Reason for replacing mifare classic As you know there occured some successful attacks again mifare classic tags (manufactured by NXP) and therefore NXP recommends to replace these tags with some newer products like mifare pro desfire and others.

My question is why don't we encrypt data (with AES, ...) and then translate and store this data to the mifare classic's block, with present 32bit micro controllers it doesn't need so calculation time and there will be a good saving in cost . Why don't companies implement this approach ?

EDIT

tnx all answers.
at first my mind from 32 bit mcu was not tag's built in processor,i mean first we separately encrypt plaintext  to ciphertext in the mcu based on AES.
I have an extra scenario, lets find it's bugs:


as you know each tag has a Unique ID(UID) &amp; we define a key for transacting data between tag &amp; reader/writer chip that controlled by the mcu( PCD chip).
when tag entered to PCD's operation field,pcd read tag's UID
mcu make ENC_KEY with irregular combination of AES_INIT_KEY &amp; uid (each tag will has seprate ENC_KEY)
mcu calculate plaintext'hash with its hash function(TEXT_HASH).
mcu encrypt plaintext based on AES with ENC_KEY(it makes CIPH_TEXT).
mcu transfer &amp; stored CIPH_TEXT &amp; TEXT_HASH to the tag's (mifare classic) via mifare algorithm .


then there are not problems mentioned in answer1, each tag's has unique key &amp; same credit in different has not same CIPH_TEXT



in the second part:

mcu read tag's uid,make ENC_KEY with tag's uid &amp; AES_INIT_KEY ,read CIPH_TEXT &amp; TEXT_HASH with crypto1 &amp; its key from tag,decrypt CIPH_TEXT &amp; TEXT_HASH with ENC_KEY &amp; checked plaintext integrity with its hash value.



i accepted  Maarten Bodewes's answer about price case .(tnx a lot)
I'm really concerned to read this approach's baugs
WBR
Enciphering data using AES and moving that to a card without effective security is not enough to solve some very real security issues; including most preeminently the possibility of


returning a prepaid card to its original state, re-crediting it after use (an adversary can copy the original enciphered data, and save it for future use when the genuine card is empty)
making duplicates indistinguishable form the original (which can multiply stored value, or allow to forge duplicates of an access cards)




Update: it was suggested that the content of the card be enciphered and integrity-protected using a symmetric key derived by the reader from the tag's Unique IDentifier and a master key. This effectively prevents an adversary (not knowing the master key) from altering at will the content of a card, or copying the content of a card to another card with different UID. It however gives no protection at all against attack 1 (which copies the original enciphered and integrity-protected data back to the original card); and only blocks attack 2 if an adversary has some difficulty making a duplicate card with the same UID as the original.



Note: Mifare Classic cards do not use a micro-controller, much less a 32-bit micro-controller (as I wrongly thought the question suggested), which are currently used in rather high-end Smart Cards; many Smart Cards ICs used in mobile phones SIMs, bank cards, transport cards, identity documents.. still use an 8-bit CPU, or 16-bit evolved from 8-bit, like these or these.