Encryption Algorithms

Symmetric Cryptography

What is it

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DES a symmetric algorithm

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Asymmetric Cryptography

What is it

The concept of Asymetric Cryptography, also called Public Key Cryptography, was described for the first time by Whitfield Diffie and Martin Hellman in 1976. In contrast to the Symmetric Cryptography in which we have the same secret key for encryption and decryption, we now have one public key $e_{P}$ (ecncryption key) and one private key $d_{P}$ (decryption key) for each person $P$ . While the public key $e_{P}$ can be published to the whole world the private key $d_{P}$ is to be treaten as a secret and only the person $P$ knows it. An important characteristic of such a crypto system is that it is computationally infeasible to determine the private key given the corresponding public key.

The idea of Public Key Cryptography can be visualised like a mailbox. Generally the address of someone is public and everybody can send him a letter, but only the reciever owns the key for opening the mailbox and reading the letters. Remember that Symmetric Cryptography works like a safe. For instance assume that Alice wants to send a message m to Bob. She encrypts the plaintext m with Bob's public key e , computes the ciphertext c=f(m,e) and sends c to him. Now Bob can decrypt the recieved cipher c with his private key d and computes the plaintext m=f'(c,d).

The biggest advantage of Asymetric Cryptography is the enormously reduced effort for the key management. Imagine a company with 100 staffers gets a new employee Bob and you have to offer Bob the possibility to communicate securely to each other person. Using a symmetric crypto system means that you must create 100 new keys and distribute them. With a asymetric system you only have to create the key pair $(e_{Bob},d_{Bob})$ , publish $e_{Bob}$ and tell Bob his private key $d_{Bob}$ .

A disadvantage is the velocity. Comparing DES and RSA the most popular representatives of both worlds, we get that DES is at least 100 times as fast in software and between 1,000 and 10,000 times as fast in hardware. Generally we can say that asymetric algorithms are significant slower than symmetric algorithms.

A solution for this are Hybride Systems which work as follows:

• First asymmetric cryptography is used for the key exchange of a session key $k_{s}$ .
• Then symmetric cryptography is used for encryption and decryption of the data with the exchanged key $k_{s}$.

So these method offers the comfortable key management as well as the velocity.