more explanation of HMAC construction

Author: drbruced12

crypto.rst

@@ -602,7 +602,19 @@ HMAC = H((K⊕opad) || H((K⊕ipad) || text))
 
 H is the hash function, K is the key, and opad (output pad) and ipad
 (input pad) are well-known strings that are XORed (⊕) with the key. ||
-represents concatenation. 
+represents concatenation.
+
+A deep explanation of this HMAC function is beyond the scope of this
+book. However, this approach has been proved to be secure as long as
+the underlying hash function H has the appropriate
+collision-resistance properties outlined above. Note that the HMAC
+takes a hash function *H* that is not keyed, and turns it into a keyed
+hash by using the key (XORed with another string, *ipad*) as the first
+block to be fed into the hash function. The output of
+the keyed hash is then itself subjected to another keyed hash (again
+by XORing the key with a string and using that as the first block fed
+to the hash.) The two passes of the keyed-hash function are important
+to the proof of security for this HMAC construction. 
 
 .. let's delete this incorrect pic for now
   .. _fig-macAndHmac: