PII Detection & Redaction — In Depth
The #1 security topic in FDE interviews. Know the pipeline cold.
Three Detection Methods
| METHOD | HOW | PROS | CONS |
|---|---|---|---|
| Regex / Pattern | SSN: \d{3}-\d{2}-\d{4}, Credit card: Luhn check | Fast, deterministic, zero cost | Misses context-dependent PII (names, addresses) |
| NER (Named Entity Recognition) | spaCy, Cloud DLP's 150+ infotypes | Catches names, orgs, locations | False positives on common names |
| LLM-based | Ask a cheap model: "List all PII in this text" | Catches novel patterns | Slow, expensive, non-deterministic |
The Gateway Pattern
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