Outsourcing the Re-encryption Key Generation: Flexible Ciphertext-Policy Attribute-Based Proxy Re-encryption

In this paper, we introduce a new proxy re-encryption (PRE) in that the re-encryption key generation can be outsourced, in attribute-based encryption. We call this new notion

flexible

ciphertext-policy attribute-based proxy re-encryption (flexible CP-AB-PRE). In ordinary PRE scheme, re-encryption keys are generated by using user’s decryption key and an access structure. So, whenever the access structure is changed, a PRE user has to generate new different re-encryption keys. In order to overcome this disadvantage of the ordinary PRE, the re-encryption key generation of the proposed scheme is divided into the following two steps. First, a user generates

universal

re-encryption key

urk

S

which indicates delegator’s attributes set

S

. Second, an authority who has re-encryption secret key

rsk

generates ordinary re-encryption key

${\sf rk}_{S\rightarrow{\Bbb M}'}$

by using

urk

S

,

rsk

, and an access structure

${\Bbb M}'$

. The user has only to generate single

urk

S

for all re-encryption keys. By this “outsourcing”, the task of re-encryption key generation for a user is reduced only to generate one

urk

S

. Furthermore, supposing a Private Key Generator (PKG) generates

urk

simultaneously at the time of decryption key generation, the load of re-encryption key generation for users almost vanishes.