Member Avatar for vijaysrivatsa

Hello Friends...we r doin our UG project named Watermarking relational database using optimization based techniques....here we r supposed to use a PJW hash function to encode the values inside a database with the help of a secret key....Can anyone pls explain me the logic for this hash function and i ll be very happy if u quote it with an example....Thank you

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asked for the PJW hash logic and an example; brief context: PJW (Peter J. Weinberger) is a non-cryptographic string hash used for symbol tables (the common ELF hash is a variant). It folds high bits back into the low bits to avoid losing information on overflow. It is not a keyed or secure MAC — do not use PJW to provide secrecy. See for background.

Algorithm (plain description): start with hash = 0. For each input byte do hash = (hash << 4) + byte. Extract the high bits with high = hash & 0xF0000000. If high != 0 then fold them in with hash ^= (high >>> 24) and clear them hash &= ~high. At the end you can return hash or a non-negative version hash & 0x7FFFFFFF, and take hash % tableSize for bucket indices. The constants correspond to OneEighth = 4, ThreeQuarters = 24 on a 32-bit word.

Short Java implementation (use bytes to match the original C behavior):

import java.nio.charset.StandardCharsets;

public static int pjwHash(byte[] data) {
    int hash = 0;
    for (byte b : data) {
        hash = (hash << 4) + (b & 0xFF);
        int high = hash & 0xF0000000;
        if (high != 0) {
            hash ^= (high >>> 24);
            hash &= ~high;
        }
    }
    return hash & 0x7FFFFFFF;
}

public static int pjwHash(String s) {
    return pjwHash(s.getBytes(StandardCharsets.UTF_8));
}

Notes and cautions: Java char is 16-bit, so iterate bytes (UTF-8) for consistent results. If a secret key is required for watermarking or integrity, use a keyed cryptographic construction (for example HMAC-SHA256) rather than trying to “salt” PJW — HMAC is designed for secrecy and integrity (HMAC). As and implied, PJW is simple to implement and good for hashing/indexing, but it is not a substitute for cryptographic primitives.

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Member Avatar for jwenting

no, we're not your friends.
We're also not going to read anything that's obviously concocted by someone who doesn't care whether his/hers/its writing is at all understandable to others.
And most of all we're not here to do your homework for you.

Member Avatar for vijaysrivatsa

:D hey that was great.......thanks duckman...i din want u to do my homework..i just wanted to know if anyone can teach me the logic of PJW hash function or atleast wat it does....if u din know..then u need not have replied.....

Member Avatar for Ezzaral

Well, you are asking someone to teach you how to do your project, which amounts to the exact same thing. Make some effort.

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