How to generate tokens from a secret key?
Digital Signatures are the solution.

A very simple algorithm is


Just like RSA, generate a public key $(N, e)$ for everyone, and a private key $d$.
Whatever message $m$ you post, also post a digital signature $\sigma = m^d ~(mod ~ N)$
People can verify the post is by you by checking that $\sigma^e = m ~ (mod ~ N)$


For more rigorous schemes, see Digital Signature and Digital Signature Algorithm