Method and result
For a one-character input, the first weight is 7. A has value 10: 10 × 7 = 70, giving remainder 0. K has value 20: 20 × 7 = 140, also giving remainder 0. The calculation follows the check-digit method specified by ICAO. ICAO Doc 9303, Part 3: check-digit method
| Input | Value | Weighted total | Digit |
|---|---|---|---|
| A | 10 | 70 | 0 |
| K | 20 | 140 | 0 |
| 0 | 0 | 0 | 0 |
| < | 0 | 0 | 0 |
Why the collision occurs
If a single-position value increases by 10, the weighted total increases by a multiple of 10. Its remainder modulo 10 stays the same. This is a property of the arithmetic, not a finding about a particular passport or vendor.
Reproduce the demonstration
- Open the check-digit calculator and enter A.
- Record the displayed total and digit.
- Replace the input with K.
- Observe a different total and the same digit.
- Use the dataset below to check the other listed inputs.
Run the calculation
Inspect every value, weight and product.
Explore resourceDownload the ten-case CSV
Fictional arithmetic cases with explicit expected totals and digits.
Explore resourceScope and limitations
This dataset contains ten deliberately chosen synthetic data elements. It is not sampled from identity documents. It supports the narrow claim that different input strings can share a check digit. It does not estimate error-detection rates, fraud prevalence or the performance of any verification service.
The automated tests in this project's source check the listed expected totals and digits and confirm the illustrated collision. No document-upload service, commercial vendor or government-document authentication process was tested.