RSA-OAEP Input Parameters

When creating padding for RSA using OAEP, a message is prepared as follows:

Hash(Input Parameter) || Zeros || 1 || Message


My question is, what should the Input Parameter be? It must be known to both sides, I understand it can be an empty array (string, whatever). In that case there is no need to calculate the hash, hashing an empty input always produces the same result.

Is there a recommendation regarding what to use as the input parameter?

I'm thinking about hashing the public modulus n, which is already known by all parties. It's not necessary I suppose since the random seed from the other part of the OAEP procedure will change the final value anyway, but I'm still curious what is considered "best practice", or some requirement that I missed.

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See PKCS#1v2.1 which calls that a Label, and states it might be empty, or in some specified syntax. See here, in particular Remark 3, for a discussion on possible uses. – fgrieu Mar 11 '12 at 19:52
@fgieu That might be considered an answer. Answered questions will receive less attention from other experts (and nutcases such as me :) ). – Maarten Bodewes Mar 21 '12 at 0:53
@owlstead: Feel free to expand this into an answer. – Paŭlo Ebermann Apr 15 '12 at 21:20