Key server (cryptographic)

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In computer security, a key server is a computer — typically running special software — which provides cryptographic keys to users or other programs. The users' programs can be working on the same network as the key server or on another networked computer.

The keys distributed by the key server are almost always provided as part of a cryptographically-protected identity certificate containing not only the key but also 'entity' information about the owner of the key. The certificate is usually in a standard format, such as the OpenPGP public key format, the X.509 certificate format, or the PKCS format. Further, the key is almost always a public key for use with an asymmetric key encryption algorithm.

1 History
2 Public versus private keyservers
3 Privacy concerns
4 Problems with keyservers
5 References
6 See also
7 External links

[edit] History

Key servers were developed as a result of the invention of public key cryptography. In public key cryptography an individual is able to generate a key pair which included two related keys. One of the keys in the pair is meant to be kept private while the other is meant to be distributed. Public key cryptosystems are designed in a way such that the distribution of the public key of the key pair should not significantly weaken the security provided by encryption with the key pair. If an individual has the public key of a keypair, they are able to use that key to initiate secret communications with the holder of the matching secret key. The need to have the public key of a key pair in possession in order to start communication or verify signatures is a bootstrapping problem. Locating keys on the web or writing to the individual asking them to transmit their public keys can be time consuming. Key servers act as central repositories to alleviate the need to individually transmit public keys.

The first web-based PGP keyserver was written for a thesis by Marc Horowitz, while he was studying at MIT. Horowitz's keyserver was called the HKP Keyserver after a web-based protocol it used to allow people to interact with the keyserver. Users were able to upload, download, and search keys either through the HTTP-based HKP protocol on port 11371, or through web pages which ran CGI scripts. Before the creation of the HKP Keyserver, keyservers relied on email processing scripts for interaction.

[edit] Public versus private keyservers

The most important universally accessible key servers are those computers, located around the world, which store and provide OpenPGP keys over the Internet for users of that cryptosystem. In this instance, the computers can be, and are, mostly run by individuals as a pro bono service, facilitating the web of trust model PGP uses. There are also multiple proprietary public key infrastructure systems which maintain key servers for their users; only their users are likely to be aware of them at all.

[edit] Privacy concerns

For many individuals, the purpose of using cryptography is to obtain a higher level of privacy in personal interactions and relationships. It has been pointed out that allowing a public key to be uploaded in a key server when using decentralized web of trust based cryptographic systems, like PGP, may reveal a good deal of information that an individual may wish to have kept private. Since PGP relies on signatures on an individual's public key to determine the authenticity of that key, potential relationships can be revealed by analyzing the signers of a given key. In this way, models of entire social networks can be developed.

[edit] Problems with keyservers

The OpenPGP keyservers developed in the 1990s suffered from a few problems. Once a public key has been uploaded, it is difficult to remove. Some users stop using their public keys for various reasons, such as when they forget their pass phrase, or if their private key is compromised or lost. In those cases, it was hard to delete a public key from the server, and even if it were deleted, someone else can upload a fresh copy of the same public key to the server. This leads to an accumulation of old fossil public keys that never go away, a form of keyserver "plaque". Another problem is that anyone can upload a bogus public key to the keyserver, bearing the name of a person who in fact does not own that key. The keyserver had no way to check to see if the key was legitimate.

To solve these problems, PGP Corp developed a new generation of key server, called the PGP Global Directory. This keyserver sent an email confirmation request to the putative key owner, asking that person to confirm that the key in question is his. If he confirms it, the PGP Global Directory accepts the key. This can be renewed periodically, to prevent the accumulation of keyserver plaque. The result is a higher quality collection of public keys, and each key has been vetted by email with the key's apparent owner.

These are some keyservers that are often used for looking up keys with "gpg --recv-key"