Message authentication and dynamic passwords

The security of transactions flowing across a communications network is of ever increasing importance. In many such circumstances it is important not only to protect the messages from passive interception but also, and often of greater importance, to be able to detect any active attack against messages. An active attack may take the form of an interceptor tampering with the message: altering it, adding information, removing information and so on. While it is almost impossible to prevent an active attack there are many mechanisms to ensure, with a high probability, that such an attack may be detected and hence rendered harmless. The techniques to allow detection and thus audit take many forms of which the most common are normally cryptographically based and depend upon the generation, before transmission of the message, of a check-sum which is then appended to the message. The theory underlying this approach works on the basis that if a would-be fraudster changes any part of the message in any way then the check-sum will no longer be correct and thus the recipient of such message can compute, for himself, the expected check-sum, compare it with that received in the message and if they disagree will know the message has been altered. If on the other hand the expected and received check-sums agree then he knows with a high probability that the message has not been altered. This probability is dependent upon the amount of information within the check-sum (i.e. the longer it is) the lower the probability of an undetected alteration.