Data Security - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 216

participants in the communication check the other party's knowledge of a secret (secret
cryptological key).
In this procedure, all the transponders and receivers that form part of an application
are in possession of the same secret cryptological key K ( symmetrical procedure).

When a transponder first enters the interrogation zone of a reader it cannot be assumed
that the two participants in the communication belong to the same application. From the
point of view of the reader, there is a need to protect the application from manipulation
using falsified data. Likewise, on the part of the transponder there is a need to protect
the stored data from unauthorised reading or overwriting.
The mutual authentication procedure begins with the reader sending a GET
CHALLENGE command to the transponder. A random number R A is then gener-
ated in the transponder and sent back to the reader (response → challenge - response
procedure). The reader now generates a random number R B . Using the common secret
key K and a common key algorithm e k , the reader calculates an encrypted data block
(token 1), which contains both random numbers and additional control data, and sends
this data block to the transponder.

→

Token 1 = e K (R B || R A || ID A || Text1 )

The received token 1 is decrypted in the transponder and the random number R A
contained in the plain text is compared to the previously transmitted R A .Ifthetwo
figures correspond, then the transponder has confirmed that the two common keys
correspond. Another random number R A2 is generated in the transponder and this is
used to calculate an encrypted data block (token 2), which also contains R B and control
data. Token 2 is sent from the transponder to the reader.

Token 2 = e K (R A2 || R B || Text2 )

The reader decrypts token 2 and checks whether R B , which was sent previously,
corresponds with R B , which has just been received. If the two figures correspond, then
the reader is satisfied that the common key has been proven. Transponder and reader
have thus ascertained that they belong to the same system and further communication
between the two parties is thus legitimised (Figure 8.1).
To sum up, the mutual authentication procedure has the following advantages:

• The secret keys are never transmitted over the airwaves, only encrypted random

numbers are transmitted.

GET_CHALLENGE

Random A

Reader Transponder

Token 1

Token 2

Key K Key K

Figure 8.1 Mutual authentication procedure between transponder and reader