Biometric solutions can be divided into two groups, based on the type of biometric factor they use solutions based on a physiological factor, examples are fingerprint recognition and iris recognition solutions based on a behavioral factor, examples are voice pattern recognition and keystroke dynamics There are many factors which can in theory be used as a biometric solution to be applied for authentication or identification. For such a factor to be suitable, it must meet the following criteria:

• Universality – the biometric factor must be something each person has.
• Uniqueness – with the factor it must be possible to separate individuals from another, this must be possible and not be overly expensive with existing technical means.
• Permanence – how well the biometric factor resists the effect of time, generally speaking “aging”.
• Collect-ability – it must be possible and not overly expensive or time consuming to measure the factor.
• Acceptability – biometrics is not always very well accepted. This is generally dependent on people’s view is on how invasive a certain technique is. For example retina scans have low acceptability with the general public because retina scans require direct contact with the reader and retina scans can demonstrate pregnancy and other medical conditions such as high blood pressure.
• Circumvention – how easily it is to imitate the biometric factor.
• Performance – in general terms the speed, the accuracy and the robustness.