The Need for Cryptography

Security often requires that data be kept safe from unauthorized access. And the best line of defense is physical security (placing the machine to be protected behind physical walls). However, physical security is not always an option (due to cost and/or efficiency considerations). Instead, most computers are interconnected with each other openly, thereby exposing them and the communication channels that they use.

This problem can be broken down into five requirements that must be addressed:

1. Confidentiality: assuring that private data remains private.
2. Authentication: assuring the identity of all parties attempting access.
3. Authorization: assuring that a certain party attempting to perform a function has the permissions to do so.
4. Data Integrity: assuring that an object is not altered illegally.
5. Non-Repudiation: assuring against a party denying a data or a communication that was initiated by them.

Modern cryptosystems are typically classified as either public-key or private-key. Private-key encryption methods, such as the Data Encryption Standard (DES), use the same key to both encrypt and decrypt data. The key must be known only to the parties who are authorized to encrypt and decrypt a particular message. Public-key cryptosystems, on the other hand, use different keys to encrypt and decrypt data. The public-key is globally available. The private-key is kept confidential.