Public key fingerprint

A public key fingerprint is a small binary integer used to identify a larger digital public key. It is typically the last (rightmost) 64 or 32 bits of the full key.

For example, a typical RSA public key is 1,024 bits or longer. Extracting the lower 160 bits results in a much shorter indentification tag, such as:

F0 03 FA 05 56 0D CC 86 90 20 B3 1B 74 BD 44 23 8A CE 3E 79

Extracting just the lower 32 bits produces an even shorter fingerprint:

8A CE 3E 79

The shorter fingerprint can be used as an index for the larger full key, such as in a database of network users. This works because in general the probability that two users will have the same 32-bit fingerprint is n × 2⁻³² for population size n. For populations of a few hundred or a few thousand users, the chances of a collisions are fairly low.

Note that while the fingerprint can be used to identify a user or his public key, it is too short to be used as an encryption key by itself.