I have a series of 20-digit long case-sensitive alpha-numerical strings and their corresponding 10-digit long hex strings.

I want to find the constant key that's used to calculate the hex from the alpha-numerical strings.

How do I do that?

Example string pairs:
String:
12345678901234567890
Hex:
01FF25E1BC

String:
12345678901234567891
Hex:
3EF4EFF2E9

String:
abababababababababab
Hex:
6C027B9F19

String:
abababababababababaa
Hex:
2D0034ED50

Thanks!