Encryption Project

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PUBLIC KEY CRYPTOGRAPHY

Encryption

For centuries the making and breaking of codes has been of vital concern in the military and diplomatic arena. Recent dramatic advancements in electronic commerce and communication have made secure communication more essential than ever. A method for encoding and decoding messages is called a cryptosystem. A cryptosystem consists of two steps the first encoding a message and the next decoding any encoded message.

Up until 1976 all cryptosystems, no matter how powerful, suffered from one abysmal flaw: if all the details of the encoding process (i.e., the encoding function) were known, decoding a message was as easy as encoding it (i.e., the decoding function was completely determined by the encoding function). For example, if a coded message read "dwwdfn dw gdzq", and you knew that it was coded by shifting the letters forward by 3 places, then all you had to do was shift the letters back by 3 places, revealing the hidden message "attack at dawn"!

The moment the encoding function (shifting every alphabet forward by 3 places) is known, the decoding function (shifting back by 3 places) is immediately and completely determined. Thus in these cryptosystems, the encoding function had to be kept secret, which resulted in their safe use being limited to a very small trusted group.

Public Key Encryption

In 1976, W.Diffie and M. Hellman introduced Public Key Cryptography. The encoding function here is a trapdoor function one whose inverse is impractical to implement, unless some extra information is available. This extra information (called the decoding key) is not required for encoding the message, yet is essential for decoding it in reasonable time. This makes it much easier to encode messages than to decode them. The beauty of such a system is that the encoding process need not be kept secret. Each user has his own or a personal encoding function, which is public information (hence the name Public Key), and a decoding key, which he keeps secret.

Another advantage of a Public Key cryptosystem is that it has the following four desirable properties-

Confidentiality: It should not be possible for anyone other than the receiver to decode a message.

Authenticity: It should be possible to verify that the purported sender has actually sent the message.

Integrity: The receiver should know that the received message has not been tampered with in any way.

Non-repudiation: Having sent a message, the sender should not be able to deny having sent it.