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javastringhashreduction

Java convert hash to random string

I'm trying to develop a reduction function for use within a rainbow table generator.

The basic principle behind a reduction function is that it takes in a hash, performs some calculations, and returns a string of a certain length.

At the moment I'm using SHA1 hashes, and I need to return a string with a length of three. I need the string to be made up on any three random characters from:

abcdefghijklmnopqrstuvwxyz0123456789

The major problem I'm facing is that any reduction function I write, always returns strings that have already been generated. And a good reduction function will only return duplicate strings rarely.

Could anyone suggest any ideas on a way of accomplishing this? Or any suggestions at all on hash to string manipulation would be great.

Thanks in advance

Josh

Solution

  • Applying the KISS principle:

    • An SHA is just a String
    • The JDK hashcode for String is "random enough"
    • Integer can render in any base

    This single line of code does it:

    public static String shortHash(String sha) {
    return Integer.toString(sha.hashCode() & 0x7FFFFFFF, 36).substring(0, 3);
}

    Note: The & 0x7FFFFFFF is to zero the sign bit (hash codes can be negative numbers, which would otherwise render with a leading minus sign).

    Edit - Guaranteeing hash length

    My original solution was naive - it didn't deal with the case when the int hash is less than 100 (base 36) - meaning it would print less than 3 chars. This code fixes that, while still keeping the value "random". It also avoids the substring() call, so performance should be better.

    static int min = Integer.parseInt("100", 36);
static int range = Integer.parseInt("zzz", 36) - min;

public static String shortHash(String sha) {
    return Integer.toString(min + (sha.hashCode() & 0x7FFFFFFF) % range, 36);
}

    This code guarantees the final hash has 3 characters by forcing it to be between 100 and zzz - the lowest and highest 3-char hash in base 36, while still making it "random".