A PKI system uses symmetric and asymmetric encryption algorithms and digital certificates to provide security for multiple applications and uses. It provides a more secure environment for both organizations and end users. The security that is provided through PKI relies on processes and services that are fundamental. These security services allow for a secure solution to be developed for delivery of information across the Internet. The security services are:

• Confidentiality—Confidentiality ensures that only the intended recipient can read a message. Confidentiality is provided in both symmetric and asymmetric cryptosystems through the use of encryption.
• Integrity—Integrity ensures that the recipient of a message can be certain that the message received was the message sent. Integrity is provided in asymmetric cryptosystems through the use of digital signatures (discussed later in this chapter). If a message is digitally signed and the recipient verifies the digital signature, he or she can be certain that the message received is identical to the message sent. In symmetric cryptosystems, integrity is provided by virtue of the fact that a message decrypts properly.
• Authentication—Authentication allows someone to prove his or her identity to another. In asymmetric cryptography, digital certificates may be used for authentication. In symmetric cryptography, limited authentication is possible when a shared secret key is known only to two people. If one receives a valid message from the other that is encrypted with the shared secret, the recipient knows that the message must have come from the sender.
• Nonrepudiation—Nonrepudiation ensures that any objective third party can verify that a message came from the purported sender and was not forged by the recipient or anyone else. Only asymmetric cryptosystems provide nonrepudiation, and they do so through the use of digital signatures.

Use of Digital Signatures

Digitally signing an email allows the receiver to verify the contents were not modified after the data were sent. Digital signatures also provide nonrepudiation. That is, they allow the recipient to conclusively prove to a third party that the sender actually sent the message. Digital signatures can also be used to identify if a user has signed off or approved a particular document.

Creating a digital signature requires that a cryptographic hash function be applied to the message, resulting in a message digest or hash value. The original hash value is encrypted using the signer’s private key and decrypted using the signer’s public key. If the two hash values match, the signature is verified. When the sender’s email client appends the digital signature to the original message, the sender is digitally signing the document. FIGURE 11-8 walks you through the signing process.

Verifying the digitally signed document requires the receiver to apply the same cryptographic hash function to the document in order to produce the message digest or hash value. The hash value in the digital signature is decrypted with the signer’s public key. If the two hash values match, the digital signature is verified. This process is illustrated in FIGURE 11-9.