Cryptography Reference
In-Depth Information
well aware of the fact that a ciphertext attack would have been possible
otherwise. In practice, however, operators worked under stress and inadver-
tently passed the ball to the adversary by selecting bad message keys (naturally
other weaknesses of the Enigma played a role, too).
For the sake of completeness, I should mention that there is yet another possi-
bility to distribute keys securely: through so-called centralized key servers .A
universal key known only to the legitimate users is used to distribute a session
key at one of these users' request. A good example is the so-called wide-mouth
frog protocol described in Section 6.1.1.
Exchanging Keys With Asymmetric Methods
The method of distributing key parts separately, as described above, can become
cumbersome and slow if you have to exchange encrypted messages with many
conversers. It can also become costly, for example, if your converser happens
to work in New Zealand; or it can become unnerving if your Japanese business
partner writes English much better than he speaks it.
Things look much simpler when using asymmetric methods: everybody who
wants to receive encrypted messages creates a corresponding key pair and pub-
lishes their public keys. To send an encrypted message (even without previously
announcing it) to a 'key owner', we can use a similar approach:
1. We get the receiver's public key.
2. We create a random session key.
3. We use this session key and a symmetric method to encrypt the message;
then we use the receiver's public key and an asymmetric method to
encrypt the session key.
4. We send both ciphers to the receiver.
5. The receiver is the only one who can recover the session key, since he,
and only he, knows the private key.
6. The receiver can decrypt the message using the session key and the
symmetric method (Figure 4.15).
(As a sideline, the description of such an approach is called cryptographic
protocol . Chapter 6 is entirely dedicated to this topic.) We generally speak of
hybrid methods , because they use both symmetric and asymmetric algorithms.
Nothing can go wrong any more! In fact, a plaintext attack against the asym-
metric method is not doable, since random session keys are encrypted (it would
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