Strings need to be encoded into bytes before the RSA module can operate
on them.

Store blinding factor + its inverse, so that they can be reused & updated
on every blinding operation. This avoids expensive computations.

The reuse of the previous blinding factor is done via squaring (mod n), as
per section 9 of 'A Timing Attack against RSA with the Chinese Remainder
Theorem' by Werner Schindler, https://tls.mbed.org/public/WSchindler-RSA_Timing_Attack.pdf

Crypto length and blocksize are public info, so don't need side-channel
free comparison.

Use `bytes.find()` instead of `bytes.index()`, as the former doesn't raise
an exception when the to-be-found byte doesn't exist.