Security Model

Security Model

Threat Model

Assumptions

Trusted:

• Institution's private key security
• Database integrity (PostgreSQL)
• Cache integrity (Redis)

Untrusted:

• Network communication
• Client software
• External observers

Adversary Capabilities

• Network eavesdropping
• Man-in-the-middle attacks
• Client compromise
• Double-spend attempts

Cryptographic Guarantees

Confidentiality

• Network: TLS 1.3 encryption
• Database: At-rest encryption (operator configured)
• Keys: HSM storage recommended for production

Integrity

• Signatures: RSA-3072 provides existential unforgeability
• Transactions: PostgreSQL ACID guarantees
• Messages: SHA-256 collision resistance

Anonymity

• Unlinkability: Blind signatures prevent linking withdrawal to spending
• Privacy: Server cannot determine which client spent which token

Double-Spend Prevention

Two-Tier Protection

Redemption Request
       ↓
   Redis Check (fast)
       ↓
   PostgreSQL Check (reliable)
       ↓
   Atomic Insert

Properties:

• Redis: O(1) lookup, in-memory
• PostgreSQL: ACID guarantees, persistent
• Atomic: Both succeed or both fail

Race Condition Protection

PostgreSQL transaction isolation:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- Check and insert
COMMIT;

Attack Resistance

Forgery Attack

Attack: Create valid token without withdrawal
Defense: RSA signature unforgeability (3072-bit)

Double-Spend Attack

Attack: Spend same token twice
Defense: Atomic duplicate detection (Redis + PostgreSQL)

Linkability Attack

Attack: Link withdrawal to spending
Defense: Blind signature protocol unlinkability

Replay Attack

Attack: Reuse intercepted token
Defense: Serial number recorded, second use detected

Operational Security

Key Management

• Private key: HSM storage (production)
• Key rotation: Not implemented (future work)
• Backup: Encrypted offline storage

Monitoring

• Failed verification attempts
• Double-spend attempts
• Unusual withdrawal patterns
• Database replication lag

Audit Trail

All transactions logged with:

• Transaction ID
• Timestamp
• Amount
• Token serial (after redemption)