Digital signature

Digital signatures provide source authentication for online documents, messages or records, in a manner analogous to what a signature provides for a paper document.

Two cryptographic techniques are used together to produce a digital signature, a cryptographic hash and a public key cryptosystem.

The steps for the sender are as follows: Steps for the receiver are: If both the hash and the public key system used are secure, and no-one except the sender knows his private key, then the signatures are trustworthy.
 * calculate a hash or message digest from the message
 * encrypt that hash with the sender's private key (which only he should know)
 * append the encrypted hash to the message as a signature
 * obtain the sender's public key and verify its validity
 * decrypt the signature, using the sender's public key, to get the hash value; call it H1
 * hash the message body yourself to get another hash value, H2
 * compare H1 and  H2
 * if they are identical, accept the signature as valid