Ghost Vectors: Soft-Deleted Embeddings Remain Reconstructible in HNSW Vector Databases
This study addresses a critical compliance gap in HNSW-based vector databases: their soft deletion mechanisms fail to meet stringent data erasure requirements under regulations such as GDPR and HIPAA, as sensitive information—including personally identifiable information (PII)—can be reconstructed with near-perfect accuracy (empirically achieving 100% PII recovery) from residual embeddings. To mitigate this risk, the authors propose Epoch Key Rotation, a novel approach that encrypts embeddings upon deletion and immediately discards the encryption key, thereby rendering the data irrecoverable. The method further incorporates ECDSA signatures to generate cryptographically verifiable proofs of deletion. Experimental results demonstrate that this technique reduces PII recovery rates to 0% while maintaining high efficiency, with per-record deletion latency of approximately 0.005 milliseconds, thus simultaneously ensuring regulatory compliance, performance, and auditability.