question: public static DeterministicKey createMasterPrivateKey ( byte [ ] seed ) throws HDDerivationException { checkArgument ( seed . length > 8 , "Seed is too short and could be brute forced" ) ; byte [ ] i = HDUtils . hmacSha512 ( MASTER_HMAC_SHA512 , seed ) ; checkState ( i . length == 64 , i . length ) ; byte [ ] il = Arrays . copyOfRange ( i , 0 , 32 ) ; byte [ ] ir = Arrays . copyOfRange ( i , 32 , 64 ) ; Arrays . fill ( i , ( byte ) 0 ) ; DeterministicKey masterPrivKey = createMasterPrivKeyFromBytes ( il , ir ) ; Arrays . fill ( il , ( byte ) 0 ) ; Arrays . fill ( ir , ( byte ) 0 ) ; return masterPrivKey ; }
target:  Generates a new deterministic key from the given seed, which can be any arbitrary byte array. However resist the temptation to use a string as the seed - any key derived from a password is likely to be weak and easily broken by attackers (this is not theoretical, people have had money stolen that way). This method checks that the given seed is at least 64 bits long.