Page 95 - Intelligent Communication Systems

78 INTELLIGENT COMMUNICATION SYSTEMS
• Fast Encryption Algorithm (FEAL): The fast encryption algorithm (FEAL)
was invented by NTT, Japan, in 1985. Its key length is 64 bits. The text is broken
into 64-bit-long components and each component encrypted and decrypted the
same way at source and destination, respectively. The several versions of FEAL
(FEAL-4, FEAL-8, FEAL-16, and FEAL-32) differ in the number of times the algo-
rithm is applied. In the case of FEAL-4, for example, the encryption algorithm is
applied four times to get the scrambled text.

7.9.3.2 Problems with Symmetric Key Encryption
The major problem with symmetric key encryption is how to keep the symmetric
key secret and how to send it to the destination safely, without attack. There is a
great possibility that the key could be stolen by a hacker while sending it to the
destination over the Internet. To avoid this, unencrypted text should never be trans-
mitted without first being scrambled. Another problem is that the number of keys
is increasing as the number of customers increases.

7.9.3.3 Public Key Encryption Algorithm
To overcome the problems with the symmetric key method, the asymmetric
(public) key encryption algorithm was invented by W. Diffie and M. E. Hellman
of Stanford University in the 1970s. In this method, different keys are used for
encryption and decryption. The encryption key is public, but the decryption key is
private and secret. The decryption key is a prime factor obtained from factoring
the encryption key. That is, the number of keys is twice the number of customers.
The customer keeps his or her own key secret.
With the Rivest-Shamir-Adleman encryption method (RSA), which was
invented by R. L. Rivest, A. Shamir, and L. Adleman in 1977 and based on the
Diffie-Hellman method, it takes a lot of time to find a prime factor from the public
key. For example, for a number with 200 digits it would take over 380,000 years
to discover a prime factor, even using a supercomputer. Thus, rinding a prime factor
is practically impossible. This is the principle behind the RSA encryption method.
Its major defect is that encryption and decryption take more time than with the
symmetric key method.

7.9.3.4 Hybrid Method
The hybrid method is a combination of symmetric and asymmetric key encryption.
In this method, a public key is given. A symmetric key is encrypted by means of
the public key. At the same time, the text is encrypted with the symmetric key. Then
the scrambled symmetric key and the scrambled text are sent to the destination
over the Internet. At the destination, using the private key that corresponds to the
public key, the scrambled symmetric key is decrypted to produce the original sym-
metric key. Then the scrambled text is decrypted using the symmetric key. Finally,
the original text is obtained.