Cryptography Reference
In-Depth Information
6.1 Symmetric vs. Asymmetric Cryptography
In this chapter we will see that asymmetric, i.e., public-key, algorithms are very dif-
ferent from symmetric algorithms such as AES or DES. Most public-key algorithms
are based on number-theoretic functions. This is quite different from symmetric ci-
phers, where the goal is usually
not
to have a compact mathematical description
between input and output. Even though mathematical structures are often used for
small blocks
within
symmetric ciphers, for instance, in the AES S-Box, this does
not mean that the entire cipher forms a compact mathematical description.
Symmetric Cryptography Revisited
In order to understand the principle of asymmetric cryptography, let us first recall
the basic symmetric encryption scheme in Fig. 6.1.
Fig. 6.1
Principle of symmetric-key encryption
Such a system is symmetric with respect to two properties:
1. The
same secret key
is used for encryption and decryption.
2. The encryption and decryption
function
are very similar (in the case of DES they
are essentially identical).
There is a simple analogy for symmetric cryptography, as shown in Fig. 6.2.
Assume there is a safe with a strong lock. Only Alice and Bob have a copy of the
key for the lock. The action of encrypting of a message can be viewed as putting the
message in the safe. In order to read, i.e., decrypt, the message, Bob uses his key
and opens the safe.
Fig. 6.2
Analogy for symmetric encryption: a safe with one lock