Digital signature

Mathematical algorithms can also be used to create digital signatures that, in effect, place a "seal" on a digitally represented work. Generating a digital signature is referred to as "signing" the work. The algorithms can be implemented through software or hardware, or both.

The digital signature serves as means for authenticating the work, both as to the identity of the entity that authenticated or "signed" it and as to the contents of the file that encodes the information that constitutes the work. Thus, by using digital signatures one will be able to identify from whom a particular file originated as well as verify that the contents of that file have not been altered from the contents as originally distributed.

A digital signature is a unique sequence of digits that is computed based on (1) the work being protected, (2) the digital signature algorithm being used, and (3) the key used in digital signature generation. Generating a digital signature uses cryptographic techniques, but is not encryption of the work; the work may remain unencrypted so it can be accessed and used without decryption. In fact, digital signatures and encryption can be used simultaneously to protect works.

Generally, a signature is computed for a copyrighted work first and then the work (including the seal) is encrypted. When the work is to be used, the work is decrypted, then the signature (i.e., the seal) is verified to be sure the work has not been modified (either in its original or encrypted form). If the work is never changed, the seal need never be removed or changed. If the work is changed, a new seal must be computed based on the revised information.

Typically, the digital signature is incorporated in some manner in the transmission that constitutes the work. Often, the sender will also distribute his public key as well. The signature serves as a "seal" for the work because the seal enables the information to be independently checked for unauthorized modification. If the seal is verified (independently computed signature matches the original signature), then the work is a bona fide copy of the original work &mdash; i.e., nothing has been changed in the file that constitutes the work.