Why do DESFire cards and Mifare Plus cards offer a Random UID feature? What risks should it mitigate or what features should it offer?
The random UID feature means that the UID is new every time it is powered on, which means every time it is used. To randomise the UID means that everytime the card is used, a new UID is recorded. This each activity of the card cannot be linked to the user. The chain of activity cannot be traced to the card.
RID is optional and should be used to protect privacy. In case RID is enabled, there is a defined and confidential way to retrieve the UID for each product.
The TSF shall ensure that non authorized subjects are unable to determine whether any operation of the TOE were caused by the same user.
Why use IDs
IDs can used to link the card to the user
Why randomise the ID
If the IDs are sequential then an attacker can just guess them, starting at 0, a valid number, or just a guess.
Why avoid using the ID
If an attacker can obtain a valid ID and nothing else is used then they can spoof the card with hardware allowing them to set the ID