Asymmetric Encryption

Asymmetric key  (Public-key)  cryptography provides a  radical departure from all that has gone before. For one thing, asymmetric-key algorithms are based on mathematical functions rather than on substitution and permutation. More important, asymmetric-key cryptography is involving the use of two separate keys,  in contrast, to symmetric encryption, which uses only one key. The use of two keys has profound consequences in the areas of confidentiality, key distribution, and authentication.

Asymmetric algorithms rely on one key for encryption and a  different but related key for decryption.  These  algorithms  have  two  important  characteristics: 

•  It  is  computationally infeasible to  determine the  decryption  key  given  only  knowledge  of  the  cryptographic algorithm and
•  The encryption key.

Either of the two related keys can be used for encryption, with the other used for decryption.

Asymmetric Encryption Model

A asymmetric-key encryption scheme has six ingredients:

The essential steps are the following

1. Each user generates a pair of keys to be used for the encryption and decryption of messages
2. Each user places one of the two keys in a public register or another accessible file. This is the public key. The companion key is kept private. As the above figure suggests, each user maintains a collection of public keys obtained from others.
3. If Bob wishes to send a confidential message to Alice, Bob encrypts the message using Alice's public key
4.  When Alice receives the message, she decrypts it using her private key.No another recipient can decrypt the message because only Alice knows Alice's private key.

With this approach, all participants have access to public keys, and private keys are generated locally by each participant and therefore need never be distributed. As long as a user’s private key remains protected and secret, the incoming communication is secure. At any time, a system can change its private key and publish the companion public key to replace its old public key.

The two keys used for asymmetric encryption are referred to as the public key and the private key.Invariably, the private key is kept secret, but it is referred to as a private key rather than a secret key to avoid confusion with symmetric encryption.