Asymmetric Cryptography

Asymmetric key cryptography:-

           In asymmetric cryptography, there are two keys: a private key and a public key. The private key is kept by the receiver. The public key is announced to the public.

 

public key:-

              The development of public-key cryptography is the greatest and perhaps the only true revolution in the entire history of cryptography. It is asymmetric, involving the use of two separate keys, in contrast to symmetric encryption, which uses only one key. Public key schemes are neither more nor less secure than private key (security depends on the key size for both). Public-key cryptography complements rather than replaces symmetric cryptography. Both also have issues with key distribution, requiring the use of some suitable protocol.

The concept of public-key cryptography evolved from an attempt to attack two of the most difficult problems associated with symmetric encryption:

1.) key distribution – how to have secure communications in general without having to trust a KDC with your key

2.) Digital signatures – how to verify a message comes intact from the claimed sender

Public-key/two-key/asymmetric cryptography involves the use of two keys: 

• a public-key, which may be known by anybody, and can be used to encrypt messages, and verify signatures 

• a private-key, known only to the recipient, used to decrypt messages, and sign (create) signatures. 

• is asymmetric because those who encrypt messages or verify signatures cannot decrypt messages or create signatures

Public-Key algorithms rely on one key for encryption and a different but related key for decryption. These algorithms have the following important characteristics: 

• It is computationally infeasible to find decryption key knowing only algorithm & encryption key 
• It is computationally easy to en/decrypt messages when the relevant (en/decrypt) key is known 

The following figure illustrates public-key encryption process and shows that a public-key encryption scheme has six ingredients: plaintext, encryption algorithm, public & private keys, ciphertext& decryption algorithm.

                          

The essential steps involved in a public-key encryption scheme are given below: 

1) Each user generates a pair of keys to be used for encryption and decryption.

2) Each user places one of the two keys in a public register and the other key is kept private.

3) If B wants to send a confidential message to A, B encrypts the message using A‟s public key.

4) When A receives the message, she decrypts it using her private key. Nobody else can decrypt the 

message because that can only be done using A‟s private key (Deducing a private key should be infeasible).

5) If a user wishes to change his keys –generate another pair of keys and publish the public one: no interaction with other users is needed.

The first attack on Public-key Cryptography is the attack on Authenticity. An attacker may impersonate user B: he sends a message and claims in the message to be B –A has no guarantee this is so. To overcome this, B will encrypt the message using his private key. Receiver decrypts using B’s public key. This shows the authenticity of the sender because (supposedly) he is the only one who knows the private key. The entire encrypted message serves as a digital signature. This scheme is depicted in the following figure: