Encryption and signatures only help if you have the genuine public key of the person or site you mean, not an impostor's.
The idea: Every protocol needs a first anchor of trust that math can't provide. Browsers ship a list of certificate authorities. SSH remembers the first key it saw. Signal lets you compare safety numbers in person. DNSSEC builds a chain of signatures up to one root key. Everything after the anchor is checked by cryptography, which makes the anchor the weak link.
Why it matters: Every encrypted channel hands that job to someone: certificate authorities, your first connection, or a fingerprint you compare.
The details
On the map: certificate authorities (TLS), trust on first use (SSH), keys swapped out of band (WireGuard), the key server and safety numbers (Signal), the root key (DNSSEC)
Public-key cryptography protects you only if you are using the right public key. If an attacker gets you to use theirs, the math works perfectly: you encrypt to the attacker, or accept their signatures.
Each protocol hands this problem to someone:
TLS: certificate authorities vouch for a site's key, and any of the more than a hundred trusted root authorities (and the intermediates they certify) can issue a certificate for any domain. DigiNotar's 2011 compromise[1] produced fake certificates for Google that were used against users in Iran. Certificate Transparency logs now make such misissuance detectable[2, 3].
SSH: trust on first use[4]. The first connection asks you to accept the server's key fingerprint. An attacker in the middle at that moment wins.
WireGuard: peers exchange public keys out of band, through whatever channel set up the configuration.
Signal: the server distributes keys, and comparing safety numbers detects a swapped one[5].
DNSSEC: each zone's key is vouched for by its parent, up to the root key, which is managed in public key-signing ceremonies[6, 7].
It is a trust assumption, not math, and several of the most famous real-world attacks went through it.
Rests on
If any of these breaks, this breaks too.
Nothing below it on the map: this is a starting point that the rest builds on.
If it falls
If this breaks, these break with it.
Impersonation risk for all four encrypted channels: 4 boxes show them
Fox-IT. Black Tulip: Report of the investigation into the DigiNotar Certificate Authority breach. Report for the Dutch Ministry of the Interior, 2012. zoek.officielebekendmakingen.nl/blg-191225.pdf
B. Laurie, A. Langley and E. Kasper. Certificate Transparency. RFC 6962, 2013. rfc-editor.org/rfc/rfc6962
B. Laurie, E. Messeri and R. Stradling. Certificate Transparency Version 2.0. RFC 9162, 2021. rfc-editor.org/rfc/rfc9162