A digital signature is a mathematical technique used to validate the authenticity and integrity of a message, software or digital document. The digital equivalent of a handwritten signature or stamped seal, a digital signature offers far more inherent security, and it is intended to solve the problem of tampering and impersonation in digital communications.
Digital signatures can provide the added assurances of evidence of origin, identity and status of an electronic document, transaction or message and can acknowledge informed consent by the signer.
In many countries, including the United States, digital signatures are considered legally binding in the same way as traditional document signatures. The United States Government Publishing Office publishes electronic versions of the budget, public and private laws, and congressional bills with digital signatures.
How digital signatures work
Digital signatures are based on public key cryptography, also known as asymmetric cryptography. Using a public key algorithm, such as RSA, one can generate two keys that are mathematically linked: one private and one public.
Digital signatures work because public key cryptography depends on two mutually authenticating cryptographic keys. The individual who is creating the digital signature uses their own private key to encrypt signature-related data; the only way to decrypt that data is with the signer's public key. This is how digital signatures are authenticated.
Digital signature technology requires all the parties to trust that the individual creating the signature has been able to keep their own private key secret. If someone else has access to the signer's private key, that party could create fraudulent digital signatures in the name of the private key holder.
How to create a digital signature
To create a digital signature, signing software -- such as an email program -- creates a one-way hash of the electronic data to be signed. The private key is then used to encrypt the hash. The encrypted hash -- along with other information, such as the hashing algorithm -- is the digital signature.
The reason for encrypting the hash instead of the entire message or document is that a hash function can convert an arbitrary input into a fixed length value, which is usually much shorter. This saves time as hashing is much faster than signing.
The value of a hash is unique to the hashed data. Any change in the data, even a change in a single character, will result in a different value. This attribute enables others to validate the integrity of the data by using the signer's public key to decrypt the hash.
If the decrypted hash matches a second computed hash of the same data, it proves that the data hasn't changed since it was signed. If the two hashes don't match, the data has either been tampered with in some way -- integrity -- or the signature was created with a private key that doesn't correspond to the public key presented by the signer -- authentication.
A digital signature can be used with any kind of message -- whether it is encrypted or not -- simply so the receiver can be sure of the sender's identity and that the message arrived intact. Digital signatures make it difficult for the signer to deny having signed something -- assuming their private key has not been compromised -- as the digital signature is unique to both the document and the signer and it binds them together. This property is called nonrepudiation.
Digital signatures are not to be confused with digital certificates. A digital certificate, an electronic document that contains the digital signature of the issuing certificate authority, binds together a public key with an identity and can be used to verify that a public key belongs to a particular person or entity.
Most modern email programs support the use of digital signatures and digital certificates, making it easy to sign any outgoing emails and validate digitally signed incoming messages. Digital signatures are also used extensively to provide proof of authenticity, data integrity and nonrepudiation of communications and transactions conducted over the internet.
Digital signature vs. electronic signature
While digital signature is a technical term, defining the result of a cryptographic process that can be used to authenticate a sequence of data, the term electronic signature -- or e-signature -- is a legal term that is defined legislatively.
For example, in the United States, the term was defined in the Electronic Signatures in Global and National Commerce Act, passed in 2000, as meaning "an electronic sound, symbol, or process, attached to or logically associated with a contract or other record and executed or adopted by a person with the intent to sign the record."
This means that a digital signature -- which can be expressed digitally in electronic form and associated with the representation of a record -- can be a type of electronic signature. More generally, though, an electronic signature can be as simple as the signer's name being entered on a form on a webpage.
To be considered valid, electronic signature schemes must include three things:
- a way to verify the identity of the entity signing it;
- a way to verify that the signing entity intended to affirm the document being signed; and
- a way to verify that the electronic signature is indeed associated with the signed document.
A digital signature can, on its own, fulfill these requirements to serve as an electronic signature:
- the public key of the digital signature is linked to the signing entity's identification;
- the digital signature can only be affixed by the holder of the public key's associated private key, which implies the entity intends to use it for the signature; and
- the digital signature will only authenticate if the signed data -- document or representation of a document -- is unchanged. If a document is altered after being signed, the digital signature will fail to authenticate.
While authenticated digital signatures provide cryptographic proof that a document was signed by the stated entity and that the document has not been altered, not all electronic signatures can provide the same guarantees.