Objective of Modern Cryptography Systems

No system of cryptography, called a cryptosystem, can be considered absolutely unbreakable or beyond compromise. However improbable a successful attack might seem, there is always some facet of the cryptosystem that can be attacked. The history of cryptography is full of examples of cryptosystems that were once considered invincible, and yet people were able to break the security and compromise them. Because cryptographers are not omniscient, they cannot design cryptosystems that are guaranteed to have no weaknesses or that are impervious to unforeseeable methods of attack. Furthermore, cryptosystems must be implemented in the real word, so they are subject to real-world limitations and constraints. All information security systems, including cryptography-based security, have weak links that can be attacked and potentially exploited to compromise the system.

The objective of modern cryptosystems is not to provide perfect or risk-free security. Rather, the objective of cryptography-based security is to protect information resources by making unauthorized acquisition of the information or tampering with the information more costly than the potential value that might be gained. Because the value of information usually decreases over time, good cryptography-based security protects information until its value is significantly less than the cost of illicit attempts to obtain or tamper with the information. Good cryptography, when properly implemented and used, makes attempts to violate security cost-prohibitive.

For example, many modern cryptosystems make it very difficult but not impossible for an attacker to determine the decoding key. Even though the key might be eventually determined by a skilled decoder, given enough time and effort, cryptosystems can still provide ample security to protect valuable information. By the time an attacker can feasibly determine the right decoding key, the cost of the attack would be much higher than the value of the information that is being protected by the key.

For well-designed and analyzed cryptosystems with no known flaws, the primary defense against attack is the length of the encoding key. All cryptosystems with encoding keys shorter than the plaintext message are subject to exhaustive search attacks where the attacker tries all possible combinations of the key until the key is found. For large encoding keys, an exhaustive search for the key usually requires sophisticated and expensive computing equipment to conduct the search, and the search can take hundreds, thousands, or even trillions of years to complete. Cryptosystems can defend against exhaustive key search attacks by simply making the encoding key long enough to make the search computationally infeasible or cost-prohibitive.

Another objective of all information security systems, including cryptography-based security systems, is to protect information resources at less cost than the value of the information that is being protected. A cryptography-based security system must provide information security at acceptable costs. Determining acceptable costs involves weighing the cost of the security versus the benefits of the security. For more information about evaluating the costs and benefits of security solutions, see "Choosing Security Solutions That Use Public Key Technology" in this book.