Vulnarex | Elite Cybersecurity Learning Platform

#Proving Identity at the Gateway#link

Now that we have the IKE framework, we need to decide how the two gateways (or a client and a gateway) prove their identities. In IPsec, this is the 'Authentication' part of the handshake. Depending on the method, your VPN is either a fortress or a screen door.

Pre-Shared Keys (PSK): The Simple but Risky Way

PSK is the most common method. Both sides are configured with the exact same secret string. While easy to deploy, PSK has a massive scaling problem: if one admin leaks the key, *every* tunnel using that key is compromised. Furthermore, in IKEv1 Aggressive Mode, this key is vulnerable to offline cracking.

info

💡 PSK should only be used for temporary lab setups or extremely small environments where keys can be rotated daily.

bash

# Conceptual PSK Configuration
set ike-psk "S3cur3_Pr0t0c0l_2025!"
set peer 203.0.113.10

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Using X.509 certificates (which we studied in Module 2) is the only secure way to scale IPsec. Each peer has its own private key and a certificate signed by a trusted CA. During the IKE handshake, the peers exchange certificates and sign a challenge. This eliminates the 'Shared Secret' problem and allows for easy revocation.

STRICT SECURE AUDIT RULE

⚠️ The biggest failure in certificate-based VPNs is neglecting the CRL (Certificate Revocation List) or OCSP. If a laptop is stolen, the certificate must be revoked, or the thief has a permanent key to the network.

Method	Scalability	Security Level	Key Risk
PSK	Poor	Low/Medium	Key Leakage / Brute-force
Certificates	Excellent	Very High	CA Compromise / Revocation failure
EAP (Extensible Auth)	High	High	Backend Server (RADIUS) downtime

EAP and Multi-Factor Authentication

For Remote Access VPNs (where employees connect from home), certificates aren't enough. We use EAP (Extensible Authentication Protocol) to integrate with RADIUS or Active Directory. This allows the VPN to require a username, a password, and a TOTP token before the IKE tunnel is fully established.

▪Use X.509 certificates for Site-to-Site tunnels
▪Implement EAP-TLS for remote employee access
▪Rotate PSKs every 30 days if they must be used
▪Integrate VPN logs with a SIEM to detect authentication spikes

STRICT SECURE AUDIT RULE

Avoid using the same PSK for multiple peer-to-peer tunnels. Every single tunnel should have a unique, complex key to prevent a single compromise from collapsing the entire network.

quiz BLOCK (★ 50 XP)

You are the security architect for a company with 500 remote employees. Why do you choose X.509 certificates over PSK for their VPN connections?

Select your proof vectors above

challenge BLOCK (★ 100 XP)

The PSK Crack