How cryptography can be applied to an ICT system and data security.

Cryptography is a method of storing and transmitting data in a certain way where only people who are intended to use it, can read and process it. The term is also associated with scrambling plain text (ordinary text) into cipher text (encryption), and then back again (decryption). To decrypt cipher text users will need a key which transforms the cipher text back to the plain text. Cryptography is an effective way of protecting sensitive information as it is stored on media or transmitted through network communication paths. 

Cryptography is also used on the internet with browsers, if you visit a website and it has HTTPS in the URL bar, then you will know that you are on a secure legitimate website using cryptography. This verifies that you are on a legitimate website.  HTTPS works with data encryption and decryption, whenever someone connects to the server the data exchange then starts. The exchange between the client and the server is then encrypted, therefore the client first encrypts the data and sends it to the server which then decrypts it, does whatever is needed and then encrypts the necessary information and sends it back to the client. 

Public Key Infrastructure.

Public key Infrastructure is a system required to provide public key encryption and digital signature services. Its purpose is to manage keys and certificates, by managing these through a PKI an organisation can establish and maintain a trustworthy networking environment. Certificate Authoroties (CA) such as Verisign issue and manage security credentials and public keys for message encryption. As part of a PKI, the CA checks with the registration authority (RA) to verify information provided by the requester of a digital certificate.

More information on PKI can be found here:

http://www.entrust.com/what-is-pki/

http://searchsecurity.techtarget.com/definition/certificate-authority