Continuing the series of enhancing Groovy’s core language classes, here is a snippet that allows a String to be hashed using the SHA1 algorithm. Let’s look at the code first.

import java.security.*

def source = "Hello. This is my secret message. That should be hashed."

String.metaClass.toSHA1 = { salt = "" ->
   def messageDigest = MessageDigest.getInstance("SHA1")

   messageDigest.update(salt.getBytes())
   messageDigest.update(delegate.getBytes())

   /*
    * Why pad up to 40 characters? Because SHA-1 has an output
    * size of 160 bits. Each hexadecimal character is 4-bits.
    * 160 / 4 = 40
    */
   new BigInteger(1, messageDigest.digest()).toString(16).padLeft(40, '0')
}

println "Source = ${source}"
println "SHA1 = ${source.toSHA1('salty')}"

This script starts innocently enough with a source string that contains a message to be hashed. We then extend the String class with a new function named toSHA1(). This method uses the MessageDigest class from the java.security package to run the source string through the SHA1 hash routine. Also notice the optional salt argument. A salt is used to make the hash a little more secure and can complicate various hack attacks (http://en.wikipedia.org/wiki/Salt_%28cryptography%29). We then pass the byte array of the hashed string, as returned by messageDigest.digest(), into the constructor of a BigInteger, and in turn translate that into a string of Base 16 digits. Also notice the padLeft. That makes sure that we have 40 digits, each hexadecimal character being 4-bits, giving us 160-bits. Check Wikipedia and you’ll see that the SHA1 algorithm requires an output of 160-bits.

This technique could be easily applied to any other hashing routine such as MD5. Happy coding, and happy extending Groovy!