Cryptographic Protocols - Everyday Cryptography - page 296

r B || It's Bob, are you OK?

Alice Bob

r B || Bob || Yes, I'm OK

MAC K ( r B || Bob || Yes, I'm OK )

Figure 9.2. Protocol 1

If Alice and Bob do not already share a symmetric key then they will need to first
run a different protocol in order to establish a common symmetric key K . We will
discuss suitable protocols in Section 9.4. Technically, if Alice and Bob have not
already agreed on the use of a strong MAC algorithm to compute the MAC then
Alice could indicate the choice of MAC algorithm that she is using in her reply .

PROTOCOL DESCRIPTION
Protocol 1 consists of the following steps:

1. Bob conducts the following steps to form the request:

(a) Bob generates a nonce r B (this is an implicit action that is not described in
Figure 9.2, as is the fact that he stores it for later checking purposes).
(b) Bob concatenates r B to the text It's Bob, are you OK? . This combined data
string is the request.
(c) Bob sends the request to Alice.

2. Assuming that she is alive and able to respond, Alice conducts the following
steps to form the reply:

(a) Alice concatenates the nonce r B to identifier Bob and the text Yes, I'm OK .
We will refer to this combined data string as the reply text .
(b) Alice computes a MAC on the reply text using key K (this is an implicit
action). The reply text is then concatenated to the MAC to form the reply.
(c) Alice sends the reply to Bob.
3. On receipt of the reply, Bobmakes the following checks (all of which are implicit
actions that are not shown in Figure 9.2):

(a) Bob checks that the received reply text consists of a valid r B (which he can
recognise because he generated it and has stored it on a local database)