Fix code scanning alert - Use of RSA algorithm without OAEP

[xokker]

I'm not sure how serious these warning are, but it's probably a good idea to keep track of them here.

Tracking issue for:

• https://github.com/google/data-transfer-project/security/code-scanning/6
• https://github.com/google/data-transfer-project/security/code-scanning/7

[Anilreddy2009]

@xokker can you assign this issue

[lisad]

Discussed today in maintainer's meeting.

• We don't think this is a severe vulnerability for this project.
• An update to the algorithm might not cause any breakage but of course that's so hard to tell that even just updating this algorithm would require folks who use it to test.