It is written in the  paper that the ipad=0x36 and opad=0x5C were chosen such that it maximize the  of the part of the key used in the inner and outer part of the HMAC process.  Since 0x36=0b0110110 and 0x5c=0b1011100, it seems that the Hamming distance is 4, but I would expect that ipad and opad with Hamming distance of 8 would have been used.  Do you know how does those exact ipad and opad values make the scheme stronger?
Wikipedia lists the following pseudocode for HMAC:  function hmac (key, message)     if (length(key) &gt; blocksize) then         key = hash(key) // keys longer than blocksize are shortened     end if     if (length(key) &lt; blocksize) then         // keys shorter than blocksize are zero-padded ('∥' is concatenation)          key = key ∥ [0x00 * (blocksize - length(key))]      end if      // Where blocksize is that of the underlying hash function        o_key_pad = [0x5c * blocksize] ⊕ key      // Where ⊕ is exclusive or (XOR)     i_key_pad = [0x36 * blocksize] ⊕ key       // Where '∥' is concatenation        return hash(o_key_pad ∥ hash(i_key_pad ∥ message))  end function   Why are they there?  How were they chosen?  It seems to me that  $hash(key + hash(message))$ (where + is concatenation) would suffice.    Obviously I'm missing something.  What is it?
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