On Decentralized Cloud Storage Security and an Efficient Post-Quantum Encryption Scheme

In this thesis, we address three main security problems related to cryptography and
cloud storage. To tackle the challenge posed by a quantum computer, we need
encryption that is resistant to quantum computers. This category of cryptography
is called post-quantum cryptography. In the first paper, we solve a challenge
in one of the lattice-based cryptographic protocols called Nth-degree Truncated
polynomial Ring Unit (NTRU) namely how to reduce the key size while keeping
the desired security level. We propose a solution that reduces the key size significantly.Our proposed solution allows a practical implementation of NTRU with
fast polynomial multiplications.
Next, we move to solve a long-standing problem arising in any cloud storage
namely the reduction of storage cost of redundant data and maintaining security
and privacy at the same time. Data deduplication is considered to be a tool that
can be used to eliminate redundant data and store only one of its copies. But
data deduplication also means that the file cannot go through client-side encryption
which opens up new possibilities of adversarial threats. In order to tackle this
challenge, we propose a new architecture where we perform client-side deduplication
along with dynamic erasure protection by introducing a third-party assistant.
We also performed an erasure analysis to quantitatively analyze the probability of
loss of a file when a large number of replicas are deleted at random.
Finally, we shift our interest to Decentralized Cloud Storage (DCS). DCS
solutions like Filecoin, Storj, and Arweave are gaining more popularity in the Web 3.0 ecosystem. But they are not without challenges. The robustness of the DCS
protocols remains a challenging ground. Since the file in a DCS protocol is stored
in a decentralized manner among different nodes, a Distributed Denial of Service
(DDoS) attack would render the system vulnerable to data loss. Therefore, it is
important to analyze the robustness of decentralized architecture against DDoS
attacks. In our last paper, we perform a similar erasure analysis to that of the
second paper but in a decentralized setup, where the adversary aims to disrupt
the system by deleting a file from the network. Storj is one of the leading players
in the DCS space. We have created an adversarial model capturing the real Storj
network scenario and simulated our model using real-time data obtained from the
Storj network. We obtain resource budget figures for DDoS on Storj using our
model. Also, we propose a better parametric value for the erasure piece distribution
in Storj which suits well when there is a large portion of so-called unvetted nodes
in the network.