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