Ein kryptographisches Vertrauensnetzwerk aufbauen

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• Ich will jemandem ein vertrauliches, d.h. verschlüsseltes E-Mail schreiben
• Der Empfänger stellt sein öffentlicher Schlüssel zur Verfügung, mit dem ich meine Nachricht für ihn verschlüsseln kann
• Entschlüsseln kann sie nur der Empfänger, und zwar mit seinem privaten Schlüssel
• Öffentliche Schlüssel werden auf Verzeichnissen publiziert
• Problem: Jemand kann unter dem gleichen Namen des Empfängers einen Schlüssel ins Verzeichnis stellen und sich als Empfänger ausgeben

Wie stellen wir sicher, dass der Empfänger auch wirklich derjenige ist, als den er sich ausgibt? Ich habe nicht mit ihm persönlich Schlüssel getauscht.

Lösung: Über ein Web of Trust; einen Vertrauenspfad.

Wenn genügend Personen seine Identität bestätigt haben, kann ich davon ausgehen, dass der Empfänger auch wirklich derjenige ist, als den er sich ausgibt. Das ganze funktioniert dezentral, wir haben also nicht das Problem, dass eine Firma für «Vertrauen aussprechen» Geld verlangen kann oder durch eine hierarchische Struktur bei einem Angriff auf diese Firma das Vertrauen zerschlagen wird.

Keysigning Parties

An einer Keysigning Party treffen sich die Leute, um gegenseitig ihre Identität zu bestätigen. So auch gestern an der UnFUCK 2012 in Furtwangen.

Jeder generiert sich einen Schlüssel:

gpg --gen-key

Und lädt diesen auf den Server hoch:

gpg --keyserver wwwkeys.de.pgp.net --send-key 0xDEADBEEF

Jetzt erstellt sich jeder einen Fingerabdruck seines Schlüssels:

gpg --fingerprint C5846309

Ein Fingerabdruck sieht so aus: 4C28 7958 6F47 ACAF EBFB A96A 3DCD AF42 C584 6309

Den druckt oder schreibt sich jeder auf, und geht damit an die Keysigning Party. Dort werden die Fingerprints von allen Anwesenden verteilt.
Jetzt liest jeder seinen Fingerprint in der Runde vor, und jeder kontrolliert auf seinem Blatt, ob er übereinstimmt. Danach stehen sich alle in einer Reihe gegenüber, und jeder überprüft von jedem Teilnehmer sein Lichtbildausweis.

Wenn alle Teilnehmer sich so überprüft haben, geht jeder zurück an den Rechner und signiert alle soeben überprüften Schlüssel.

Schlüssel vom Server holen:
gpg --recv-keys 0xDEADBEEF

Fingerprint überprüfen:
gpg --fingerprint 0xDEADBEEF

Schlüssel signieren:
gpg --sign-key 0xDEADBEEF

Signierten Schlüssel auf den Server hochladen:
gpg --send-keys 0xDEADBEEF

Wenn das alle Teilnehmer gemacht haben, kann man sich deren Signaturen holen:

gpg --refresh-keys

Mit sig2dot kann man daraus nette Graphen erstellen:

gpg --list-sigs 2> /dev/null | sig2dot -a -t "UnFUG keysigning party" | neato -T png > truststats.png

Hellblau: Gegenseitig signiert
Rot: Wurde signiert, hat selber noch nicht
Dunkelgrün: Nicht im Vertrauensnetzwerk

Noch sind nicht so viele Schlüssel gegenseitig signiert. Am nächsten Morgen sind es schon mehr:

Alternative Darstellung: (neato mit circo ersetzen im Befehl oben)

Anhand dieser Darstellung kann ich jetzt sehen: Claude, Florian, Rudi und Nicolas haben sich gegenseitig signiert. Ich will als fremder nun Claude eine verschlüsselte Nachricht senden. Ich kenne Rudi persönlich und sehe, dass er Florian kennt, und der wiederum Claude. So kann ich über diese Vertrauenspfade sehen, dass dieser Schlüssel von Claude wirklich zu ihm gehört und sein Schlüssel und seine Identität von allen verbundenen Teilnehmern überprüft wurde.