Public Key Cryptography

Contributed from Anoop MS

The data transferred from one system to another over public network can be protected by the method of encryption. On encryption the data is encrypted/scrambled by any encryption algorithm using the ‘key’. Only the user having the access to the same ‘key’ can decrypt/de-scramble the encrypted data. This method is known as private key or symmetric key cryptography. There are several standard symmetric key algorithms defined. Examples are AES, 3DES etc. These standard symmetric algorithms defined are proven to be highly secured and time tested. But the problem with these algorithms is the key exchange. The communicating parties require a shared secret, ‘key’, to be exchanged between them to have a secured communication. The security of the symmetric key algorithm depends on the secrecy of the key. Keys are typically hundreds of bits in length, depending on the algorithm used. Since there may be number of intermediate points between the communicating parties through which the data passes, these keys cannot exchanged online in a secured manner. In a large network, where there are hundreds of system connected, offline key exchange seems too difficult and even unrealistic. This is where public key cryptography comes to help. Using public key algorithm a shared secret can be established online between communicating parties with out the need for exchanging any secret data.

In public key cryptography each user or the device taking part in the communication have pair of keys, a public key and a private key, and a set of operations associated with the keys to do the cryptographic operations. Only the particular user/device knows the private key whereas the public key is distributed to all users/devices taking part in the communication. Since the knowledge of public key does not compromise the security of the algorithms, it can be easily exchanged online.

A shared secret can be established between two communicating parties online by exchanging only public keys and public constants if any. Any third party, who has access only to the exchanged public information, will not be able to calculate the shared secret unless it has access to the private key of any of the communicating parties. This is key agreement and is defined in section 2.

Apart from Key Agreement the other important applications of public key cryptography are Data Encryption and Digital Signature, which are explained in sections 3 and 4 respectively.

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