A digital guarantee that information has not been modified, as if it were protected by a tamper-proof seal that is broken if the content were altered. The two major applications of digital signatures are for setting up a secure connection to a Web site and verifying the integrity of files transmitted (more below).
An Encrypted Digest
The digital signature is an encrypted digest of the file (message, document, driver, program) being signed. The digest is computed from the contents of the file by a one-way hash function such as MD5 or SHA-1 (see MD5 and SHA-1) and then encrypted with the private part of a public/private key pair (see RSA). To prove that the file was not tampered with, the recipient uses the public key to decrypt the signature back into the original digest, recomputes a new digest from the transmitted file and compares the two to see if they match. If they do, the file has not been altered in transit by an attacker. See MD5.
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.
