SSL Weak Key Length

What This Check Does

The SSL Weak Key Length check inspects the public key embedded in your SSL/TLS certificate and verifies that it meets modern cryptographic strength requirements. For RSA keys, DNS Spy flags certificates with keys shorter than 2048 bits. For ECDSA keys, keys shorter than 256 bits are flagged. Keys below these thresholds are considered cryptographically weak.

Why It Matters

Weak keys can be factored or broken by sufficiently powerful adversaries, including nation-state actors and well-resourced criminal organizations. A 1024-bit RSA key, once considered adequate, is now considered broken by NIST standards. An attacker who factors your key can impersonate your server, decrypt past traffic captured via passive monitoring, or forge certificates.

Modern certificate authorities will no longer issue certificates with RSA keys below 2048 bits, but older certificates — particularly those issued before 2012 — may still be in use on legacy systems. PCI DSS, NIST SP 800-52, and most compliance frameworks require minimum key lengths that align with DNS Spy's thresholds.

Good vs. Bad Configuration

Bad Configuration

The certificate for example.com uses a 1024-bit RSA key. This key length is below the minimum recommended by NIST and is considered cryptographically weak. An attacker with sufficient compute resources could potentially factor this key.

Good Configuration

The certificate for example.com uses a 2048-bit RSA key (or better, a 4096-bit RSA key or a P-256 ECDSA key). These key sizes meet current NIST recommendations and are accepted by all modern browsers and TLS implementations.

How DNS Spy Monitors This

DNS Spy retrieves your SSL certificate during each monitoring cycle and inspects the public key type and bit length. If a weak key is detected, an alert is raised. Resolving this check requires replacing the certificate with one that uses a sufficiently long key — DNS Spy will automatically detect the new certificate and resolve the alert.