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

Efficient Unlinkable Sanitizable Signatures from Signatures with Re-Randomizable Keys

In a sanitizable signature scheme the signer allows a designated third party, called the sanitizer, to modify certain parts of the message and adapt the signature accordingly. Ateniese et al. (ESORICS 2005) introduced this primitive and proposed five security properties which were formalized by Brzuska et al.~(PKC 2009). Subsequently, Brzuska et al. (PKC 2010) suggested an additional security notion, called unlinkability which says that one cannot link sanitized message-signature pairs of the same document. Moreover, the authors gave a generic construction based on group signatures that have a certain structure. However, the special structure required from the group signature scheme only allows for inefficient instantiations. Here, we present the first efficient instantiation of unlinkable sanitizable signatures. Our construction is based on a novel type of signature schemes with re-randomizable keys. Intuitively, this property allows to re-randomize both the signing and the verification key separately but consistently. This allows us to sign the message with a re-randomized key and to prove in zero-knowledge that the derived key originates from either the signer or the sanitizer. We instantiate this generic idea with Schnorr signatures and efficient $\Sigma$-protocols, which we convert into non-interactive zero-knowledge proofs via the Fiat-Shamir transformation. Our construction is at least one order of magnitude faster than instantiating the generic scheme of Brzuska et al. with the most efficient group signature schemes

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

Unlinkable Policy-based Sanitizable Signatures

In CT-RSA 2020, P3S was proposed as the first policy-based sanitizable signature scheme which allows the signer to designate future message sanitizers by defining an access policy relative to their attributes rather than their keys. However, since P3S utilizes a policy-based chameleon hash (PCH), it does not achieve unlinkability which is a required notion in privacy-preserving applications. Moreover, P3S requires running a procedure to share the secret trapdoor information for PCH with each new sanitizer before sanitizing a new message. We further observe that in order to maintain the transparency in P3S’s multiple-sanitizers setting, the signature size should grow linearly with the number of sanitizers. In this work, we propose an unlinkable policy-based sanitizable signature scheme (UP3S) where we employ a rerandomizable digital signature scheme and a traceable attribute-based signature scheme as its building blocks. Compared to P3S, UP3S achieves unlinkability, does not require new secrets to be shared with future sanitizers prior to sanitizing each message, and has a fixed signature size for a given sanitization policy. We define and formally prove the security notions of the generic scheme, propose an instantiation of UP3S utilizing the Pointcheval-Sanders rerandomizable signature scheme and DTABS traceable attribute-based signature scheme, and analyze its efficiency. Finally, we compare UP3S with P3S in terms of the features of the procedures, scalability, and security models

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

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

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

Policy-Based Sanitizable Signatures

Sanitizable signatures are a variant of signatures which allow a single, and signer-defined, sanitizer to modify signed messages in a controlled way without invalidating the respective signature. They turned out to be a versatile primitive, proven by different variants and extensions, e.g., allowing multiple sanitizers or adding new sanitizers one-by-one. However, existing constructions are very restricted regarding their flexibility in specifying potential sanitizers. We propose a different and more powerful approach: Instead of using sanitizers\u27 public keys directly, we assign attributes to them. Sanitizing is then based on policies, i.e., access structures defined over attributes. A sanitizer can sanitize, if, and only if, it holds a secret key to attributes satisfying the policy associated to a signature, while offering full-scale accountability

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