
German: 
Hier das, was ich für die beste Antwort halte:
Die beste Möglichkeit ist, dass wir einen mathematischen Beweis haben,
der Standard-Beweisregeln folgt, der zeigt , dass das Chiffrierverfahren sicher ist.
Es gibt sehr wenige Chiffrierverfahren, für die wir das bekommen können.
Eines ist tatsächlich das One-Time Pad.
Wir sehen später in dieser Unit, wie man mathematisch beweist,
dass es eine sehr starke Sicherheits-Eigenschaft hat.
Das nächstbeste ist das letzte,
nämlich formal zu zeigen,
warum dieses Chiffrierverfahren mindestens so hart wie ein andres Problem ist.
Über Reduktionsbeweise werden wir später in diesem Kurs noch mehr sprechen.
Die wesentliche Voraussetzung hier ist, dass es ein anderes Problem gibt, von dem wir mit gutem Grund
glauben, dass es bereits immer hart ist.
Dann werden wir zeigen, dass, wenn wir das Chiffrierverfahren knacken können,
wir auch das andere Problem, das wir bereits mit gutem Grund für hart gehalten haben, lösen können.
Das nächstbeste ist dieses:
dass viele schlaue, hochmotivierte Leute versucht haben, es zu knacken. Aber sie konnten es nicht.
Das ist häufig das beste, was wir tun können.

English: 
Here what I think the best answer is.
The best possibility is that we do have a mathematical proof
that follows standard rules of deduction that shows that the cipher is secure.
There are very few ciphers that we can actually get that for.
One actually is the one-time pad.
We'll see later this unit how to prove in a mathematical sense
that it has a very strong security property.
The next best one is the last one,
which is to show in a formal way
why this cipher is at least as hard as some other problem.
We'll talk about reduction proofs more later in this course.
The basic premise here is that there is some other problem that we have good reasons
to believe is always already hard.
Then we're going to show that if we could break the cipher,
we could solve that other problem that we already have good reasons to believe is hard.
The next best is this one--
that many smart highly-motivated people tried to break but couldn't.
This is often the best we can do.

German: 
Für die besten symmetrischen Chiffrierverfahren, die heute benutzt werden,
ist das tatsächlich der Grund, warum sie als sicher gelten.
Es mag formale Argumente geben, die zeigen, warum sie speziellen Angriffen widerstehen,
Und das ist

English: 
For the best symmetric ciphers that are in use today
this is really the reason that they're argued secure.
There may be formal arguments that show why they resist particular attacks,
and that's part of smart people trying to break ciphers,
knowing all the known best-case attacks and trying them against the cipher
and seeing that the cipher resists them.
But ultimately the best we can do is show that
we think it's secure because it has these properties,
and lots of smart people weren't able to break it.
But that's not a very satisfying way to know it's secure.
We're certainly much rather have the strong mathematical proof.
The worst possible argument here is the key-space argument.
This one is often made incorrectly.
The number of keys gives you an upper bound on the difficulty to break the cipher,
because at worst the attacker could try all the keys.
That's not true for the one-time pad, as we saw.
Trying keys gives you perfectly sensible messages.
You'll eventually see all possible messages.
You can't know which key is correct.
For ciphers where the key space is smaller than the message space,
you could try keys and have a good likelihood

English: 
if the key leads to a sensible message, that that's the right message.
This gives you an upper bound on how hard it is to break a cipher.
It doesn't give you a lower bound.
The fact that you have a large number of possible keys doesn't mean the cipher is secure.
We can always add to the key space without increasing the difficulty in breaking a cipher.
We'll see many ciphers with very large key spaces
that are completely insecure.
Our best possible argument is to have a mathematical proof.
That's what Claude Shannon was able to do for the one-time pad.
It's really a fairly unusual case where we can get

English: 
a mathematical proof that's that strong.
