Module 8: Capstone Parser Quiz

1. Your parser receives input that starts with '30313030'. How can you tell if this is a hex string representation of '0100' or actual binary data?

Check if all characters are valid hex digits (0-9, A-F) Check if the length is divisible by 2 Both of the above, plus verify it decodes to valid ISO8583 You can't tell - always require the user to specify

2. You're parsing a message and the first 16 bytes after the MTI are '723A448108E18000'. What encoding is this bitmap using?

Binary - these are raw bitmap bytes Hex-ASCII - these are printable hex characters representing the bitmap BCD - each nibble is a bitmap value Cannot determine without knowing the network

3. Your parser finds DE2 = '4532101234567890' (16 digits). What should the pretty-printer output?

4532101234567890 (show full PAN) 453210******7890 (mask middle digits) [REDACTED] (never show any part) 4532-1012-3456-7890 (add dashes)

4. You're parsing a LLVAR field and read '16' as the length prefix. The next bytes are '54321012345678901234'. What's wrong?

Nothing - read 16 bytes as the value The length prefix should be BCD, not ASCII The data is 20 bytes but length says 16 - possible corruption LLVAR can only have 2-digit lengths

5. Your parser encounters field 52 (PIN Data). What's the correct way to handle and display it?

Decode as ASCII and display the PIN Keep as binary, display hex representation Keep as binary, display '[ENCRYPTED PIN DATA]' - never show actual bytes Skip it entirely - PIN data shouldn't be in messages

6. Your parser is processing a message but runs out of data halfway through field 35. What should happen?

Return an error and fail completely Return what was successfully parsed, add a warning about truncation Pad the field with zeros and continue Assume the remaining fields aren't present

7. The bitmap indicates field 99 is present, but your spec doesn't have a definition for field 99. What should the parser do?

Fail with 'unknown field' error Skip the field and add a warning Try to detect the field format (LLVAR/LLLVAR) and parse anyway Assume it's a fixed 10-byte field

8. When displaying DE4 (Amount) = '000000010000', what should the decoded output show?

000000010000 (raw value) 10000 (strip leading zeros) 100.00 (add decimal, last 2 are cents) $100.00 (add currency symbol)

9. You're building a parser configuration. Why is it better to make parsing logic data-driven (specs in a map) rather than hard-coded?

It's faster at runtime Adding support for a new network means adding a new spec, not changing parser code It uses less memory It's required by the ISO8583 specification

10. Your parser reads a bitmap byte as 0x72. Using the formula byte=(N-1)/8, bit=7-((N-1)%8), which fields in positions 1-8 are present?

Fields 1, 2, 3, 4 Fields 1, 2, 4, 6 Fields 2, 3, 5, 6, 7 Fields 1, 2, 5, 6, 7

11. When comparing a request (0100) with its response (0110), which fields would you expect to remain identical?

All fields - response is just a copy with added DE39 DE2, DE3, DE4, DE11, DE32 - key transaction identifiers Only DE11 (STAN) - everything else can change DE37, DE38, DE39 - these are the response fields

12. Your parser needs to handle both BCD and ASCII length prefixes. Given bytes '15' (0x31 0x35 in ASCII), how do you interpret the length?

Always 15 - same in both encodings ASCII: 15, BCD: 0x15 = 21 decimal ASCII: 15, BCD: 15 (read as hex literal) Check spec.LengthEncoding to determine interpretation

13. When validating a 0400 (reversal) message, which field is specifically required that wouldn't be in a 0100 (auth)?

DE38 (Authorization Code) DE39 (Response Code) DE90 (Original Data Elements) DE64 (MAC)

14. Your JSON output mode needs to represent DE55 (ICC Related Data). What's the best approach?

Output as a single hex string Output as base64 encoded Parse the EMV TLV tags and output as nested objects Skip it - binary data doesn't belong in JSON

15. You're debugging why your parser gives different results than a colleague's. The input is identical. What's the most likely cause?

Different CPU architectures Different message specification (field definitions) Random variation in parsing Time-of-day affects parsing