## 6 Limitation and Future Works  
**Integrity verification mechanism:** Our proposed scheme ensures safe key distribution and secure data delivery through a layered structure that combines Kyber-based key encapsulation, QKD-based masking, and optional Shamir secret sharing. (1) Each AES key is never exposed in plaintext. It is encapsulated via PQC, while the corresponding secret key is QKD-masked and optionally fragmented, making unauthorized key reconstruction infeasible even under partial node compromise. (2) All encrypted subsets are transmitted only after local encryption with securely reconstructed keys, and communication occurs over pre-encrypted data without relying on secure channels, thereby preventing eavesdropping and key leakage in practice. Additionally, each subset is encrypted in CTR mode using a randomly generated nonce, which supports freshness and prevents ciphertext duplication across sessions. However, our study lacks cryptographic integrity verification to detect tampering or replay attacks on encrypted subsets or keys, such as when a compromised slave node encrypts and transmits manipulated data. Our plan for future work is to implement post-quantum digital signatures, such as Dilithium [61], to authenticate the origin and ensure tamper resistance of both key materials and encrypted subsets. To do this, each key and encrypted subset will be signed at the master and verified by each slave node, enabling full-path integrity checking from key generation to distributed encryption. In addition, we will bind the existing AES nonce to the digital signature or augment it with explicit timestamps to strengthen freshness guarantees and replay protection. **Non-elastic communication structure in cloud systems:** Our proposed scheme adopts MPI-based communication between a master and fixed slave nodes (e.g., number of nodes: 2, 4, 6, 8), which has shown effective in stable, high-throughput performance under controlled cloud workloads such as YCSB (e.g., workload A, B, C, D). (1) This structure enables efficient coordination and predictable communication by leveraging deterministic message passing, resulting in minimal runtime variability across nodes and strong throughput scalability as demonstrated in our evaluation. (2) While this design meets the requirements of large-scale file encryption in a fixed-resource environment, it provides limited elasticity in dynamic cloud-native platforms such as Kubernetes [90], where autoscaling, container migration, and fault-tolerant recovery are essential. Our plan for future work is to support elastic and fault-tolerant execution in cloud-native systems by modularizing communication and coordination using gRPC [91] and distributed key-value stores such as etcd. To achieve this, we will decouple encryption scheduling and metadata exchange from static MPI bindings, enabling seamless adaptation to elastic cloud infrastructure while maintaining the stable performance characteristics of the existing design. **Centralized decryption and performance-security trade-off:** Our decryption strategy is performed at the master node using the AES key recovered through Kyber decapsulation and QKD unmasking, with both operations strictly confined to the master node to minimize key exposure and ensure coordinated trust. Although all slave nodes are capable of reconstructing the AES key from QKD-masked secret shares, only a selected slave nodes retain the key during execution, and the rest discard it immediately after encryption. As a result, decryption cannot be performed by slave nodes alone, which effectively eliminates unauthorized decryption paths and strengthens security against node compromise and replay-based attacks. However, centralizing decryption introduces a performance bottleneck. As the number of slave nodes increases, the master node receives more encrypted subsets via MPI. For large files, this leads to concentrated communication, index-based reordering, and sequentialdecryption, creating I/O and compute contention that limits scalability.