Public key encryption using Hash digest

In public key cryptography, some elements of security are missing. Although the message's confidentiality is assured, there is no authentication of the sender, no nonrepudiation (the sender could deny sending), and no assurance the message was not altered in transit.

A more sophisticated use of public key cryptography can achieve authentication, nonrepudiation, and integrity. To check the integrity of a message, a hash function is used first to create a digest of the message. A hash function is an algorithm that produces a fixed-length number called a hash or message digest. Standard hash functions include MD4 and MD5, which produce 128- and 160-bit hashes.

The sender encrypts both the hash result and the original message using the recipient's public key, producing a single block of cipher text. Then, to ensure the authenticity of the message and nonrepudiation, the sender encrypts the entire block of cipher text one more time using the sender's private key. This produces a digital signature (also called an e-signature) or 'signed' cipher text that can be sent over the Internet.

A digital signature is a close parallel to a handwritten signature — only one person presumably possesses the private key. When used with a hash function, the digital signature is unique to both the individual and the document, and changes for every document. The recipient first uses the sender's public key to authenticate the message, then uses his or her own private key to obtain the hash result and original message, and finally re-applies the hash function to verify the message has not been changed.