My contribution is here: https://github.com/paragonie/constant_time_encoding
https://galois.com/blog/2013/09/high-assurance-base64/
Also, anyone wanting constant time implementation might just run a verified implementation through something like FaCT or Jasmin:
https://cseweb.ucsd.edu/~dstefan/pubs/cauligi:2017:fact.pdf
https://acmccs.github.io/papers/p1807-almeidaA.pdf
My contribution is here: https://github.com/paragonie/constant_time_encoding