Proxy cryptosystems are classified into proxy decryption systems and proxy re-encryption systems on the basis of a proxy's role. In this paper, we propose an ID-based proxy cryptosystem with revocability and hierarchical confidentialities. In our scheme, on receiving a ciphertext, the proxy has the rights to perform the following three tasks according to the message confidentiality levels of the sender's intention: (1) to decrypt the ciphertext on behalf of the original decryptor; (2) to re-encrypt the ciphertext such that another user who is designated by the original decryptor can learn the message; (3) to do nothing except for forwarding the ciphertext to the original decryptor. Our scheme supports revocability in the sense that it allows proxy's decryption and re-encryption rights to be revoked even during the valid period of the proxy key without changing the original decryptor's public information. We prove that our proposal is indistinguishable against chosen identity and plaintext attacks in the standard model. We also show how to convert it into a system against chosen identity and ciphertext attacks by using the Fujisaki-Okamoto transformation.
Proxy cryptosystems are classified into proxy decryption systems and proxy re-encryption systems on the basis of a proxy's role. In this paper, we propose an ID-based proxy cryptosystem with revocability and hierarchical confidentialities. In our scheme, on receiving a ciphertext, the proxy has the rights to perform the following three tasks according to the message confidentiality levels of the sender's intention: (1) to decrypt the ciphertext on behalf of the original decryptor; (2) to re-encrypt the ciphertext such that another user who is designated by the original decryptor can learn the message; (3) to do nothing except for forwarding the ciphertext to the original decryptor. Our scheme supports revocability in the sense that it allows proxy's decryption and re-encryption rights to be revoked even during the valid period of the proxy key without changing the original decryptor's public information. We prove that our proposal is indistinguishable against chosen identity and plaintext attacks in the standard model. We also show how to convert it into a system against chosen identity and ciphertext attacks by using the Fujisaki-Okamoto transformation.
A proxy re-encryption scheme allows Alice to temporarily delegate the decryption rights to Bob via a proxy. Alice gives the proxy a re-encryption key so that the proxy can convert a ciphertext for Alice into the ciphertext for Bob. Recently, ID-based proxy re-encryption schemes are receiving considerable attention for a variety of applications such as distributed storage, DRM, and email-forwarding system. And a non-interactive identity-based proxy re-encryption scheme was proposed for achieving CCA-security by Green and Ateniese. In the paper, we show that the identity-based proxy re-encryption scheme is unfortunately vulnerable to a collusion attack. The collusion of a proxy and a malicious user enables two parties to derive other honest users' private keys and thereby decrypt ciphertexts intended for only the honest user. To solve this problem, we propose two ID-based proxy re-encryption scheme schemes, which are proved secure under CPA and CCA in the random oracle model. For achieving CCA-security, we present self-authentication tag based on short signature. Important features of proposed scheme is that ciphertext structure is preserved after the ciphertext is re-encrypted. Therefore it does not lead to ciphertext expansion. And there is no limitation on the number of re-encryption.
In an identity-based proxy re-encryption scheme, a semi-trusted proxy can convert a ciphertext under Alice's public identity into a ciphertext for Bob.The proxy does not know the secret key of Alice or Bob, and also does not know the plaintext during the conversion.In identity-based proxy re-encryption, the collusion of the proxy and a delegatee may result in the decryption of ciphertext for delegator.In this paper, we present a new scheme which can avoid the collusion of proxy and delegatee.Our scheme improves the scheme of Chu and Tzeng while inheriting all useful properties such as unidirectionality and non-interactivity.In our scheme, we get the security by using added secret parameter and change the secret key and re-encryption key.Our sheme is secure against Chosen-Ciphertext Attack (CCA) and collusion attack in the standard model.
In proxy re-encryption (PRE), a semi-trusted proxy can convert a ciphertext originally intended for Alice into one which can be decrypted by Bob, while the proxy cannot know the corresponding plaintext. PRE can be classified as single-use PRE and multi-use PRE according to the times the ciphertext can be transformed. In multi-use PRE schemes, the ciphertext can be transformed from A to B and to C and so on. In CCS’09 (post session), Wang et al. proposed a multi-use unidirectional CCA-secure proxy re-encryption scheme. Unfortunately, we show their proposal is not CCA-secure in the corresponding security models by giving concrete attacks. In 2010, Ren et al. proposed a hierarchical identity-based proxy re-encryption scheme without random oracles, and claimed their scheme was also multi-use and CCA-secure, we also show their scheme is not secure.
The notion of proxy cryptography is very useful in cases when one user needs to perform sensitive operation without holding the necessary secret key. Recently, proxy reencryption scheme received much attention due to its application in information storing, secure email, etc. In this paper, we propose a proxy re-encryption used to divert ciphertext from one group to another. The scheme is bidirectional and any member can independently decrypt the ciphertexts encrypted to its group. We discuss the security of the proposed scheme and show that our scheme withstands chosen ciphertext attack in standard model.
Proxy re-encryption (PRE) allows a proxy to use the reencryption keys to convert ciphertexts encrypted under the delegators' public keys into ones that can be decrypted using the delegatees' secret keys, without revealing the underlying plaintext. However, PRE is limited to an all-or-nothing re-encryption, that is, the delegatee can either decrypt the re-encrypted ciphertext to obtain the whole message, or it learns nothing about the underlying message. To address this limitation, Zhou et al. (Asiacrypt 2023) proposed a new variant of PRE called fine-grained PRE (FPRE). FPRE extends PRE to support fine-grained re-encryption, allowing the proxy to convert any ciphertext encrypting a message m to a new ciphertext encrypting f (m) with a fine-grained re-encryption key for any function f. This enables the delegator to flexibly control the sharing of messages by specifying functions. Unfortunately, their scheme achieves only CPA security and requires a common random/reference string (CRS) during key generation. This requirement may weaken the syntax of (F)PRE, which allows users to independently generate their own keys. In this paper, we propose a new FPRE for bounded linear functions that we prove is CCA1 secure and does not require a CRS, based on the Learning with Errors (LWE) assumption. When the linear function is set to be the identity function, our construction immediately yields the first PRE scheme without a CRS, with CCA1 security under adaptive corruptions in the standard model. The main technique behind our construction is the design of a deterministic preimage computation algorithm for the decryption oracle in the security proof. To demonstrate the broader applicability of our proposed algorithm beyond the context of (F)PRE, we use it to prove the selective-identity CCA1 security of Agrawal et al.'s (Eurocrypt 2010) identity-based encryption (IBE) scheme, which was previously only known to be selective-identity CPA secure.
In a proxy re-encryption scheme, a semi-trusted proxy converts a ciphertext for Alice into a ciphertext for Bob without seeing the underlying plaintext. A number of solutions have been proposed in public key settings. Hierarchical identity-based cryptography is a generalization of identity-based encryption that mirrors an organizational hierarchy, which allows a root private key generator to distribute the workload by delegating private key generation and identity authentication to lower-level private key generators. In this paper, we propose a hierarchical identity-based proxy re-encryption (HIBPRE) scheme which achieves IND-PrID-CCA2 security without random oracles. This is the first HIBPRE scheme up to now, and our scheme satisfies unidirectionality, non-interactivity and permits multiple re-encryptions.