[pdurrant]

Originally Posted by jackie_w

1. a random digit (1 in 10 chance of not being scrambled)
2. a digit guaranteed to be different from the correct one
3. use a single digit everywhere, e.g. all digits displayed as zero
4. replace all digits with a random alpha A-Z char
5. something else I haven't thought of

From a cryptographic point of view, (2) is the worst one. Since then you have some information about the original digit left.