CODEWITHSUNDEEP
contactless-smartcardemv
TalaT
TalaT

Reputation: 953

emv tag 0x9F37 unpredictable numbers length

I have noticed that in some of the cases in emv transactions, the tag 9f37(TAG_UNPREDICTABLE_NUMBER) length is not 4 bytes, It is a read only tag so I cannot set it. Please someone explain me is it must be 4 bytes or it can be of any length upto 4 bytes. And also please guide me how this number is generated and what can cause its length.

Upvotes: 2

Views: 3170

Answers (1)

Adarsh Nanu
Adarsh Nanu

Reputation: 2211

As the name denotes it should not be predictable by any means and you can use any random number generation algorithm to create a value whether you developing a card application or terminal app as explained below.

  1. Unpredictable number is used during Offline Enciphered PIN verification to ensure that PIN block generated is different at all times. This is generated by the chip and length is 8 bytes(image 1). This unpredictable number you will not see at host and you will need a tool like FIME Smartspy or Keolab Nomadlab to get the value.

  2. Another is Unpredictable number generated by Terminal which is used in cryptogram generation ensuring a different cryptogram is generated every time even when all other CDOL elements are same. Its length is 4 bytes(image 2)

enter image description here

image 2

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Upvotes: 5

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