Digest Message
A digest message is a conventional message consisting of a header and body conforming to RFC 822 as clarified in RFC 1123. There is no fixed size. Limitations may exist in intermediate mail gateways which restrict the size. The typical digest size is 15,000 characters. The following three formats are available in the URLSs as stated. RFC means Request For Comment.
The message digest function is in RFC 1186 standards, MD4.
The message digest function is in RFC 1321 (MD5). The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input.
The message digest function is in RFC1319 (MD2), This document describes the MD2 message-digest algorithm. The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input
Question
What is authentication?
Authentication is any process through which one proves and verifies certain information. Sometimes one may want to verify the origin of a document, the identity of the sender, the time and date a document was sent and/or signed. For example, you received a phone call from your classmate saying that you got a distinction in Internet Security. How do you know that he/she is really your classmate? He/she might say I am John only. You need to verify?Another example is related to credit card. How do you know that the cardholder is really the person who owns the card? All of them are related to authentication.
What is a digital signature?
A digital signature is a cryptographic means through which many of these may be verified. The digital signature of a document is a piece of information based on both the document and the signer's private key. It is typically created through the use of a hash function and a private signing function (encrypting with the signer's private key). You can regard that it is a signature that is used in cyber world.
What is message digest?
Suppose Alice (sender) wants to send a signed document (encrypted document) or message (encrypted or not encrypted) to Bob. The first step is generally to apply a hash function to the message, creating what is called a message digest. The message digest is usually considerably shorter than the original message. In fact, the job of the hash function is to take a message of arbitrary length and shrink it down to a fixed length. Alice sends Bob the encrypted message digest and the message (this message may or may not be encrypted.) In order for Bob to authenticate the signature he must apply the same hash function as Alice to the message she sent him, decrypt the encrypted message digest using Alice’s public key and compare the two. If the two are the same he has successfully authenticated the signature. The above is available from
In this case, Bo receives two documents (encrypted digest message and the original message ). Assume the original message is not encrypted, he can apply the same procedure to generate the digest message and then decrypt the encrypted digest message he received from Alice. He then compares it to verify it is actually sent from Alice.
(This covers the definition on CA, PKI and Certificate)