GDPR Engineering Skill

Actionable GDPR reference for engineers, architects, DevOps, and tech leads. Inspired by CNIL developer guidance and GDPR Articles 5, 25, 32, 33, 35.

Golden Rule: Collect less. Store less. Expose less. Retain less.

For deep dives, read the reference files in references/:

• references/data-rights.md — user rights endpoints, DSR workflow, RoPA
• references/security.md — encryption, hashing, secrets, anonymization
• references/operations.md — cloud, CI/CD, incident response, architecture patterns

---

1. Core GDPR Principles (Article 5)

Principle	Engineering obligation
Lawfulness, fairness, transparency	Document legal basis for every processing activity in the RoPA
Purpose limitation	Data collected for purpose A MUST NOT be reused for purpose B without a new legal basis
Data minimization	Collect only fields with a documented business need today
Accuracy	Provide update endpoints; propagate corrections to downstream stores
Storage limitation	Define TTL at schema design time — never after
Integrity & confidentiality	Encrypt at rest and in transit; restrict and audit access
Accountability	Maintain evidence of compliance; RoPA ready for DPA inspection at any time

---

2. Privacy by Design & by Default

MUST

• Add CreatedAt, RetentionExpiresAt to every table holding personal data at creation time.
• Default all optional data collection to off. Users opt in; they never opt out of a default-on setting.
• Conduct a DPIA before building high-risk processing (biometrics, health data, large-scale profiling, systematic monitoring).
• Update the RoPA with every new feature that introduces a processing activity.
• Sign a DPA with every sub-processor before data flows to them.

MUST NOT

• Ship a new data collection feature without a documented legal basis.
• Enable analytics, tracking, or telemetry by default without explicit consent.
• Store personal data in a system not listed in the RoPA.

---

3. Data Minimization

MUST

• Map every DTO/model field to a concrete business need. Remove undocumented fields.
• Use separate DTOs for create, read, and update — never reuse the same object.
• Return only what the caller is authorized to see — use response projections.
• Mask sensitive values at the edge: return ****1234 for card numbers, never the full value.
• Exclude sensitive fields (DOB, national ID, health) from default list/search projections.

MUST NOT

• Log full request/response bodies if they may contain personal data.
• Include personal data in URL path segments or query parameters (CDN logs, browser history).
• Collect dateOfBirth, national ID, or health data without an explicit legal basis.

---

4. Purpose Limitation

MUST

• Document the purpose of every processing activity in code comments and in the RoPA.
• Obtain a new legal basis or perform a compatibility analysis before reusing data for a secondary purpose.

MUST NOT

• Share personal data collected for service delivery with advertising networks without explicit consent.
• Use support ticket content to train ML models without a separate legal basis and user notice.

---

5. Storage Limitation & Retention

MUST

• Every table holding personal data MUST have a defined retention period.
• Enforce retention automatically via a scheduled job (Hangfire, cron) — never a manual process.
• Anonymize or delete data when retention expires — never leave expired data silently in production.

Recommended defaults

Data type	Max retention
Auth / audit logs	12–24 months
Session / refresh tokens	30–90 days
Email / notification logs	6 months
Inactive user accounts	12 months after last login → notify → delete
Payment records	As required by tax law (7–10 years), minimized
Analytics events	13 months

SHOULD

• Add RetentionExpiresAt column — compute at insert time.
• Use soft-delete (DeletedAt) with a scheduled hard-delete after the erasure request window (30 days).

MUST NOT

• Retain personal data indefinitely "in case it becomes useful later."

---

6. API Design Rules

MUST

• MUST NOT include personal data in URL paths or query parameters.
  • GET /users/{userId}
• Authenticate all endpoints that return or accept personal data.
• Extract the acting user's identity from the JWT — never from the request body.
• Validate ownership on every resource: if (resource.OwnerId != currentUserId) return 403.
• Use UUIDs or opaque identifiers — never sequential integers as public resource IDs.

SHOULD

• Rate-limit sensitive endpoints (login, data export, password reset).
• Set Referrer-Policy: no-referrer and an explicit CORS allowlist.

MUST NOT

• Return stack traces, internal paths, or database errors in API responses.
• Use Access-Control-Allow-Origin: * on authenticated APIs.

---

7. Logging Rules

MUST

• Anonymize IPs in application logs — mask last octet (IPv4) or last 80 bits (IPv6).
  • 192.168.1.xxx
• MUST NOT log: passwords, tokens, session IDs, credentials, card numbers, national IDs, health data.
• MUST NOT log full request/response bodies where PII may be present.
• Enforce log retention — purge automatically after the defined period.

SHOULD

• Log events not data: "User {UserId} updated email" not "Email changed from a@b.com to c@d.com".
• Use structured logging (JSON) with userId as an internal identifier, not the email address.
• Separate audit logs (sensitive access, admin actions) from application logs — different retention and ACLs.

---

8. Error Handling

MUST

• Return generic error messages — never expose stack traces, internal paths, or DB errors.
  • "Column 'email' violates unique constraint on table 'users'"
  • "A user with this email address already exists."
• Use Problem Details (RFC 7807) for all error responses.
• Log the full error server-side with a correlation ID; return only the correlation ID to the client.

MUST NOT

• Include file paths, class names, or line numbers in error responses.
• Include personal data in error messages (e.g., "User john@example.com not found").

---

9. Encryption (summary — see references/security.md for full detail)

Scope	Minimum standard
Standard personal data	AES-256 disk/volume encryption
Sensitive data (health, financial, biometric)	AES-256 column-level + envelope encryption via KMS
In transit	TLS 1.2+ (prefer 1.3); HSTS enforced
Keys	HSM-backed KMS; rotate DEKs annually

MUST NOT allow TLS 1.0/1.1, null cipher suites, or hardcoded encryption keys.

---

10. Password Hashing

MUST

• Use Argon2id (recommended) or bcrypt (cost ≥ 12). Never MD5, SHA-1, or SHA-256.
• Use a unique salt per password. Store only the hash.

MUST NOT

• Log passwords in any form. Transmit passwords in URLs. Store reset tokens in plaintext.

---

11. Secrets Management

MUST

• Store all secrets in a KMS: Azure Key Vault, AWS Secrets Manager, GCP Secret Manager, or HashiCorp Vault.
• Use pre-commit hooks (gitleaks, detect-secrets) to prevent secret commits.
• Rotate secrets on developer offboarding, annual schedule, or suspected compromise.

.gitignore MUST include: .env, .env.*, *.pem, *.key, *.pfx, *.p12, secrets/

MUST NOT

• Commit secrets to source code. Store secrets as plain-text environment variable defaults.

---

12. Anonymization & Pseudonymization (summary — see references/security.md)

• Anonymization = irreversible → falls outside GDPR scope. Use for retained records after erasure.
• Pseudonymization = reversible with a key → still personal data, reduced risk.
• When erasing a user, anonymize records that must be retained (financial, audit) rather than deleting them.
• Store the pseudonymization key in the KMS — never in the same database as the pseudonymized data.

MUST NOT call data "anonymized" if re-identification is possible through linkage attacks.

---

13. Testing with Fake Data

MUST

• MUST NOT use production personal data in dev, staging, or CI environments.
• MUST NOT restore production DB backups to non-production without scrubbing PII first.
• Use synthetic data generators: Bogus (.NET), Faker (JS/Python/Ruby).
• Use @example.com for all test email addresses.

---

14. Anti-Patterns

Anti-pattern	Correct approach
PII in URLs	Opaque UUIDs as public identifiers
Logging full request bodies	Log structured event metadata only
"Keep forever" schema	TTL defined at design time
Production data in dev/test	Synthetic data + scrubbing pipeline
Shared credentials across teams	Individual accounts + RBAC
Hardcoded secrets	KMS + secret manager
Access-Control-Allow-Origin: * on auth APIs	Explicit CORS allowlist
Storing consent with profile data	Dedicated consent store
PII in GET query params	POST body or authenticated session
Sequential integer IDs in public URLs	UUIDs
"Anonymized" data with quasi-identifiers	Apply k-anonymity, test linkage resistance
Mixing backup regions outside EEA	Explicit region lockdown on backup jobs

---

15. PR Review Checklist

Data model

• Every new PII column has a documented purpose and retention period.
• Sensitive fields (health, financial, national ID) use column-level encryption.
• No sequential integer PKs as public-facing identifiers.

API

• No PII in URL paths or query parameters.
• All endpoints returning personal data are authenticated.
• Ownership checks present — user cannot access another user's resource.
• Rate limiting applied to sensitive endpoints.

Logging

• No passwords, tokens, or credentials logged.
• IPs anonymized (last octet masked).
• No full request/response bodies logged where PII may be present.

Infrastructure

• No public storage buckets or public-IP databases.
• New cloud resources tagged with DataClassification.
• Encryption at rest enabled for new storage resources.
• New geographic regions for data storage are EEA-compliant or covered by SCCs.

Secrets & CI/CD

• No secrets in source code or committed config files.
• New secrets added to KMS and secrets inventory document.
• CI/CD secrets masked in pipeline logs.

Retention & erasure

• Retention enforcement job or policy covers new data store or field.
• Erasure pipeline updated to cover new data store.

User rights & governance

• Data export endpoint includes any new personal data field.
• RoPA updated if a new processing activity is introduced.
• New sub-processors have a signed DPA and a RoPA entry.
• DPIA triggered if the change involves high-risk processing.

---

Golden Rule: Collect less. Store less. Expose less. Retain less.

Every byte of personal data you do not collect is a byte you cannot lose, cannot breach, and cannot be held liable for.

---

Inspired by CNIL developer GDPR guidance, GDPR Articles 5, 25, 32, 33, 35, ENISA, OWASP, and NIST engineering best practices.