Safety number.
The two-minute habit that rules out a man in the middle. After pairing, both phones derive the same number from the two public keys. Compare it once — in person or over a channel you trust — and you know your envelopes go where you think.
A safety number is a human-comparable digest computed from the public keys of both participants in a conversation. Matching numbers on both phones prove both apps hold the same two keys — no interception, no substitution.
What it is
Open the conversation's details and each phone shows the number. Read it aloud face to face, over a phone call, or across any channel an attacker is unlikely to control simultaneously. Match means verified; mismatch means stop and re-pair.
Why it matters
Pairing over a network is trust-on-first-use: secure against everyone except an attacker present at that exact moment. Verifying the safety number upgrades the pairing from probably-fine to cryptographically confirmed.
Related terms
Common questions.
Do I have to verify to use the app?
No — messaging works immediately after pairing. Verification is the extra step that removes the remaining sliver of first-contact risk, and it's strongly recommended for anything sensitive.
The numbers don't match. Now what?
Stop, delete the pairing, and pair again through a channel you trust — ideally in person. A mismatch means the keys on the two phones aren't the pair you think they are.
Get CarrierPony
Free private messaging and file transfer for iPhone. No accounts, no tracking, no phone numbers.