Sodium_fmt.Secret_boxSecret_box defines formatters for types defined in Sodium.Secret_box.
secret_key formats a secret_key in this fashion: (Secret_box.secret_key AAAAAA==).
nonce formats a nonce in this fashion: (Secret_box.nonce AAAAAA==).
module type S = sig ... endBytes offers versions of each of the above formatters, which get their value from a bytes value.