Is it possible to limit the hashcode into specific number of characters in Java

Question

I have written a method to convert a plain text into it's hashcode using MD5 algorithm. Please find the code below which I used.

public static String convertToMD5Hash(final String plainText){
            MessageDigest messageDigest = null;

            try {
                messageDigest = MessageDigest.getInstance("MD5");
            } catch (NoSuchAlgorithmException e) {
                LOGGER.warn("For some wierd reason the MD5 algorithm was not found.", e);
            }

            messageDigest.reset();
            messageDigest.update(plainText.getBytes());
            final byte[] digest = messageDigest.digest();
            final BigInteger bigInt = new BigInteger(1, digest);
            String hashtext = bigInt.toString(8);

            return hashtext;
}

This method works perfectly but it returns a lengthy hash. I need to limit this hash text to 8 characters. Is there any possibilities to set the length of the hashcodes in Java?

Answer 1

Yes and No. You can use a substring of the original hash if you always cut the original hash-string similary (ie. 8 last/first characters). What are you going to do with that "semi-hash" is another thing.

Whatever it is you're going to do, be sure it has nothing to do with security.

Here's why: MD5 is 128-bit hash, so there's 2^128 = ~340,000,000,000,000,000,000,000,000,000,000,000,000 possible permutations. The quite astronomical amount of permutations is the thing that makes bruteforcing this kind of string virtually impossible. By cutting down to 8 characters, you'll end up with 32-bit hash. This is because a single hex-value takes 4 bits to represent (thus, also 128-bit / 4 bit = 32 hex-values). With 32-bit hash there's only 2^32 = 4,294,967,296 combinations. That's about 79,228,162,514,264,337,593,543,950,336 times less secure than original 128-bit hash and can be broken in matter of seconds with any old computer that has processing power of an 80's calculator.