# How to calculate hash sum of a string (using java)?

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Sebastian Holmqvist on 6 Aug 2012
Answered: Oliver Woodford on 5 May 2016
Couldn't find this very easily (without including a bunch of c-mex files etc) so I thought I'd post it here for future reference. I prefer using java methods to avoid dependencies.
Question:
Using java, how to create a MD5 (or SHA1 etc) hash sum of a string?

Sebastian Holmqvist on 6 Aug 2012
Edited: Sebastian Holmqvist on 6 Aug 2012
Solution:
The trick here is to input an ascii representation of your string (hence, the double(string) call). MessageDigest outputs 16 bytes which represents 32 chars. So we use BigInteger to convert the bytes to that radix.
import java.security.*;
import java.math.*;
md = MessageDigest.getInstance('MD5');
bi = BigInteger(1, hash);
char(bi.toString(16))
ans =
b99e5935368933bafefed10b99bb0489

#### 1 Comment

Nick Hilton on 14 Oct 2015
I found a bug, BigInteger.toString(16) won't fill with zeros. For example, if the BigInter's hex representation was '0abc', toString(16) will only return 'abc'.
Fixing this requires using the java.lang.String.format method:
import java.lang.String;
char(String.format('%032x', bi));
MD5 returns 32 hex values, but if one were to change the message digest to 'SHA-256', then one needs to use:
char(String.format('%064x', bi));
This will guarantee a fixed width hash filled with '0'.

Oliver Woodford on 5 May 2016
A similar solution:
%STRING2HASH Convert a string to a 64 char hex hash string (256 bit hash)
%
% hash = string2hash(string)
%
%IN:
% string - a string!
%
%OUT:
% hash - a 64 character string, encoding the 256 bit SHA hash of string
function hash = string2hash(string)
persistent md
if isempty(md)
md = java.security.MessageDigest.getInstance('SHA-256');
end
hash = sprintf('%2.2x', typecast(md.digest(uint8(string)), 'uint8')');
end