Potential side-channels during en-/decoding of private-key

[blochberger]

There are multiple places where an RSA private-key is encoded or decoded with Base64 using standard Swift library functions. Since these functions do not guarantee constant-time processing of the input, there is a potential side-channel that may leak information about the private-key.

I did not investigate the code thoroughly, but here are the potentially insecure uses I found at first glance:

https://github.com/IBM-Swift/BlueRSA/blob/9435e102af2838aa29c1f52a843ee1886876c379/Sources/CryptorRSA/CryptorRSAKey.swift#L392

https://github.com/IBM-Swift/BlueRSA/blob/9435e102af2838aa29c1f52a843ee1886876c379/Sources/CryptorRSA/CryptorRSAKey.swift#L631

https://github.com/IBM-Swift/BlueRSA/blob/9435e102af2838aa29c1f52a843ee1886876c379/Sources/CryptorRSA/CryptorRSAKey.swift#L676

https://github.com/IBM-Swift/BlueRSA/blob/9435e102af2838aa29c1f52a843ee1886876c379/Sources/CryptorRSA/CryptorRSAKey.swift#L739