fix: resolve security issues #800 #803 #804 #806

[JudeDaniel6]

Summary

Resolves all four open security issues assigned to @JudeDaniel6.

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#800 — EncryptionService uses SHA-256 instead of a proper KDF

Problem: Key derivation via crypto.createHash('sha256') has no work factor, making brute-force trivial.

Fix:

• Replace with crypto.scryptSync(secret, salt, 32, { N: 16384, r: 8, p: 1 })
• Add ENCRYPTION_SALT env var (startup fails fast if absent)
• Document migration path in commit message
• Unit tests verify scrypt is used (and SHA-256 is not)

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#803 — AuthService logout does not blacklist the current access token

Problem: logout() only nulls the refresh token; the access token stays valid until natural expiry.

Fix:

• Add jti claim to access token in generateTokens()
• Update logout(userId, accessToken?) to decode the token, extract jti, and call tokenBlacklistService.addToBlacklist(jti, remainingTtlMs)
• Update AuthController.logout to extract the Bearer token and pass it to the service
• Tests verify immediate invalidation on logout

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#804 — GDPR erasure does not cascade-delete financial records and active sessions

Problem: eraseUserData() only nulls profile fields, leaving orphaned payments/enrollments/audit logs and usable sessions.

Fix:

• Revoke all active sessions immediately (before transaction)
• Wrap all erasure in a TypeORM DataSource.transaction()
• Anonymize payments, enrollment, audit_logs, and notifications tables inside the transaction
• GdprModule updated to import TypeOrmModule (provides DataSource)
• Tests: cascade anonymization, NotFoundException, idempotency (second call succeeds)

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#806 — FraudDetectionService has hardcoded thresholds and no behavioral signals

Problem: Three hardcoded threshold checks are trivially bypassed; no velocity or geo signals.

Fix:

• Introduce FraudSignalProvider interface for pluggable signal sources
• IpRateSignalProvider, NewDeviceSignalProvider, LargeTransactionSignalProvider — existing logic, now configurable via ConfigService
• VelocitySignalProvider — flags users exceeding FRAUD_MAX_PURCHASES_PER_HOUR (configurable)
• GeoAnomalySignalProvider — flags when purchase country differs from registration country
• All thresholds read from env vars (FRAUD_IP_RATE_THRESHOLD, FRAUD_NEW_DEVICE_AMOUNT_THRESHOLD, FRAUD_LARGE_TX_THRESHOLD, FRAUD_MAX_PURCHASES_PER_HOUR)
• Unit tests for each provider independently + the aggregate service

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Testing

All 4 target test suites pass (37 new/updated tests):

• encryption.service.spec.ts — 6 tests ✅
• auth.service.spec.ts — 10 tests ✅
• gdpr.service.spec.ts — 5 tests ✅
• fraud-detection.service.spec.ts — 16 tests ✅

Closes #800
Closes #803
Closes #804
Closes #806

[RUKAYAT-CODER]

kindly resolve conflict