Before we can define what a digital signature is, we must first talk about a broader concept, the electronic signature. Wikipedia defines it as:
An electronic signature, or e-signature, refers to data in electronic form, which is logically associated with other data in electronic form and which is used by the signatory to sign.
In this sense, an electronic signature can be understood as a wide range of processes for associating a signer to an electronic document through some kind of commitment (for instance the commitment to comply with the document's contents, or the commitment of having witnessed an agreement).
The terms signatory and signer are synonyms. We use the latter.
For instance, the printout of a document that is first signed by hand and later faxed over might be considered an electronic signature. However, it may be argued that such a signature can be easily forfeit. Different electronic signature processes offer different guarantees, or assurances.
Electronic signature processes can be categorized into different assurance levels. The eIDAS standard defines three increasing levels of assurance:
- Electronic signatures
- Advanced electronic signatures
- Qualified electronic signatures
For the intents and purposes of this documentation, a digital signature is an implementation of electronic signature using X.509 public key certificates and adhering to a specific signature standard in order to achieve the highest levels of assurance.
The term digital signature should not be confused with digital signature algorithm, which is a type of asymmetric cryptographic algorithm (for instance, the RSA algorithm). Usually, digital signature algorithms are a small part of the implementation of digital signatures.
In all other articles, when we say simply signature, we mean a digital signature as defined above.
The main digital signature standards in use today are: