Use constant time encoding / decoding of security critical code (#3699)

Use constant time encoding / decoding of security critical code (#3699)

* Use `Hex::decode` to convert hex2bin
Previously we used the internal PHP function `hex2bin` which has the problem with cache-timing leaks. The Hex class converts the given string without cache-timing leaks.

* Use `Hex::encode` to convert bin2hex
Previously we used the internal PHP function `bin2hex` which has the problem with cache-timing leaks. The Hex class converts the given string without cache-timing leaks.