Research On Key Cryptographic Technologies Of Access Control Over Outsourced Data In Cloud Computing Environments

As a promising computing pattern, cloud computing possesses strong computing ability, on-demand services, high reliability, low investment in IT infrastructure and other characteristics. and has attracted more and more attentions in academia and industry. Cloud computing is facing challenges from security concerns, and frequently-occurring cloud computing security incidents severely affect the confidence of using cloud computing services.Observe that in many cloud computing services, regardless of how the cloud com-puting platforms are built, data storing and sharing are the core functions of cloud computing, and also the most vulnerable link that could be attacked. This is due to the fact that once users outsource their data to cloud service providers, they lose the physical control of the data, and they have reasons to worry about illegal accesses to their data stored on the cloud servers. Cryptographical encryption has been considered as a fundamental approach to protect data privacy, in that it gives cloud users the power to enable access control on outsourced data, and allows users to formulate access policies for the data to prevent illegal accesses from unauthorized users and malicious cloud service providers. The existing solutions that leverage cryptographic encryption to enforce access control over outsourced data suffer from the problems of key manage-ment, key leakage, the asymmetric computing ability among access devices and so on. This dissertation addresses these problems and has the contributions as follows.1. In view of the requirements of fine-grained access control over outsourced data in cloud computing, an analysis for attribute-based encryption (ABE) is conducted. This analysis demonstrates the difficulty of key distribution and the leakage of users' sensitive attributes in the applications of ABE in cloud computing. To address such issues, a new cryptographic primitive referred to as hierarchical attribute-based encryp-tion (HABE) is proposed. HABE provides unrestricted key delegation functionality, compared to the limited key delegation in traditional attribute-based encryption. Due to this advantageous functionality, HABE greatly reduces the burden of key generation center and can be applied to cloud computing to share sensitive files. According to the classification of ABE, ciphertext-policy HABE (CP-HABE) and key-policy HABE (KP-HABE) are defined and their security models are accordingly formalized. A CP-HABE scheme and a KP-HABE scheme are constructed and proved to be fully secure (adaptively secure) in the standard model. The CP-HABE scheme is especially suited for applications where the key generator cannot directly generate keys for users unwill-ing to reveal sensitive personal attributes to untrusted third party; and the KP-HABE is especially suitable to cloud computing systems where a priori specification of keys' access policies is too rigid or simply unavailable.2. In the use of attribute-based encryption to achieve fine-grained access control on outsourced data, there is a subtle issue that the access credentials could be leaked. To address such problems, a leaked access credentials tracing model is proposed. This mod-el, incorporating the advantages of attribute-based encryption, allows users to enforce flexible access control over outsourced data in that the users do not need to specify the identities of authorized visitors in encrypting data. Further, by introducing the tracing mechanism into access control, the leaked access credentials tracing model can find out the leaked credentials, which provides evidences for countability and legal suits if nec-essary. A leaked access credentials tracing scheme with short ciphertexts is proposed to trace in black-box manner the leaked credentials used in illegal accesses. A formal security analysis shows the semantic security and traceability of the scheme. Experi-mental analysis shows that the introduced tracing mechanism does not affect the most frequent data outsourcing or access procedures.3. An analysis is conducted to illustrate the challenges of access control on out-sourced data in mobile cloud environments. A content-based revocable access control model is proposed to address the problem of access control failure caused by lost mobile devices and the problem of limited computing capability of mobile devices. The mod-el introduces a proxy to reduce the decryption burden of mobile devices in accessing outsourced data; when there are mobile devices compromised, the model can revoke the access credentials involved in the compromised devices to protect data security. The revocation procedure can be run without any help of non-revoked users, which enhances user experience and system efficiency. By using attribute-based encryption, a provably secure content-based revocable access control scheme is proposed to achieve fine-grained access control on outsourced data. Moreover, the proposed scheme takes the keywords of data contents rather than users'personal information as attributes, hence users privacy is well protected even the access credentials involved in mobile devices are compromised.4. For the asymmetric computing resources of access devices in cloud computing, an analysis is conducted to demonstrate the computing requirements of identity-based broadcast encryption (IBBE) and identity-based encryption (IBE) applied to secure out-sourced data. To address the conflict between the high complexity of IBBE and the low computing ability of mobile devices, an asymmetric cross-cryptosystem re-encryption model is proposed. This model employs IBBE for resource-adequate computing de-vices (e.g., desktop computers, servers) to provide feature-rich access control, allowing data owners to specify a group of users to access their data. In the meanwhile, the model provides simple-yet-efficient IBE for resource-limited computing devices (e.g., mobile phones, tablets) to protect data security. The asymmetric cross-cryptosystem re-encryption model provides ciphertext transformation mechanism from complicated-yet-versatile IBBE to simple-yet-efficient IBE, so that resource-limited devices can use their own IBE secret keys to decrypt IBBE ciphertexts. A provably secure asymmetric cross-cryptosystem re-encryption scheme is constructed to reduce the costs of resource-limited devices in accessing the data encrypted by complicated IBBE, which resolves the problem of sharing data in devices with asymmetric computing capability.