Question: How is a 4-digit Hidden Key (e.g., "1234") coded for storage in the EFHiddenkey file on a USIM?
Option 1: As '34 12 FF FF' using BCD coding with swapped digits.
Option 2: Directly as ASCII values without any padding or swapping.
Option 3: As '12 34 FF FF' using BCD coding with unused digits padded with 'F'.
Option 4: By scrambling the digits and then applying a proprietary encryption algorithm.
Explanation: According to specifications, the Hidden Key is coded on 4 bytes using BCD (Binary Coded Decimal) coding. A 4-digit key like "1234" is specifically coded as '12 34 FF FF', where unused digits are padded with 'F' and no digit swapping occurs, ensuring compatibility and straightforward verification by the Mobile Equipment (ME).