The pin is compared in the spelling the mesh writes it

The builder hashed the certificate and compared a bare digest against a
fingerprint written as `sha256:` followed by 64 hex characters. It could never
match — and it failed as "this is not the broker this builder was told about",
which is the one thing this check exists to report truthfully. A check that
cries wolf on every correct broker is worse than no check, because the first
thing anybody does is remove it.

The error now prints what was expected beside what arrived, the way the host's
has always done: without both, the message describes a mismatch nobody can
confirm.

And the pin check has its own test, driven against a real TLS handshake — it
accepts the certificate whose fingerprint the mesh wrote and refuses another.
A pin only ever exercised through a live broker is a pin nothing tests.

cmd/mesh-builder/where_test.go

@@ -1,10 +1,20 @@
 package main
 
 import (
+	"crypto/ed25519"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/hex"
+	"math/big"
+	"net"
 	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 )
 
 // Where a builder publishes.
@@ -141,3 +151,73 @@ func TestABuilderWithNoCredentialAtAllSaysSo(t *testing.T) {
 		t.Fatal("a builder with no broker reported one")
 	}
 }
+
+// The pin is compared in the spelling the mesh writes it.
+//
+// A bare digest against a written fingerprint never matches, and the failure is indistinguishable
+// from being pointed at the wrong broker — which is the one thing this check exists to report
+// truthfully. It cost a lab run.
+func TestThePinIsComparedInTheSpellingTheMeshWritesIt(t *testing.T) {
+	certificate, key := aServerCertificate(t)
+	der := certificate.Certificate[0]
+	sum := sha256.Sum256(der)
+	written := "sha256:" + hex.EncodeToString(sum[:])
+
+	listener, err := tls.Listen("tcp", "127.0.0.1:0", &tls.Config{
+		Certificates: []tls.Certificate{certificate}, MinVersion: tls.VersionTLS12,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer listener.Close()
+	go func() {
+		for {
+			conn, err := listener.Accept()
+			if err != nil {
+				return
+			}
+			_ = conn.(*tls.Conn).Handshake()
+			conn.Close()
+		}
+	}()
+	_ = key
+
+	// The pin the mesh wrote must be accepted.
+	if err := handshakeWith(listener.Addr().String(), written); err != nil {
+		t.Fatalf("the broker this builder was told about was refused: %v", err)
+	}
+	// And a different one refused, or the check reports nothing.
+	other := "sha256:" + strings.Repeat("ab", 32)
+	if err := handshakeWith(listener.Addr().String(), other); err == nil {
+		t.Fatal("a broker this builder was not told about was accepted")
+	}
+}
+
+// handshakeWith runs the builder's own pin check against an address.
+func handshakeWith(address, pin string) error {
+	conn, err := tls.Dial("tcp", address, pinning(pin))
+	if err != nil {
+		return err
+	}
+	return conn.Close()
+}
+
+func aServerCertificate(t *testing.T) (tls.Certificate, ed25519.PrivateKey) {
+	t.Helper()
+	public, private, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	template := &x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject:      pkix.Name{CommonName: "a broker"},
+		NotBefore:    time.Now().Add(-time.Hour),
+		NotAfter:     time.Now().Add(time.Hour),
+		IPAddresses:  []net.IP{net.ParseIP("127.0.0.1")},
+	}
+	der, err := x509.CreateCertificate(rand.Reader, template, template, public, private)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return tls.Certificate{Certificate: [][]byte{der}, PrivateKey: private}, private
+}