A digital signature is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, such that the sender cannot successfully deny having sent the message (authentication and non-repudiation), and that the message was not altered in transit (integrity). Digital signatures are often used to implement electronic signatures, a legally binding equivalent to traditional handwritten signatures.
Digital signatures are usually based on public key cryptography, a method by which two keys (a public key and a private key) are generated which can be mathematically related to each other but are not easily derived from each other. The private key is kept secret; it is the individual's credentials to sign documents digitally. The public key may be widely distributed without compromising its associated private key.
The validity of digital signatures is based on security proofs such as those provided by RSA cryptography or hash functions like SHA-1, which are believed to be hard to break.