If the message is encrypted symmetrically...

Joseph Oreste Bruni jbruni at mac.com
Wed Jun 20 20:47:37 CEST 2007

Correct. If I'm sending a message that I want protected, I hash the  
contents with something like SHA-1. I encrypt this hash with my  
private key and attach the encrypted hash to the document.

Recipients can then compute their own hash of the document, decrypt  
the attached, encrypted hash using my public key, and compare the  
results. If the hashes match, the document is good, and non- 
repudiation has been established since it was encrypted with MY  
private key.

To extend our discussion, suppose I wish to send an encrypted message  
to multiple recipients. I would then encrypt the (randomly generated)  
symmetric key to each recipient's public key in turn. All of the  
encrypted copies (of the symmetric key) are attached. A valid  
recipient will be able to encrypt his (and only his) copy of the  
symmetric key and then decrypt the document.

On Jun 20, 2007, at 11:30 AM, Newton Hammet wrote:

> I am not exactly sure how the sig fits in but it is a hash value of  
> either
> the original message or the encrypted message depending on the  
> order of
> signing and encryption.  This hash is encrypted with sender's  
> private key
> part of his own public key. This part I am not sure of so others can
> correct me if I am wrong with this part.
> I believe signatures are encrypted with private key and decrypted with
> public key.  In order to protect against some exploits it is best  
> to have
> your public key consist of one signing key and a different key for  
> message
> encryption.
> -Newton

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