def genKw(w,msk,z):
<EOL>TAG_KW = "<STR_LIT>"<EOL>b = hmac(TAG_KW, msk, z + w)<EOL>return BigInt(longFromString(b) % long(orderGt()))<EOL>
Generates key Kw using key-selector @w, master secret key @msk, and
table value @z.
@returns Kw as a BigInt.