#import "@preview/ichigo:0.1.0": config, prob
#import "@preview/algorithmic:0.1.0"
#import algorithmic: algorithm

#show: config.with(
  course-name: "SMART CARDS & NFC",
  serial-str: "k12104785",
  author-info: [
    Lukas Heiligenbrunner
  ],
  author-names: "Lukas Heiligenbrunner",
)

#prob[
  #figure(
    image("Screenshot From 2024-10-21 14-07-41.png", width: 80%),
    caption: [
      Pin try counter.
    ],
  )

][
- The PiN_TRY_COUNTER is prone to turn off attacks.
  Each time the chip resets the ram value of the counter is cleared and one gets basically infinite retries.

  Solution:
  - Store counter in non-volatile memory eg. EEPROM, flash, eMMC.
  - Store counter in a secure server backend.

- Depending on the implementation of the comparison operation, it might leak side-channel information.
  For example, if the comparison is done byte-wise, the attacker can determine the correct byte by comparing the time it takes to compare the bytes.

  Solution:
  - Implement a constant time comparison operation.
]

#prob[
  #figure(
    image("Screenshot From 2024-10-21 14-10-15.png", width: 80%),
    caption: [
      Pin comparision (PIN == REF_PIN).
    ],
  )
][
  - The comparison of the entered pin and the reference pin is array entry wise.
    If a entry doesn't match the comparison is short-handed and the function returns no match.
    This is prone to a timing side-channel attack. If a pin digit matches the comparison takes longer than if it doesn't.

    Solution:
    - Implement a constant time comparison operation. (no comparison shorthand)

    For example:
    #algorithm({
      import algorithmic: *
      Function("Constant-Time-Compare", args: ("PIN", "Ref_PIN"), {
        Cmt[Check if lengths are equal]
        If(cond: $"length" ("PIN") != "length"("Ref_PIN")$, {
          Return[false]
        })
        State[]
        Cmt[Initialize result variable to 0]
        Assign[$"result"$][$0$]
        State[]
        Cmt[Loop through each character in PIN and Ref_PIN]
        For(cond: [$i=0$; $i < "length"("PIN") - 1$], {
          Cmt[XOR corresponding characters and accumulate result]
          Assign[$"result"$][$"result" or ("PIN"[i] xor "Ref_PIN"[i])$]
        })
        State[]
        Cmt[Return true if result is 0, else false]
        Return[$"result" == 0$]
      })
    })
]