digital signature images

An Encrypted Digest

A digital signature is an encrypted digest of a file. The digest was created with a one-way hash function from the file's contents.

Signed Certificates The first major application for digital signatures is digital certificates. "Signed" digital certificates are used to verify the identity of an organization or individual. They are widely used to authenticate a Web site in order to establish an encrypted connection for credit card and other confidential data (see SSL and digital certificate). Signed Files The second major application for digital signatures is "code signing," which verifies the integrity of executable files downloaded from a Web site. Code signing also uses signed digital certificates to verify the identity of the site (see code signing and digital certificate). Also see digital envelope and electronic signature. The Illustrations Below The following two illustrations show how digital signatures are used for data integrity in both non-private and private exchanges. Because of the requirement of disseminating keys, the following methods are used mostly between two parties that communicate with each other on a regular basis and not by the public in general. The references to the man and woman are used to help explain the concept; however, all functions are automatically performed by the software.

Integrity, But No Privacy

The woman makes her message tamper proof by encrypting the digest into a "digital signature," which accompanies the message. At the receiving side, the man uses her public key to verify the signature. However, the message text is sent "in the clear" and could be read by an eavesdropper.

Message Integrity and Privacy

In this example, the woman signs her message and also encrypts the signature and message with the man's public key for privacy (confidentiality). When he receives the encrypted signed message, he decrypts it with his private key to expose the text he can now read along with the signature. He then verifies the signature to ensure the message was not tampered with.

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