optimize code against side-channel attacks

[omershlo]

make sure implementation behaviour is uniform in respect to secret data

[gbenattar]

https://www.nccgroup.trust/us/our-research/technical-advisory-return-of-the-hidden-number-problem/

[mortendahl]

Note for future: decrypting using the CRT can lead to attack vulnerabilities, see various literature on RSA and CRT side channel attacks (including faulty decryption).