Fingerprint.
A key's ID card. Public keys are long strings of bytes; a fingerprint boils one down to a short digest two humans can actually compare. Same fingerprint, same key — no exceptions.
A fingerprint is a cryptographic hash of a public key. Because the hash is collision-resistant, matching fingerprints mean matching keys, which makes fingerprints the practical unit of key verification.
What it is
When CarrierPony shows you a contact's key details, the fingerprint is the value that pins down exactly which key you're trusting. The safety number you compare after pairing is built from the fingerprints of both keys in the conversation.
Why it matters
Attacks on encrypted messaging rarely break the math — they substitute keys. Comparing fingerprints (directly, or via the safety number) is how you detect substitution: an impostor's key cannot fake the digest of the real one.
Related terms
Common questions.
Do I need to compare the whole fingerprint?
Compare all of what the app shows you. Truncated comparisons weaken the guarantee, which is why CarrierPony presents a complete safety number rather than inviting you to eyeball a prefix.
My contact's fingerprint changed — is that bad?
It means the key changed, usually a reinstall or new phone without a restored backup. Confirm with them out-of-band and verify the new safety number before continuing.
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