Miyaji Laboratory

A hash function is a mathematical algorithm that takes an input (or "message") and returns a f ixed-size string, known as the hash value. Designed to be pre-image resistant (hard to reverse) and collision-resistant (difficult to find two different inputs yielding the same hash), hash functions serve as fundamental tools in modern cryptography. They are widely used in applications such as data integrity verification, digital signatures, and secure password storage. Blockchain technology, in particular, relies on hash functions to ensure the immutability and security of its distributed ledger. However, as blockchain applications evolve, there is a growing need for redactable blockchains to address scenarios like legal compliance and error correction. Redactable blockchains require the ability to find collisions in hash values to enable block edits without compromising the integrity of the chain. To fulfill this need, the Chameleon Hash (CH) was introduced. A Chameleon Hash is a specialized hash function incorporating a public and private key pair. It is collision-resistant for users without access to the private key but allows users with the private key to generate collisions intentionally. This unique capability makes CH ideal for enabling editable blockchains, as it permits selective block modification while preserving the chain's overall integrity. Despite its use fulness, CH has a critical limitation: its security relies entirely on the secrecy of the private key. If the private key is compromised, the collision resistance of the CH function is lost, undermining the security of the system. To address this limitation, Matsubara et al. proposed the forward-secure Chameleon Hash (FSCH), which ensures that previous hash values remain secure even if the private key is exposed. However, their solution requires modifying the construction of CH and relies on specific security assumption, limiting the universality. In this work, we propose a generic construction that capable to transform any given CH into an FSCH, which is called Generic Forward-Secure Chameleon Hash (G-FSCH) scheme. By employing CH and forward-secure public key encryption (FSPKE) in a black-box composition, G-FSCH preserves the original CH structure and cryptographic assumptions, making it easy to integrate into existing cryptographic protocols based on various security assumptions.