Oblivious Identity-based Encryption (IBE Secure Against an Adversarial KGC)

Identity-Based Encryption (IBE) was introduced in order to reduce the cost associated with Public Key Infrastructure systems. IBE allows users to request a trusted Key Generation Centre (KGC) for a secret key on a given identity, without the need to manage public keys. However, one of the main concerns of IBE is that the KGC has the power to decrypt all ciphertexts as it has access to all (identity, secret key) pairs. To address this issue, Chow (PKC 2009) introduced a new security property against the KGC by employing a new trusted party called the Identity Certifying Authority (ICA). Emura et al. (ESORICS 2019) formalized this notion and proposed construction in the random oracle model. In this work, we first identify several existing IBE schemes where the KGC can decrypt a ciphertext even without knowing the receiver\u27s identity. This paves the way for formalizing new capabilities for the KGC. We then propose a new security definition to capture an adversarial KGC including the newly identified capabilities and we remove the requirement of an additional trusted party. Finally, we propose a new IBE construction that allows users to ask the KGC for a secret key on an identity without leaking any information about the identity to the KGC that is provably secure in the standard model against an adversarial KGC and corrupted users. Our construction is achieved in the composite order pairing groups and requires essentially optimal parameters

Locally Reconstructable Non-Malleable Secret Sharing

Non-malleable secret sharing (NMSS) schemes, introduced by Goyal and Kumar (STOC 2018), ensure that a secret $m$ can be distributed into shares $m_1,...,m_n$ (for some $n$), such that any $t$ (a parameter $<=n$) shares can be reconstructed to recover the secret $m$, any $t-1$ shares doesn\u27t leak information about $m$ and even if the shares that are used for reconstruction are tampered, it is guaranteed that the reconstruction of these tampered shares will either result in the original $m$ or something independent of $m$. Since their introduction, non-malleable secret sharing schemes sparked a very impressive line of research. In this work, we introduce a feature of local reconstructability in NMSS, which allows reconstruction of any portion of a secret by reading just a few locations of the shares. This is a useful feature, especially when the secret is long or when the shares are stored in a distributed manner on a communication network. In this work, we give a compiler that takes in any non-malleable secret sharing scheme and compiles it into a locally reconstructable non-malleable secret sharing scheme. To secret share a message consisting of $k$ blocks of length $l$ each, our scheme would only require reading $l + log k$ bits (in addition to a few more bits, whose quantity is independent of $l$ and $k$) from each party\u27s share (of a reconstruction set) to locally reconstruct a single block of the message. We show an application of our locally reconstructable non-malleable secret sharing scheme to a computational non-malleable secure message transmission scheme in the pre-processing model, with an improved communication complexity, when transmitting multiple messages