9F64
NATC(Track1)Number of ATC Digits, Track 1 is the companion figure to the position mask in PUNATC(Track1) (9F63): where 9F63 says which digit positions in the track 1 discretionary data carry dynamic material, 9F64 says how many of those positions specifically belong to the Application Transaction Counter portion rather than to the Unpredictable Number sharing the same discretionary data. It's a single byte, but it's load-bearing in a very literal sense - a host that gets this count wrong will split the extracted digits between ATC and Unpredictable Number in the wrong place, again with no field-level error to flag the mistake, since both quantities decode to ordinary-looking digits either way. Because this whole mag-stripe-mode mechanism exists specifically for legacy host compatibility, a bug here tends to be discovered not by the terminal but by whichever back-office system later tries to use the ATC value for velocity checking and finds numbers that don't behave the way a real transaction counter should. See EMV Contactless Book C-2.
Interactive decoder
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Binary (1): 05
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Decoded example
Example value: 05
Related tags
Properties
| Tag | 9F64 |
|---|---|
| Name | NATC(Track1) |
| Format | Binary |
| Length | 1 bytes |
| Source | Card (ICC) |
| Templates | 70 |
| Books | EMV Contactless Book C-2 |
Frequently asked questions
- What is EMV tag 9F64?
- Number of ATC Digits, Track 1 is the companion figure to the position mask in PUNATC(Track1) (9F63): where 9F63 says which digit positions in the track 1 discretionary data carry dynamic material, 9F64 says how many of those positions specifically belong to the Application Transaction Counter portion rather than to the Unpredictable Number sharing the same discretionary data. It's a single byte, but it's load-bearing in a very literal sense - a host that gets this count wrong will split the extracted digits between ATC and Unpredictable Number in the wrong place, again with no field-level error to flag the mistake, since both quantities decode to ordinary-looking digits either way. Because this whole mag-stripe-mode mechanism exists specifically for legacy host compatibility, a bug here tends to be discovered not by the terminal but by whichever back-office system later tries to use the ATC value for velocity checking and finds numbers that don't behave the way a real transaction counter should. See EMV Contactless Book C-2.
- What format and length does EMV tag 9F64 use?
- Tag 9F64 uses the Binary format and is normally 1 bytes long.
- Is tag 9F64 provided by the card or the terminal?
- Tag 9F64 (NATC(Track1)) is provided by the Card (ICC).
Sources
- C-2-Kernel-2-V2.11-Final-June-2023, p. 398
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