What is a digital certificate? 

This is digital signature ? I understood digital certificates are different, where a CA is involved.

Very good explanation, thanks.

It thought it went like this: 1) Alice applies hashing algorithm to the message to get unique and irreversible representation of the message 2) Alice applies Asym Encryption Algorithm, where the parameters are the Hash value and her private key to get an encrypted hash. 3 Alice sends Bob the message (plain text) and the encrypted hash. 4) Bob gets both items 5) Bob firstly applies an asymmetric decryption algorithm (using Alice's public key) to the encrypted hash to get the unencrypted hash value (H1). 6) Bob then applies the same hashing algorithm to the plain text data as Alice did to get (H2). Result: If H1 = H2 then Bob can be assured of 2 things: 1) The message is definitely from Alice 2) The message was not altered during transmission. Private key encryption verifies Data Integrity and Authentication. It does not ensure Confidentiality, that is achieved with Public key encryption.

nicely explained , thanks

What kind of gel does Bob put on his hair? Thanks for the video.

But if Alice encrypt the message with her private key to create a certificate, an attaker with her public key could decrypt the certificate an read the message. That is not cool. I would use a hash of the message and then create the certificate from the hash.

Hi Andre, I think the digital certificate is encrypted along with the message so only Bob will be able to access this particular digital certificate after decryption. I am also not totally clear between digital signature and digital certificate (with CA). I guess this case involves a self signed certificate rather than using a CA. The digital certificate contains the public key for Bob to reply and authenticates the message came from Alice. A digital signature would only authenticate the message came from Ann but contain no public key for Bob to reply with.

thank you

Allright. So Alice uses public key of bob to encrypt the message. Alise als aso signs with her own private key. Bob wants to use the public key of Alice to de crypt the value, and also uses the public key to dycrypt the signature.. but in both cases, Bob still has to use the public key of Alice. So the man in the middle could still give his own public key and sign with his own private key, intercepting the communication?? I dont get it.. So by saying "Bob can only validate the signature with the public key of Alice".. but that is the whole problem, how does Bob know what public key to use?

Couldn’t get the one....

this is digital signature not certificate