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A PKI (public key infrastructure) enables users of a basically unsecure public network such as the Internet to securely and privately exchange data and money through the use of a public and a private cryptographic key pair that is obtained and shared through a trusted authority. The public key infrastructure provides for a digital certificate that can identify an individual or an organization and directory services that can store and, when necessary, revoke the certificates. Although the components of a PKI are generally understood, a number of different vendor approaches and services are emerging. Meanwhile, an Internet standard for PKI is being worked on.

The public key infrastructure assumes the use of public key cryptography, which is the most common method on the Internet for authenticating a message sender or encrypting a message. Traditional cryptography has usually involved the creation and sharing of a secret key for the encryption and decryption of messages. This secret or private key system has the significant flaw that if the key is discovered or intercepted by someone else, messages can easily be decrypted. For this reason, public key cryptography and the public key infrastructure is the preferred approach on the Internet. (The private key system is sometimes known as symmetric cryptography and the public key system as asymmetric cryptography.)

A public key infrastructure consists of:

-A certificate authority (CA) that issues and verifies digital certificates. A certificate includes the public key or information about the public key

-A registration authority (RA) that acts as the verifier for the certificate authority before a digital certificate is issued to a requestor

-One or more directories where the certificates (with their public keys) are held

-A certificate management system

How Public and Private Key Cryptography Works

In public key cryptography, a public and private key are created simultaneously using the same algorithm (a popular one is known as RSA) by a certificate authority (CA). The private key is given only to the requesting party and the public key is made publicly available (as part of a digital certificate) in a directory that all parties can access. The private key is never shared with anyone or sent across the Internet. You use the private key to decrypt text that has been encrypted with your public key by someone else (who can find out what your public key is from a public directory). Thus, if I send you a message, I can find out your public key (but not your private key) from a central administrator and encrypt a message to you using your public key. When you receive it, you decrypt it with your private key. In addition to encrypting messages (which ensures privacy), you can authenticate yourself to me (so I know that it is really you who sent the message) by using your private key to encrypt a digital certificate. When I receive it, I can use your public key to decrypt it.

Who Provides the Infrastructure?

A number of products are offered that enable a company or group of companies to implement a PKI. The acceleration of e-commerce and business-to-business commerce over the Internet has increased the demand for PKI solutions. Related ideas are the virtual private network (VPN) and the IP Security (IPsec) standard. Among PKI leaders are:

-RSA, which has developed the main algorithms used by PKI vendors Verisign, which acts as a certificate authority and sells software that allows a company to create its own certificate authorities

-GTE CyberTrust, which provides a PKI implementation methodology and consultation service that it plans to vend to other companies for a fixed price

-CheckPoint, which offers a product, VPN-1 Certificate Manager, that is based on the Netscape Directory Server

-Xcert, whose Web Sentry product that checks the revocation status of certificates on a server, using the Online Certificate Status Protocol (OCSP)

-Netscape, whose Directory Server product is said to support 50 million objects and process 5,000 queries a second; Secure E-Commerce, which allows a company or extranet manager to manage digital certificates; and Meta-Directory, which can connect all corporate directories into a single directory for security management

Pretty Good Privacy

For e-mail, the Pretty Good Privacy (Pretty Good Privacy) product lets you encrypt a message to anyone who has a public key. You encrypt it with their public key and they then decrypt it with their private key. PGP users share a directory of public keys that is called a key ring. (If you are sending a message to someone that doesn't have access to the key ring, you can't send them an encrypted message.) As another option, PGP lets you "sign" your note with a digital signature using your private key. The recipient can then get your public key (if they get access to the key ring) and decrypt your signature to see whether it was really you who sent the message.

For more information on: PKI/Network Infrastructure
Digital Signatures

This was first published in January 2001

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