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1. Conditional Encryption with Applications to Secure Personalized Password Typo Correction

   https://doi.org/10.1145/3658644.3690374  

   Ameri, Mohammad Hassan; Blocki, Jeremiah (December 2024, ACM)

   We introduce the notion of a conditional encryption scheme as an extension of public key encryption. In addition to the standard public key algorithms (KG, Enc, Dec) for key generation, encryption and decryption, a conditional encryption scheme for a binary predicate P adds a new conditional encryption algorithm CEnc. The conditional encryption algorithm c=CEncpk (c1,m2,m3) takes as input the public encryption key pk, a ciphertext c1 = Encpk (m1) for an unknown message m1, a control message m2 and a payload message m3 and outputs a conditional ciphertext c. Intuitively, if P(m1,m2)=1 then the conditional ciphertext c should decrypt to the payload message m3. On the other hand if P(m1,m2) = 0 then the ciphertext should not leak any information about the control message m2 or the payload message m3 even if the attacker already has the secret decryption key sk. We formalize the notion of conditional encryption secrecy and provide concretely efficient constructions for a set of predicates relevant to password typo correction. Our practical constructions utilize the Paillier partially homomorphic encryption scheme as well as Shamir Secret Sharing. We prove that our constructions are secure and demonstrate how to use conditional encryption to improve the security of personalized password typo correction systems such as TypTop. We implement a C++ library for our practically efficient conditional encryption schemes and evaluate the performance empirically. We also update the implementation of TypTop to utilize conditional encryption for enhanced security guarantees and evaluate the performance of the updated implementation. 

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2. The Local Forking Lemma and Its Application to Deterministic Encryption

   https://doi.org/10.1007/978-3-030-34618-8_21  

   Mihir Bellare, Wei Dai (December 2019, Advances in Cryptology - {ASIACRYPT} 2019 - 25th International Conference on the Theory and Application of Cryptology and Information Security, Kobe, Japan, December 8-12, 2019, Proceedings, Part III)

   Steven D. Galbraith, Shiho Moriai (Ed.)

   We bypass impossibility results for the deterministic encryption of public-key-dependent messages, showing that, in this setting, the classical Encrypt-with-Hash scheme provides message-recovery security, across a broad range of message distributions. The proof relies on a new variant of the forking lemma in which the random oracle is reprogrammed on just a single fork point rather than on all points past the fork. 

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3. Indistinguishability obfuscation from circular security

   https://doi.org/10.1145/3406325.3451070  

   Gay, Romain; Pass, Rafael (January 2021, STOC)

   We show the existence of indistinguishability obfuscators (iO) for general circuits assuming subexponential security of: (a) the Learning with Errors (LWE) assumption (with subexponential modulusto- noise ratio); (b) a circular security conjecture regarding the Gentry- Sahai-Waters’ (GSW) encryption scheme and a Packed version of Regev’s encryption scheme. The circular security conjecture states that a notion of leakage-resilient security, that we prove is satisfied by GSW assuming LWE, is retained in the presence of an encrypted key-cycle involving GSW and Packed Regev. 

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4. Dynamic Collusion Bounded Functional Encryption from Identity-Based Encryption

   Garg, Rachit; Goyal, Rishab; Lu, George; Waters, Brent (May 2022, Eurocrypt 2022)

   Functional Encryption is a powerful notion of encryption in which each decryption key is associated with a function such that decryption recovers the function evaluation . Informally, security states that a user with access to function keys (and so on) can only learn (and so on) but nothing more about the message. The system is said to be -bounded collusion resistant if the security holds as long as an adversary gets access to at most function keys. A major drawback of such "statically" bounded collusion systems is that the collusion bound must be declared at setup time and is fixed for the entire lifetime of the system. We initiate the study of "dynamically" bounded collusion resistant functional encryption systems which provide more flexibility in terms of selecting the collusion bound, while reaping the benefits of statically bounded collusion FE systems (such as quantum resistance, simulation security, and general assumptions). Briefly, the virtues of a dynamically bounded scheme can be summarized as: (i) [Fine-grained individualized selection.] It lets each encryptor select the collusion bound by weighing the trade-off between performance overhead and the amount of collusion resilience. (ii) [Evolving encryption strategies.] Since the system is no longer tied to a single collusion bound, thus it allows to dynamically adjust the desired collusion resilience based on any number of evolving factors such as the age of the system, or a number of active users, etc. (iii) [Ease and simplicity of updatability.] None of the system parameters have to be updated when adjusting the collusion bound. That is, the same key can be used to decrypt ciphertexts for collusion bound as well as . We construct such a dynamically bounded functional encryption scheme for the class of all polynomial-size circuits under the general assumption of Identity-Based Encryption. 

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5. Adaptive Security via Deletion in Attribute-Based Encryption: Solutions from Search Assumptions in Bilinear Groups

   https://doi.org/10.1007/978-3-030-92068-5_11  

   Goyal, Rishab; Liu, Jiahui; Waters, Brent (December 2021, Asiacrypt 2021)

   One of the primary research challenges in Attribute-Based Encryption (ABE) is constructing and proving cryptosystems that are adaptively secure. To date the main paradigm for achieving adaptive security in ABE is dual system encryption. However, almost all such solutions in bilinear groups rely on (variants of) either the subgroup decision problem over composite order groups or the decision linear assumption. Both of these assumptions are decisional rather than search assumptions and the target of the assumption is a source or bilinear group element. This is in contrast to earlier selectively secure ABE systems which can be proven secure from either the decisional or search Bilinear Diffie-Hellman assumption. In this work we make progress on closing this gap by giving a new ABE construction for the subset functionality and prove security under the Search Bilinear Diffie-Hellman assumption. We first provide a framework for proving adaptive security in Attribute-Based Encryption systems. We introduce a concept of ABE with deletable attributes where any party can take a ciphertext encrypted under the attribute string and modify it into a ciphertext encrypted under any string where is derived by replacing any bits of with symbols (i.e. ``deleting" attributes of ). The semantics of the system are that any private key for a circuit can be used to decrypt a ciphertext associated with if none of the input bits read by circuit are symbols and . We show a pathway for combining ABE with deletable attributes with constrained psuedorandom functions to obtain adaptively secure ABE building upon the recent work of Tsabary. Our new ABE system will be adaptively secure and be a ciphertext-policy ABE that supports the same functionality as the underlying constrained PRF as long as the PRF is ``deletion conforming". Here we also provide a simple constrained PRF construction that gives subset functionality. Our approach enables us to access a broader array of Attribute-Based Encryption schemes support deletion of attributes. For example, we show that both the Goyal~et al.~(GPSW) and Boyen ABE schemes can trivially handle a deletion operation. And, by using a hardcore bit variant of GPSW scheme we obtain an adaptively secure ABE scheme under the Search Bilinear Diffie-Hellman assumption in addition to pseudo random functions in NC1. This gives the first adaptively secure ABE from a search assumption as all prior work relied on decision assumptions over source group elements. 

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