l-5 2.3 PUBLIC-KEY ENCRYPTION

PUBLIC-KEY ENCRYPTION
Of equal importance to symmetric encryption is public-key encryption, which finds use in message authentication and key distribution.

Public-Key Encryption Structure
Public-key encryption, first publicly proposed by Diffie and Hellman in 1976 [DIFF76], is the first truly revolutionary advance in encryption. Public-key algorithms are based on mathematical functions rather than on simple operations on bit patterns, such as are used in symmetric encryption algorithms.
More important, public-key cryptography is asymmetric, involving the use of two separate keys, in contrast to symmetric encryption, which uses only one key. The use of two keys has deep consequences النتائج in the areas of confidentiality, key distribution, and authentication.
we should first mention several common misconceptions المفاهيم الخاطئة concerning public-key encryption.
One is that public-key encryption is more secure from cryptanalysis than symmetric encryption. In fact, the security of any encryption scheme depends on
(1) the length of the key and
(2) the computational work involved in breaking a cipher.
**There is nothing in principle about either symmetric or public-key encryption that makes one superior متفوقto another from the point of view of resisting cryptanalysis.
A second misconception is that public-key encryption is a general- purpose technique that has made symmetric encryption obsolete.
because of the computational overhead of current public-key encryption schemes, there seems no predictable likelihood that symmetric encryption will be disused.
Finally, there is a feeling that key distribution is trivial when using public-key encryption, compared to the rather weighty handshaking involved with key distribution centers for symmetric encryption.
For public-key key distribution, some form of protocol is needed, often involving a central agent, and the procedures involved are no simpler or any more efficient than those required for symmetric encryption.

A public-key encryption scheme has six ingredients ( Figure 2.7a ):
• Plaintext: This is the readable message or data that is fed into the algorithm as input.
• Encryption algorithm: The encryption algorithm performs various transformations
on the plaintext.
• Public and private key: This is a pair of keys that have been selected so that if one is used for encryption, the other is used for decryption. The exact transformations performed by the encryption algorithm depend on the public or private key that is provided as input.