Q: I do not know/remember the key for a certain block, can I read or write in that block?
A: No, it is not possible to access a block unless we have authenticated us in it. All cards are provided with both A and B keys by default (0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF). If the user changes them, he must remember the change. The only thing we can do without the keys of a card is to read its UID and ATQ.