# Summary: Fine-Grained Access Control using ABE and ABS

This is a summary of the paper “Realizing Fine-Grained and Flexible Access Control to Outsourced Data with Attribute-Based Cryptosystems” by Zhao et al. (ISPEC 2011). All the information is based on my understanding of the paper and may contain inaccuracies.

## Overview

The authors explore the advantages of Attribute-Based Encryption (ABE) and Attribute-Based Signature (ABS) in the context of write access control. In scenarios covering outsourcing using cloud server, although read access (“one-write-many-read”) is covered by a number of works, the case of multiple writers (“many-write-many-read”) is still not fully explored.

The system assumes an untrusted (honest-but-curious) server, and that users can collude with the server and between themselves. A trusted attribute authority (TA) is used to manage attributes and the related keys in a honest manner.

To realize read access, each resource is encrypted with a key that depends on the access policy for that particular resource. The access policy is represented by an access tree, $T_{decrypt}$, whose internal nodes are threshold gates and leaves are attributes. In order to decrypt an resource, a user’s attributes must satisfy the resource’s access tree.
Each encrypted resource is signed using ABS, thus generating another access tree for the signature: $T_{sign}$. Therefore, each resource has two access trees ($T_{decrypt}$ and $T_{sign}$).

When creating a resource, the data owner defines the access trees representing the policies for that resource, and uploads the encrypted resource (ciphertext) and its signature to the server. The period of validity of the signature is also uploaded to prevent the use of old but valid signatures. When users want to access a resource, they verify their attributes with the TA and receive the decryption keys. To update a resource, the user must be able to generate signature which will be checked by the server. If the signature is valid, the server accepts the upload.

## Contributions

The proposal offers a cryptographically-enforced method to control the write access to data. By using ABE and ABS, it is possible to reduce the complexity of the key management while offering attribute-level access.

The authors claim end-to-end data confidentiality, protection against collusion and replay attacks, and integrity verification by leveraging ABS.

In addition, they present a complexity analysis of each step and also make comparisons with related works. The comparison considers data access granularity, key management method, presence of integrity checks, consideration of write access control, and the availability of implementation.

## Limitations

The implementation of the solution is considered part of future work, and it is not presented in the paper. The use of a trusted authority can be seen as a disadvantage, since it increases the number of parties in the system and can be seen as an extra component which can be compromised in reality.