Sign.Sval secret_key : storage Fmt.tsecret_key formats a secret_key in this fashion: (Sign.secret_key AAAAAA==). If the storage value is not Sodium.Sign.secret_key_size bytes long, it will be formatted as (Sign.secret_key <size mismatch>).
val public_key : storage Fmt.tpublic_key formats a public_key in this fashion: (Sign.public_key AAAAAA==). If the storage value is not Sodium.Sign.public_key_size bytes long, it will be formatted as (Sign.public_key <size mismatch>).