A Brief Introduction to Cryptography This section introduces and briefly explains some of the terminology used in discussions of cryptography and digital certificates. It is intended for users who are new to cryptography. If you are interested in reading more about cryptographic methods and issues, you should consult the section titled “Related documentation” in any of the SET Specification books. It provides a bibliography of writings on cryptography. Cryptography is a means of keeping a message private when it may be seen by others to whom the sender does not want to reveal its contents. In other words, encrypting a message is a way of making it unreadable if it is intercepted, or simply received unintentionally, by people for whom it was not intended. Cryptography is especially valuable when a message must be transmitted over a medium that is known to be insecure. The Internet is one example of such a medium, because a message sent from one point to another on the Internet passes through many intermediate points along the way, and can be intercepted at any of those intermediate points. A message may even be intercepted between intermediate points, if the physical cable over which it is transmitted is compromised. You may sometimes hear the Internet referred to as an open wire because of this characteristic; it is like a large party-line telephone system on which many ears may be listening. When a message is encrypted, however, the clear text (the message itself) is altered to produce the cipher text, a form of the message which is scrambled, and intelligible only to someone who is able to decrypt it. Decrypting the cipher text of a message reproduces the clear text of the original message. Private- and Public-Key Cryptography In one commonly used cryptographic system, private-key or secret-key encryption, a mathematical function referred to as a key is used to encrypt the contents of a message. The message contents can then be deciphered (decrypted) only by someone who has a copy of the key used to encrypt the original message. Private-key encryption is also referred to as symmetric encryption because the same key is used to encrypt and decrypt a message. Although private-key encryption is a secure method of making messages unreadable, it can be cumbersome because it requires that all message recipients have copies of the private key. Because the private key must be accessible to all recipients, it is also difficult to ensure message security because the private key may be easily compromised when many copies are available. An alternative method, public-key encryption, is easier to manage, but at first a little more difficult to understand. In public-key encryption systems, two mathematically related keys - a private key and a public key - are used. Such systems are also referred to as asymmetric encryption systems because different keys are used to encrypt and decrypt messages, even though the keys are mathematically related. The two keys are related in such a way that a message encrypted using one can only be decrypted using the other. For example, if you encrypt a message using another person’s public key, the message can only be decrypted using that person’s private key. Thus, two people can communicate securely by exchanging public keys and encrypting all messages sent to the other person using that person’s public key. If the recipient of a message encrypted using the public key is the only person who has the private key, the recipient is the only person who can decrypt the message. Extensions of Public-Key Cryptography: Digital Envelopes and Signatures The SET protocol makes extensive use of both private-key and public-key encryption, and of two extensions intended to increase the security of the information contained in SET messages: digital envelopes and digital signatures. Digital envelopes are a means of increasing the security of messages sent over insecure public networks like the Internet. The message sender’s software randomly generates a new symmetric private key for each message it sends, thus reducing dramatically the risk should any single private key be compromised. After the message is encrypted using the randomly generated private key, the key itself is then encrypted using the public key of the message recipient, and the encrypted private key is appended to the message. This encrypted private key is referred to as the digital envelope of the message. When the recipient’s software receives the message, it first uses the private half of the recipient’s private/public key pair to decrypt the sender’s random private key contained in the digital envelope. Only then can it use the sender’s private key to decrypt the message. In addition to digital envelopes, the SET protocol requires an additional mechanism for ensuring the security of SET messages: digital signatures. Digital signatures are meant to address concerns about authenticating messages, that is, verifying that a message was, in fact, sent by the sender. A digital signature is created by passing a message through a one-way cryptographic function known as a hash function to create a message digest, a mathematical value that is unique to that message (the hash specified by the SET protocol is the SHA1 function). The message digest is encrypted using the sender’s asymmetric private key, and the encrypted digest is then appended to the message. The recipient’s software then passes the message through the same hash function to create a message digest that should be identical to that created by the sender, uses the sender’s public key to decrypt the sender’s message digest, and compares the two digests. If the two digests are identical, the recipient is assured that the message is authentic, that is, was indeed sent by the sender. The security of public-key cryptography systems depends greatly upon the ability of message recipients to verify that the public keys it uses to decrypt messages are valid. That verification is provided by certificates issued by trusted third parties.