Cryptography Reference
In-Depth Information
be reasonably assumed that many national intelligence organizations have DES
crack machines. Nevertheless, DES has been confirmed as a secure standard
by NIST repeatedly every five years. Schneier [SchnCr] wrote in 1996, 'Guess
what will happen in 1998'.
Schneier was wrong in this respect. Already at the beginning of 1997, the
NIST had begun searching for a DES successor, which turned out to be the
Advanced Encryption Standard
(
AES
). Meanwhile, this process was com-
pleted successfully. You will learn more about it in Section 5.5.
What we should take home from these lessons is that nobody should send
valuable DES-encrypted information over the Internet, never ever again. And
it's not necessary. There are DES variants, such as Triple-DES or DES with
key-dependent S-boxes, that appear to be secure. Better yet, use more secure
algorithms. We will get back to this issue in Chapter 5.
4.5 Asymmetric (Public-Key) Methods
DES was confirmed as a standard at the end of 1976 and, as you know, brought
about a radical change to cryptology. In that same year, another path-breaking
event occurred in this field: Diffie and Hellman introduced the first asymmetric
encryption method ever at a conference, while Merkle submitted his work on
the same topic at the same time. These methods introduced a new quality to
the field: they widely solve the problem of key distribution. But before we
can study this problem, we need to look at some more theory. Once you've
understood some important basic terms and the practical uses, we will discuss
three specific examples.
4.5.1 Symmetric and Asymmetric Methods
So far in this topic we have discussed only
symmetric methods
: the receiver
decrypts each message with the same key that the sender used to encrypt it.
Notice that the symmetry refers to the keys rather than to the methods them-
selves: with a few exceptions (e.g., one-time pad, ROT13, stream ciphers), the
encryption algorithm is different from the decryption algorithm. Encryption and
decryption differ even in the Caesar and Vigenere ciphers: an amount (mod-
ulo 26) is added to each character during the encryption and subtracted during
the decryption. More specifically, though they work with one key, symmetric
methods almost always use two methods.
