Security model

Assets

Asset	Where	Protection
User wallet secrets	User.stellarSecretEnc	AES-256-GCM with ENCRYPTION_KEY
Escrowed XLM	Per-trade Stellar accounts	Master key disabled; platform co-signer only signs release to buyer or merge to seller
Platform co-signer	PLATFORM_SECRET_KEY env	Secrets manager; never logged
Identity data	User.kyc*	Only last 4 digits + keyed fingerprint stored
Passwords	User.passwordHash	bcrypt, 12 rounds
Sessions	JWT	HS256, 7-day expiry, user reloaded each request

Key compromise scenarios

Compromised	Impact	Mitigation
Database only	Encrypted secrets are useless without ENCRYPTION_KEY	Keep the key outside the database and backups
ENCRYPTION_KEY + database	Attacker can decrypt custodial wallets	Split storage, restrict access, rotate with re-encryption
PLATFORM_SECRET_KEY	Attacker can co-sign open escrows	Keep offline/HSM in production; monitor escrow operations; limit open escrow value

Application controls

• Zod validation on every input; parsed values only
• Rate limiting on auth, KYC and withdrawals
• Helmet security headers; x-powered-by disabled
• CORS restricted to CLIENT_URL
• JSON body limit 100 KB; uploads 5 MB and content-sniffed
• Timing-safe login responses; no user enumeration on password reset
• Atomic trade transitions prevent double release/refund
• Session users never include secrets or hashes

Known gaps (tracked)

• No 2FA yet (Phase 2)
• No dispute resolution flow — admins can't settle a PAID trade whose seller is unresponsive (Phase 2)
• Upload URLs are unauthenticated if leaked
• Rate limit store is in-memory per instance

Reporting vulnerabilities

Email security@nexlm.app. Please don't open public issues for security reports.