TOFU stands for “trust on first use.” It is a security policy used by clients such as SSH tools, where the first server key they see is stored and treated as trusted on later connections. The model is simple: if the key changes unexpectedly, the client warns the user or blocks the session. If the key stays the same, the client assumes it is still talking to the same server.
TOFU matters because it gives systems a practical way to detect impersonation without relying on a public-key infrastructure. It is common in SSH, secure messaging, and other peer-to-peer setups. In real attacks, an active man-in-the-middle can win if the client’s first contact is intercepted, because the attacker’s key becomes the “trusted” key. Defenders reduce that risk by verifying fingerprints out of band, enforcing stricter host-key checks, and rejecting key changes unless they are explicitly approved.