Front cover - IBM Redbooks

270 Lotus Security Handbook
encryption key (the secret key, which is generally referred to as being a
session key, since a new random key is generated every time an encrypted
S/MIME message is sent) and encrypts the message with it.
2. The session encryption key is encrypted (using either Triple-DES or RC2)
with the recipient’s public key and attached to the message, which means that
only Bob’s RSA public key will be able to decrypt it.
3. The encrypted text and the encrypted key are sent to Bob via SMTP.
4. Bob’s messaging client uses Bob’s RSA private key to decrypt the encrypted
key (again, using RC2) and gets a decrypted session key. Here, secrecy is
guaranteed, because only Bob's private key can be used to decrypt the
session key needed to decrypt the message.
5. Bob’s messaging client uses the decrypted session key to decrypt the mail
message (using either Triple-DES or RC2, depending on which algorithm it
was encrypted with), resulting in the decrypted, original message that was
sent by Alice.
If Bob’s messaging client is unable to decrypt the e-mail sent by Alice, this is
probably because Bob has gotten a new X.509 certificate and the public key in
the directory Alice has access to is the old key.
Looking at the process shown in Figure 6-26, this is in effect a technique in
S/MIME often referred to as a “digital envelope,” whereby the message is actually
encrypted using the shorter symmetric cipher and the symmetric cipher is then
encrypted using the larger asymmetric key, and sent along with the encrypted
message.
This method is preferred because it is far quicker to encrypt the whole message
using the shorter symmetric key than to encrypt the message using the longer
asymmetric key. The message is still quite safe, since this approach combines
the speed of symmetric encryption with the security of asymmetric encryption.
Tamper detection
For tamper detection, or data integrity, S/MIME provides assurance that the
message can be properly validated, meaning that the message was not
tampered with during transit. This utilizes a technique known a digital signatures
and works in a similar manner to what we already discussed in Notes
messaging.
Signing: Authenticating the sender with Digital Signatures
S/MIME provides message signing through the use of digital signatures, which
permits the authentication of the message (confirming that the person who sent it
is indeed the sender) as well as tamper detection (that the message itself is
original and not a single bit of it was modified). This is illustrated in Figure 6-27.