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Digital signatures

A digital signature (not to be confused with a digital certificate) is an electronic signature that can be used to authenticate the identity of the sender of a message or the signer of a document, and possibly to ensure that the original content of the message or document that has been sent is unchanged. The ability to ensure that the original signed message arrived means that the sender cannot easily repudiate it later.
A digital signature can be used with any kind of message, whether it is encrypted or not, simply so that the receiver can be sure of the sender's identity and that the message arrived intact.
A digital certificate contains the digital signature of the certificate-issuing authority so that anyone can verify that the certificate is real.

How It Works

Assume you want to send an important message to your friend in Khatmandu town. You want to give your friend the assurance that it was not altered from what was originally sent by you.

• You type in the message for your friend and save it as a word document.
• By employing your hashing software you obtain a message hash of the saved document.
• You then use a private key that you have previously obtained from a public-private key authority to encrypt the hash.
• The encrypted hash becomes your digital signature of the message. (Note that it will be different each time you send a message.)
• You send the message document and obtained digital signature to your friend via an e-mail.
• In beutifal himalayan range, your friend receives the email.
• To make sure it's intact and from you, your friend makes a hash of the received message.
• Your friend then uses your public key to decrypt the message hash.
• If both the hashes (hash sent by you and generated by your friend) match, the received message is valid.

Digital signature standard (DSS)

DSS is the digital signature algorithm (DSA) developed by the NSA to generate a digital signature for the authentication of electronic documents. DSS was put forth by the NIST in 1994, and has become the United States government standard for authentication of electronic documents. DSS is specified in FIPS 186.

A message digest/HASH is created through the use of a hash function (called the Secure Hash Standard, or SHS, and specified in FIPS 180). The data summary is used in conjuntion with the DSA algorithm to create the digital signature that is sent with the message. Signature verification involves the use of the same hash function.