Key servers are computers that send users the public half of a cryptographic key. In cryptography, a key is used to take encoded information and turn it back into a readable message. Many modern computer systems use a two-key process where there is a public key that has little security and a private key that is known only to the user. It is the interaction of the two separate keys that allows users to decode their messages. Some organizations that rely heavily on encrypted messages will use a private key server as well.
For most of the history of cryptography, single key encryption was the norm. Messages were encoded using a single cipher or encoded multiple times with a series of ciphers. In order for anyone to decode these messages, they would need any ciphers that were used to encrypt them. This process, called symmetric cryptography, resulted in several broken codes when ciphers were intercepted while traveling to a new user.
In the 1970s, cryptologists invented asymmetric, or public key, cryptography. This process generally works using two keys, a public one and a private one. The public key is often on a key server, while the private key stays with the individual. The public key reads the private to determine its authenticity. The private key then decrypts the public key in a way only done by that specific private key—this will then allow the user to decode the message.
Accessing public key servers is vital to this system. In order for a person to send a message, they need access to the recipient’s public key. The message may then be tailor-encoded using algorithms generated by the key system. The original sender won’t even have knowledge of the specific method used to encode the message. Key servers allow access to public key information for this specific purpose.
Private key servers are much less relaxed. These servers contain the private half of an asymmetric encryption key or a decoding cipher for a symmetric system. These servers are often well-protected through both physical security and network systems. In fact, some private key servers are only online at specific times of the day or when the information is needed; they work offline the rest of the time.
Typically, only large organizations really have a need for private key servers. Small groups or individuals typically don’t require such tight security on their keys. In addition, the security and hardware is often quite expensive, further reducing the use of these systems.