Harden VNC server, IPC, and management plumbing

client/internal/approval/broker.go

@@ -28,6 +28,37 @@ const (
 	MetaExpiresAt = "expires_at"
 )
 
+// ShortKeyFingerprint formats a hex-encoded Noise_IK static pubkey as a
+// short, eyeball-able fingerprint to display in the approval dialog.
+// The dashboard-supplied display name attached to a SessionPubKey isn't
+// cryptographically asserted by the connecting client, so the prompt
+// must also show something that IS: the key fingerprint, a hash of
+// the static public key the client just proved possession of during the
+// Noise handshake. Returns the empty string when the input is too short
+// to plausibly be a hex pubkey, so the row is omitted rather than
+// rendered as a misleading partial.
+//
+// Output format: 16 hex chars grouped as XXXX-XXXX-XXXX-XXXX (64 bits of
+// fingerprint, resistant to random-prefix collisions and easy for a human
+// to compare with an out-of-band reference).
+func ShortKeyFingerprint(hexKey string) string {
+	if len(hexKey) < 8 {
+		return ""
+	}
+	src := hexKey
+	if len(src) > 16 {
+		src = src[:16]
+	}
+	var out []byte
+	for i, c := range src {
+		if i > 0 && i%4 == 0 {
+			out = append(out, '-')
+		}
+		out = append(out, byte(c))
+	}
+	return string(out)
+}
+
 // Kind values for the well-known prompt subjects. New subsystems should
 // add a constant here so the UI can dispatch on a known string.
 const (

client/internal/approval/fingerprint_test.go

@@ -0,0 +1,62 @@
+package approval
+
+import "testing"
+
+// TestShortKeyFingerprint locks in the format the VNC approval prompt
+// shows to the user. The fingerprint is the user's only cryptographic
+// anchor against a malicious management server that pushes a spoofed
+// display name, so accidental changes to its format would silently
+// undermine that defence.
+func TestShortKeyFingerprint(t *testing.T) {
+	cases := []struct {
+		name string
+		in   string
+		want string
+	}{
+		{
+			name: "full_32_byte_pubkey",
+			in:   "0123456789abcdeffedcba9876543210ffeeddccbbaa99887766554433221100",
+			want: "0123-4567-89ab-cdef",
+		},
+		{
+			name: "exactly_16_chars",
+			in:   "0123456789abcdef",
+			want: "0123-4567-89ab-cdef",
+		},
+		{
+			name: "borderline_8_chars",
+			in:   "01234567",
+			want: "0123-4567",
+		},
+		{
+			name: "too_short_returns_empty",
+			in:   "0123",
+			want: "",
+		},
+		{
+			name: "empty_returns_empty",
+			in:   "",
+			want: "",
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := ShortKeyFingerprint(tc.in)
+			if got != tc.want {
+				t.Fatalf("ShortKeyFingerprint(%q) = %q, want %q", tc.in, got, tc.want)
+			}
+		})
+	}
+}
+
+// TestShortKeyFingerprint_DistinctKeysDistinctOutputs guards against a
+// formatting bug that would collapse different prefixes onto the same
+// displayed fingerprint and let an attacker substitute their pubkey for
+// a victim's while keeping the prompt visually identical.
+func TestShortKeyFingerprint_DistinctKeysDistinctOutputs(t *testing.T) {
+	a := ShortKeyFingerprint("0123456789abcdef" + "rest_of_pubkey_ignored")
+	b := ShortKeyFingerprint("0123456789abcde0" + "rest_of_pubkey_ignored")
+	if a == b {
+		t.Fatalf("expected distinct outputs for distinct prefixes, both = %q", a)
+	}
+}