. Es proposa un esquema de protecció que combina anonimat de logs i signatures sanitizables.Todos los días, más de cinco mil millones de personas generan algún tipo de dato a través de Internet. Para acceder a esa información, necesitamos servicios de búsqueda, ya sean motores de búsqueda web o asistentes personales. En cada interacción con ellos, nuestro registro de acciones, logs, se utiliza para ofrecer una experiencia más útil. Para las empresas, también son muy valiosos, ya que ofrecen una forma de monetizar el servicio, vendiendo datos a terceros. Sin embargo, los logs podrían exponer información confidencial del usuario (identificadores, enfermedades, tendencias sexuales, creencias religiosas) o usarse para lo que se llama "life-logging": Un registro continuo de las actividades diarias. La normativa obliga a proteger esta información. Se han propuesto previamente sistemas de protección para conjuntos de datos cerrados, la mayoría de ellos trabajando con archivos atómicos o datos estructurados. Desafortunadamente, esos sistemas no se adaptan cuando se usan en el entorno de datos no estructurados en tiempo real que representan los servicios de Internet. Esta tesis tiene como objetivo diseñar técnicas para proteger la información confidencial del usuario en un entorno no estructurado de streaming en tiempo real, garantizando un equilibrio entre utilidad y protección de datos. Se han hecho tres propuestas para una protección eficaz de los logs. La primera es un nuevo método para anonimizar logs de consultas, basado en k-anonimato probabilístico y algunas herramientas de desanonimización para determinar fugas de datos. El segundo método, se ha mejorado añadiendo un equilibrio configurable entre privacidad y usabilidad, logrando una gran mejora en términos de utilidad de datos. La contribución final se refiere a los asistentes personales basados en Internet. La información generada por estos dispositivos se puede considerar “life-logging” y puede aumentar los riesgos de privacidad del usuario. Se propone un esquema de protección que combina anonimato de logs y firmas sanitizables.Every day, more than five billion people generate some kind of data over the Internet. As a tool for accessing that information, we need to use search services, either in the form of Web Search Engines or through Personal Assistants. On each interaction with them, our record of actions via logs, is used to offer a more useful experience. For companies, logs are also very valuable since they offer a way to monetize the service. Monetization is achieved by selling data to third parties, however query logs could potentially expose sensitive user information: identifiers, sensitive data from users (such as diseases, sexual tendencies, religious beliefs) or be used for what is called ”life-logging”: a continuous record of one’s daily activities. Current regulations oblige companies to protect this personal information. Protection systems for closed data sets have previously been proposed, most of them working with atomic files or structured data. Unfortunately, those systems do not fit when used in the growing real-time unstructured data environment posed by Internet services. This thesis aims to design techniques to protect the user’s sensitive information in a non-structured real-time streaming environment, guaranteeing a trade-off between data utility and protection. In this regard, three proposals have been made in efficient log protection. The first is a new method to anonymize query logs, based on probabilistic k-anonymity and some de-anonymization tools to determine possible data leaks. A second method has been improved in terms of a configurable trade-off between privacy and usability, achieving a great improvement in terms of data utility. Our final contribution concerns Internet-based Personal Assistants. The information generated by these devices is likely to be considered life-logging, and it can increase the user’s privacy risks. The proposal is a protection scheme that combines log anonymization and sanitizable signatures