Help me decode a string if I know the input and the output

My task is for educational purposes, and I have been offered the challenge from a friend, however, I'm stuck on this question:

This task may be fun for some folks. But basically, I think it's AES.

The input: G9S4DZ2 or 35419946942 (which is just base10 of first variable)

output:

OTAwSEt5SitUU3F2YnlhTk5wSEpYQT090

which is base64 for: 900HKyJ+TSqvbyaNNpHJXA==

From there, I have no idea what the encryption type is :(

Taidot: Verkkoturva, Python, C-ohjelmointi, JavaScript

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Projektin tunnus: #28994911