Why the fingerprint is the anchor
A key block is long, easy to substitute and impossible to eyeball. A fingerprint is forty hexadecimal characters derived from it, and it is the only part a person can actually check.
What a fingerprint is
A hash of the public key, rendered as forty hex characters and usually printed in groups. Two different keys cannot practically produce the same fingerprint, so comparing fingerprints is equivalent to comparing keys while being something a human can do in ten seconds.
Why comparing key blocks is useless
A key block is thousands of characters of base64. Nobody compares them by eye, and a substituted block looks exactly as convincing as a genuine one. If somebody hands you a key block and a fingerprint together and both came from the same place, you have verified that a source is internally consistent and nothing more.
Where a fingerprint should come from
- More than one independent source, compared against each other.
- Sources that predate whatever prompted you to look, since a fingerprint published today by a party that appeared today proves nothing.
- Ideally something you recorded yourself before you needed it.
Record it once, outside the machine
Forty characters on paper, or in a note you keep independently of anything connected to this. The point of an anchor is that it does not move when everything around it does, and a fingerprint stored only in the same place as the key it authenticates is not an anchor at all.
Compare the whole thing
Reading the first and last groups and assuming the middle is a habit worth breaking. Generating a key whose fingerprint matches at both ends is far harder than matching a short prefix, but the discipline costs you five extra seconds and removes the question entirely.
Writing it down is the whole point
A fingerprint recorded after you already needed it is not an anchor, it is a copy of whatever you found under pressure. Record it on a day when nothing is wrong, from sources you gathered calmly, and the check retains its value for years afterwards without any further work.