Create a random (e.g. every call a different) sequence of bytes that identidy this key. There can almost infinite
number if anonynous ids for e key (more than 1.0E77), so it is really anonymous way to identify some key. The
anonymousId for public and private keys are the same.
Anonymous ID size is 64 bytes: first are 32 random bytes, last are HMAC(key, sha256) of these random bytes.
The most important thing about anonymous ids is that every time this call generates new id for the same key,
providing anonymous but exact identification of a key.
To check that the key matches some anonymousId, use
#matchAnonymousId(byte[]).
Therefore, the private key fingerprint is its public key fingerprint. The public key fingerprint is calculated
using some hash over it's parameters, see
PublicKey#fingerprint()