Cryptography Reference
In-Depth Information
letter
A
B
courier
Alice sends Bob the universal key.
phone
carrier pigeon
S
A
Alice creates a session key, S,
and uses it to encrypt a secret
message, N.
N
A
S
B
Alice creates the universal key to
encrypt the session key and sends
both ciphers to Bob.
B
Bob uses the universal key to
decrypt the session key.
S
S
with the obtained session key, he
decrypts the message, N.
S
N
N
Figure 4.14:
Distributing a secret key more securely.
the security. This is presumably the way people always work in critical areas.
And it is the reason why a theoretically possible plaintext attack will become
practically doable if it can make do with a few Mbytes of plaintext.
There's nothing new to all of this. The work regulations for the Enigma
corresponded to it years ago! You probably remember Section 2.5.2: a univer-
sal key — the so-called 'ground setting' — was transmitted over cryptologically
secure channels (in the code book); the radio operators set the message key
themselves and encrypted it with the day key. The German cryptologists were
