Trust on first use.
How the first hello works. When you pair with someone, their key is accepted and pinned. From then on, the conversation is bound to that key: only envelopes it signed will ever verify as your contact. Verification of the safety number closes the remaining gap.
Trust on first use (TOFU) is a trust model where the first key received for a contact is remembered, and every later interaction is checked against it. It converts a one-time risk window into continuous protection.
What it is
At pairing, CarrierPony stores your contact's public key, and every incoming envelope must verify against it. A different key — new phone, reinstall, or something worse — simply is not your contact: its messages cannot verify and are never presented as them.
Why it matters
TOFU is the pragmatic backbone of SSH and most secure messengers: it requires no central authority and no ceremony to start talking. Its one weakness — an attacker present at first contact — is exactly what comparing the safety number eliminates.
Related terms
Common questions.
Is TOFU good enough on its own?
For most threats, yes — an attacker must be active at the exact moment of first contact and stay in the path forever after. For high-stakes use, verify the safety number and the question disappears.
Why not use a central directory of keys?
A directory is an account system and a target: whoever runs it can substitute keys. TOFU plus verification keeps trust between the two people it belongs to.
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