A digital signature is a one-way hash, of the original data, that has been encrypted with the signers private key. Signer calculates the hash for the data he needs to sign. The message digest is a file size small (160-bit SHA-1 now deprecated) that contains some sort of control code that refers to the document. The document is in clear text but it has the signature of the sender and can be sent so that it can be read by anyone but not altered since the digital signature guarantees also integrity of the message.”]