Confused by Public/Private Keys in Cryptography?

As you likely know, some of the reading for the upcoming module has us looking at Diffie and Landau’s excellent work in Privacy on the Line, with an emphasis on how encryption actually works. Personally, I find the discussion of public/private key exchanges invigorating, but also know that it took me some time to wrap my head around it. I came across a good posting about the Diffie-Hellmen key (you’ll read about it in the chapters assigned in Privacy on the Line) that I thought I’d share.

Public key cryptography[2] was invented in 1976 by Martin Hellman and Whitfield Diffie. It is a cryptographic system that requires two keys for each user, a public key that is known to everyone and a secret key that is known only to that user. This negates the problem with symmetric key encryption where the key must be exchanged before messages can be understood. When a user wants to send a secure message, they use the public key of the recipient to encrypt the message before the recipient then decrypts the message by using their own private key. It is important that the public and private keys are related in such a way that only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them. It is also important that the private key cannot be discovered by knowing the public key.

Public key systems are becoming very popular for transmitting information over the Internet due to the fact that they are very secure and easy to use. The main difficulty with this system is that to encrypt a message correctly it is necessary to already have the intended recipient’s public key. This means that a public server needs to be used which holds all the user’s public keys, which can be accessed by anybody. An example of a popular application that utilises the Public-key cryptography principles is the Voice over IP (VoIP) program Skype (Source).

The rest of Stuarts post is really, really good – I’d highly recommend it given that it’s written in very accessible language/terms and gives a nice, very quick, overview of the study of cryptography. Definitely helpful for orienting discussions on the topic!