question: public static byte [ ] hash ( byte [ ] data , int offset , int len ) { try { MessageDigest a = MessageDigest . getInstance ( "SHA-256" ) ; a . update ( data , offset , len ) ; return a . digest ( a . digest ( ) ) ; } catch ( NoSuchAlgorithmException e ) { throw new RuntimeException ( e ) ; } }
target:  Double SHA256 hash of arbitrary data