Principles of Crytography for Data Security | [Back]

As the amount of life spent online continues to increase, a great deal of private information is naturally - being transmitted: banking details, medical records, business correspondences. Where once these exchanges of - data would have occured between a small set of individuals in a shared space, they now happen between - continents, through hundreds of servers and over complex network infrastructures. It is a system that cannot - be fully accounted for by any individual, and so the means of transmission are insecure--much like having a - letter transmitted by a series of couriers, the data is liable to be intercepted or modified. And so both - parties in the exchange take on a risk that may prohibit especially critical data from being sent. - Cryptography is the study of data obfuscation--a means of making a message readable only by some. It is the - answer to the question "when the means of transmission cannot be trusted, how can information be conveyed - securely?

There are two classes of encryption: symmetric and asymmetric. Symmetric encryption allows a message to be - encoded and decoded with the same piece of information, or key. The ancient Caesar cipher is an example of - this; an arbitrary number acting as the key was agreed upon by both parties, and every letter in the message - was shifted through the alphabet by that amount. It could easily be decoded by anyone who knew the key by - simply shifting the letters backward through the alphabet. Simple algorithms such as the Caesar cipher are - vulnerable to various attacks due to the patterns that they create in the ciphertext. A given letter may - always be encoded in the same way, meaning that the key could be compromised if the attacker gained access - to the plaintext and ciphertext forms of the same message. And a message could be decrypted without the key - by recognizing repeated patterns in the ciphertext representing common words. More advanced symmetric - encryption methods--like the Advanced Encryption Standard (AES) algorithm, developed for the US National - Institute of Standards and Technologies--disrupt patterns in the message to prevent these types of attacks. -

Symmetric encryption is an imperfect solution in the internet age. Because it requires both parties to know - the same secret key, those parties must have a secure form of communication already established. In the days - of Caesar, this key exchange could be performed confidentially by two individuals in close proximity. When - encryption is used on the internet, it cannot be assumed that the communicating parties will have had any - physical interactions--and it would be impractical to expect every new customer of an online banking service - to perform a physical key exchange. Asymmetric encryption solves this problem by removing the key exchange. - Instead of encrypting and decrypting a message with the same key, an asymmetric encryption algorithm has a - keypair, comprising a public key, used for encrypting a message, and a private key, used for decrypting a - message. The keys are so named because the private key is never shared, while the public key can be - broadcast widely.

Typically, it demands more processing power to encrypt and decrypt messages with an asymmetric encryption - implementation than a comparable symmetric one. For this reason, it is desirable to use symmetric encryption - for most communications. An asymmetric implementation such as the Rivest--Shamir--Adleman (RSA) system is - used to perform the key exchange. One of the devices will broadcast its public RSA key to the other, which - will respond by generating an AES key, encrypting it with that RSA key, and returning it. This method of key - exchange is secure, even if every network packet is intercepted. Once the devices share an AES key, they can - communicate with the more efficient symmetric encryption method.