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Key: 91913232145
Plaintext
DES
SENDER
Ciphertext
RECEIVER
Ciphertext
Key: 91913232145
DES
Plaintext
Figure 16-10
DES: single-key encryption.
Weaknesses Despite the complexity of the encryption algorithm and the sophis-
tication of key selection, DES is not universally accepted as absolutely secure.
Critics point out the following deficiencies:
56-bit keys are inadequate. With the powerful and special hardware available
now, they are breakable. Even such expensive hardware is within the reach of
organized crime and hostile governments. However, 128-bit keys are expected
to be unbreakable within the foreseeable future. A better technique known as
PGP (pretty good privacy) uses 128-bit keys. Another possible remedy is
double application of the algorithm at each step.
Users must be given the key for decryption. Authorized users must receive the
key through secure means. It is very difficult to maintain this secrecy. This is a
major weakness.
Public Key Encryption
This technique overcomes some of the problems associated with the DES technique.
In DES you have to keep the encryption key a secret, and this is not an easy thing
to accomplish. Public key encryption addresses this problem. The public key as
well as the encryption algorithm need not be kept secret. Is this like locking the
door and making the key available to any potential intruder? Let us examine the
concept.
The widely used public key encryption technique was proposed by Rivest,
Shamir, and Adleman. It is known by the acronym RSA. The RSA model is based
on the following concepts:
Two encryption keys are used—one public and the other private.
Each user has a public key and a private key.
The public keys are all published and known openly.
The encryption algorithm is also made freely available.
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