Front cover - IBM Redbooks

26 Lotus Security Handbook
Symmetric key algorithm example
Let’s take a moment to show how symmetric encryption works. For this, we’ll
need to introduce a couple of characters that have been traditionally used when
explaining security concepts.
Normally, one would use A, B, and C to diagram the flow of information between
two points. However, since this flow of information involves people, it’s best to
give them normal names. So, Alice and Bob are the two characters we’ll use for
examples from now on. The cast of characters expands as the need dictates, but
for now, let’s concentrate on Bob and Alice.
These characters have been used for years to provide examples of how
encryption techniques work, and, as with many long-lived fictitious characters,
Bob and Alice have an amusing biography. This biography can be found in a
document titled “The Alice and Bob after-dinner speech”, given at the Zurich
Seminar in April 1984 by John Gordon. The document can be found on line at the
following URL:
http://www.conceptlabs.co.uk/alicebob.html
So, on to our example with Bob and Alice. Let’s say that Alice wants to send Bob
a message and Alice wants this message to be sent securely and wants to
ensure also that only Bob can read it. Figure 1-4 illustrates this example, with the
exchange going from left to right.
"THINK" "THINK" M0B4Q4Rg2s M0B4Q4Rg2s "THINK"
plaintext encrypt
cyphertext
Figure 1-4 Symmetric key algorithm example
decrypt
Alice key
Bob
key
plaintext
In our example, here is what happens:
1. Alice's message is encrypted using a private key.
2. Bob receives Alice's encrypted message, sees that it is encrypted and wants
to read it.