Front cover - IBM Redbooks

Figure 6-15 Digital Signatures as used in Lotus Notes
The numbered steps in the diagram are described as follows:
1. Alice decides to send a Notes e-mail to Bob. The Notes client, seeing that the
“Sign” checkbox is set, generates a hash (using MD5) of Alice’s message
(resulting in message digest d).
2. The hash is then encrypted by Notes using Alice’s RSA private key (using
RC2), which means that only her RSA public key will be able to decrypt it.
3. The encrypted hash along with the message is sent to Bob.
4. Bob’s Notes client uses Alice’s RSA Public key to decrypt the hash (again,
using RC2) and gets a decrypted hash (resulting in message digest d).
5. Bob’s Notes client computes a new hash based on the text sent by Alice
(using MD5, resulting in message digest d’).
6. Bob’s Notes client then compares the decrypted hash (message digest d) and
the newly computed hash (message digest d’) and lets Bob know whether the
digital signature is valid or not. If the two hashes are the same, the message
comes from Alice and has not been tampered with in transit. If they are
different, the message is either not from Alice or it has been tampered with in
transit.
So, the result for the user is that Notes will indicate who signed the message if
the validation of the signature is successful. Otherwise, Notes will indicate that it
cannot validate the signature.
Two things are guaranteed by this digital signature process:
Chapter 6. Public key infrastructures 223