Re: newbie: what does "sign the message digest" mean?

["Vinod Gadgoli" <vinod.infosec () gmail com>]

Hi Ravi,
There are two kinds of cryptography
1. Symmetric Key - In which same key is used for both encryption and
decryption. this key is shared between the sender and the receiver.
The sender and receiver exchanges these keys in a secure manner as
acceptable to both the parties.

2. Asymmetric key cryptography
   it uses two keys, one is called public key and the other is called
private key.  Either one can be used for encryption/decryption, and
the other key is used for the reverse operation. for example, if u
encrypt a message using public key then it can be decryped using
private key (providing confidentiality)
if u encrypt using ur private key, u can only decrypt the message
using public key. In this case it provides authenticity to the
messange (No confidentiality, bcoz public keys are displayed publicly
on users home page etc.)
In short, signing means encrypting the message using ur private key.

HTH


On 6/15/06, Ravi Malghan <rmalghan () yahoo com> wrote:
> Hi: I am very new to cryptography. I am reading a book
> and donot seem to understand the meaning of "sign the
> message digest" even after reading the chapter several
> times. Below is what the book describes
>
> A sender wants to send a message called "Message"
> securly
>
> 1. sender computes the message digest for "Message".
> 2. sender signs the message digest and attaches the
> resulting digital signature plus the certificate to
> the message. The result is Signed Message + Sender
> Certificate + Signature
> 3. sender then encrypts the result from step 2 with a
> random session key
> .
> .
> and so on
>
> What does the Step 2 mean. I understand what is
> computing a message digest.
> 1. But don't understand what is "signs the message
> digest".
> 2. How is "Signed Message" different from "Message"
> 3. What is a Signature?
> Can someone explain?
>
> Thanks
> Ravi
>
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--
Vinod Gadgoli
Systems Security Engineer  (MS Information Security)

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