Data sharing is the main way to give full play to the value
of data itself. For small amount of data, the generator directly
sends the data to the requester peer-to-peer. With the rapid
growth of user data scale, the cost of local storage, data
security and data services grow exponentially. Cloud storage
provides a cost-efficient way for data delegation, and can
deploy stable data services. However, data security turns out
to be critical since most delegated data are in plaintext. The
build-in access control and domain separation methods are
major security strategies rather than data encryption. In recent
years, several serious data leaking accidents are caused by
improper security setting in the cloud, explaining that all
sensitive data should be in fine-grained, ciphertext manner,
and the data sharing via cloud storage should be authenticated
by a safe key, which cannot be used to decrypt the original
data. Blaze et al. [1] announced the idea of proxy re-
encryption, where a proxy can convert a confidential message
encrypted by using one partyβs public key into another one
that can be decrypted by using another partyβs private key.
The Umbral project [2] propose a threshold-based proxy re-
encryption scheme following a key encapsulation mechanism
(KEM) approach. The data owner can delegate decryption
rights to any receiver for any ciphertext in intended grained,
through a re-encryption process performed by a set of N semi-
honest proxies. Under the threshold of (t, N), where at least t
out of N of these proxies participate by performing re-
encryption algorithm, and then the receiver is able to combine
these independent segments and decrypt the original message
using its private key.
In PRE schemes, the directionality and the transitivity are
critical characters for applications. In a bi-directional PRE
scheme, the re-encryption phase is reversible, which means
the proxy can use the same re-encryption key to re-encrypt
each ciphertext both from owner to receiver and receiver to
owner with zero knowledge to data. In this case, both the
owner and receiver must combine their secret keys to produce
the re-encryption key. The bi-directionality seems to bridge a
peer-to-peer data sharing relationship. On the other hand, a
unidirectional PRE means the proxy delegation is one-way.
The ciphertext can be re-encrypted from owner to receiver,
but not the reverse. Thus, the construction of re-encryption
key only requires the ownerβs secret key. As for the
transitivity character, it represents the times for a ciphertext
to be re-encrypted. Ciphertext that can be re-encrypted only
once refers to single-hop PRE scheme, while multi-hop PRE
scheme enables the unlimited-time transitivity of ciphertext
re-encryption. The permutation of types for PRE characters
allows a various of applications such as e-mail forwarding,
authorization transfer, and data distribution. However, most
PRE schemes require a centralized proxy to perform data
storage and re-encryption, which means the proxy turns out
to be the keypath of the system and has to handle all the
delegations.
We improve the system soundness by implementing a
decentralized proxy network, where each participant
possesses a key-share of re-encryption key, and can execute
the re-encrypt task separately. Meanwhile, our proposal also
involves multi-hop PRE to release data owner from re-
encryption key generation upon an authorized request, which
frequently calls the ownerβs secret key and has to be executed
locally at owner side. The distributed PRE scheme adopts
secret sharing and multi-hop re-encryption to provide reliable
and full delegation service. Moreover, this proposal achieves
a reshare function to process the continuing node variations
in decentralized environment.
The prototype of safe PRE scheme is introduced by
Mambo and Okamoto [3] by using the partial decryptions,
without offering any extra security benefits for delegatorβs
secret key. Their proposal aggregated the decrypt and re-
encrypt into an atomic execution by taking the re-encryption
key as input, during which the cipher-state original data is
never revealed. Later, Blaze, Bleumer and Strauss proposed
the ciphertext conversion algorithm [2] on the ElGamal
This work was supported by a grant from the Core Technology Research
and Development Program of Lin-Gang.
A Multi-hop and Distributed Proxy Re-encryption Scheme with
Dynamic Re-sharing
1JUNTAO CAO, 1XIN PEI, 2XIAOCHUAN WU
1The Idol Group, Shanghai, CHINA
2School of Computer Science, Fudan University, Shanghai, CHINA
Abstract: In order to achieve delegated data sharing, a reliable proxy is required for both data storage and execution of the
delegated authorization. The PRE scheme is a representative technique for delegating data sharing, which involves a single
proxy to transform the encryption by reencrypting algorithm with an auth-key, without knowing any knowledge about the
plaintext. However, most PRE schemes are performed in a centralized environment, which means the system will crash
upon the proxy is off-work. In this paper, we optimize the PRE scheme from two aspects. Firstly, the proxy acting as the
key path is decentralized in a thresholdbased network, which will provide continuous PRE service when any t out of N
nodes work. Moreover, considering the flexible entry and exit mechanism of the decentralized nodes, this proposal presents
a re-share algorithm to ensure N live nodes. Secondly, we adopt the multi-hop re-encryption strategy for transitivity of
ciphertext, so that the data owner is released from re-encryption key generation task upon user requests, and the authorized
delegatees are able to retransform the encryption to designated users by using its own secret key.
Keywords: data sharing, proxy re-encryption, KMS, reshare, multi-hop, transitivity
Received: April 25, 2021. Revised: July 15, 2022. Accepted: August 12, 2022. Published: September 13, 2022.
1. Introduction
2. Related Works
WSEAS TRANSACTIONS on INFORMATION SCIENCE and APPLICATIONS
DOI: 10.37394/23209.2022.19.18
Juntao Cao, Xin Pei, Xiaochuan Wu