Host Key Verification

To defend against Man-In-The-Middle (MITM) attacks, every time you connect to a server, the server presents its Host Key Fingerprint to prove its identity.

PhanTerm features an exceptionally strict and transparent known_hosts trust verification mechanism, prioritizing your security whenever any inconsistency occurs.

First Connection Verification

When connecting to a new, unknown host, PhanTerm displays a confirmation dialog in the center of the screen, showing:

• The server’s address and port
• The negotiated encryption algorithm (Key Type, e.g., ssh-ed25519 or ecdsa-sha2-nistp256)
• The secure hash fingerprint (SHA-256)

You must verify this fingerprint. After pressing y to accept trust, PhanTerm securely records this combination (host:port + key_type) locally in known_hosts.yaml.

Strict Trust Model

PhanTerm employs a trust mechanism isolated by specific algorithm:

1. Multi-Algorithm Trust per Host: A server may present fingerprints for multiple algorithms simultaneously. If you previously trusted a server’s RSA fingerprint but today it prefers Ed25519 for the handshake, PhanTerm still treats this as a “new key type” and asks for your confirmation again. This is secure, as a server can legitimately possess multiple keys of different algorithms.
2. Silent Pass: For host:port + key_type combinations that have already been confirmed and trusted, subsequent connections will securely pass without prompting.
3. Fail-Closed Security Block: If you previously trusted an Ed25519 key for a machine, but one day that host returns a different Ed25519 key, PhanTerm will immediately block the connection and display a red security alert.

Caution

When you see a red host key mismatch alert, it means someone might be intercepting your connection (a MITM attack), or the server administrator recently reset the OS and regenerated the SSH keys. Do not bypass this warning until you have confirmed the cause.