Entering the ciphertext and
the key in the same black box would give back the plaintext.
The encryption is said to safeguard the con?¬?dentiality of data.
The class of algorithms just described are said to perform symmetric
encryption. That is to say, the same key is used for both
encryption and decryption (see Figure 1-4). These algorithms
usually perform well and have been used for a long time
(Gerolamo Cardano, an Italian mathematician, introduced them
in the sixteenth century.) Common symmetric algorithm names
are DES, AES/Rijndael, and Blow?¬?sh.
Public Key Cryptography
A symmetric key, however, can be a nightmare to distribute. If
Alice wants to send to Bob a message, she has to know which
key she should use. If Bob owns a key, he has to give it to Alice.
This means that the exchange should be protected at all costs,
Symmetric encryption
combines text
with a secret key,
obtaining unreadable
data; only the
knowledge of the
key will allow you
to retrieve the
original text
Key distribution is a
weakness of the
symmetric encryption
schemas
40 The Problem
because restricting the knowledge of the key is the only factor
that guarantees secrecy. If by any chance the key falls into Eve??™s
hands, all future messages from Bob will hold no secrets for her.
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