Well, there is a derive-key for rc4, but it only takes keyusage as input, not a string. Defining dk interms of dr would work for rc4 if you had a reasonable definition of dr, but you currently do not. This is an issue both against the code and against your draft. The issue against your draft is best solved by including dr in the crypto profile; the issue against the code is more complex as it requires us to actually define dr for rc4. One simple but kind of sucky definition of dr might be md4. A better definition would involve the data-dependent key setup from the rest of the rc4 draft with the confounder removed.