As a security team, I need offline authentication to be secure

[guimard]

User Story

As the security team,
I need offline authentication to be cryptographically secure,
So that cached credentials cannot be easily compromised.

Acceptance Criteria

• Argon2id for password hashing (memory-hard, resists GPU attacks)
• AES-256-GCM for data encryption (authenticated encryption)
• PBKDF2 for key derivation (standard, proven)
• Secure random number generation for salts/IVs
• Cache files with restricted permissions (0600)
• Tamper detection via GCM auth tag
• Unit tests for crypto operations

Technical Details

Crypto Specifications

Password Hashing (Argon2id)

• Memory: 19 MiB (19456 KiB)
• Iterations: 2
• Parallelism: 1
• Output: 32 bytes
• OWASP recommended parameters

Key Derivation (PBKDF2-HMAC-SHA256)

• Iterations: 100,000
• Salt: 16 bytes (random per user)
• Output: 32 bytes (AES-256 key)

Encryption (AES-256-GCM)

• IV: 12 bytes (random per write)
• Auth tag: 16 bytes
• Already used by secret_store module

Security Mitigations

Threat	Mitigation
GPU/ASIC attacks	Argon2id memory-hard
Cache theft	Encrypted, unusable without password
Tampering	GCM authentication tag
Stale creds	Configurable expiration
Weak passwords	Minimum length enforcement
File access	0600 permissions, root only

Limitations

• Compromised local password = access to cached data
• No immediate revocation (must wait for expiration)
• MFA not available offline
• Should use short expiration for sensitive data

Tests to Implement

• test_offline_cache.c
  • Hash/verify password
  • Encrypt/decrypt user data
  • Expiration checking
  • Tamper detection
  • Invalid password rejection

Component

Repo: llng-pam-module (C)
Files: tests/test_offline_cache.c (new)