March 4th, 2012, 12:05 AM

Question
Can anyone help me to answer this question?
 Bob believes that he has come up with a nifty hash function. He assigns a numeric value VChar to each letter in the alphabet equal to the letter’s position in the alphabet, i.e., VA = 1, VB = 2, …, VZ = 26. For a message, he calculates the hash value H = (VChar 1 x VChar 2 x VChar 3 …x VChar N) mod(26). Bob uses this function to send a oneword message, PLANE, to his banker Bill, along with his calculated hash value for the message. Alice is able to intercept the message, and generates an alternative message that has a hash value that collides with Bob’s original hash value. Show a message that Alice may have spoofed, and demonstrate that its hash value collides with Bob’s original hash.
March 4th, 2012, 04:16 AM

An understanding of math  specifically multiplication  would be really nice to have.
Consider that 4 * 5 = 20 but so does 2 * 10 = 20.
Consider that 2 * 4 = 8 but so does 2 * 2 * 2 = 8.
Consider that 5 * 5 = 25 but so does 1 * 1 * 1 * 1 * ... * 5 * 5 = 25.
Consider that (26 * anything) mod 26 = 0.
March 4th, 2012, 10:23 PM

So What's the answer for the question?
So What's the answer for the question?
March 4th, 2012, 11:00 PM

Nobody is going to just give you the answer. That's not how "helping answer a question" works around here.
EDIT:
I just realized this was double posted yesterday (probably accidentally):
http://forums.devshed.com/securityandcryptography17/hashingquestion888725.html
If you want help with learning how to find the solution, you can start by answering my questions in your other thread. Otherwise look elsewhere for help.
Last edited by EOreo; March 4th, 2012 at 11:03 PM.
PHP FAQ
Originally Posted by Spad
Ah USB, the only rectangular connector where you have to make 3 attempts before you get it the right way around
March 5th, 2012, 11:17 AM

My answer is the following but non of the result might be the answer.
For the word "PLANE", its hash value is H = (16 * 12 * 1 * 14 * 5) mod 26 = 24.
Alice need to find an alternative message with the same hash value.
So I can try words in dictionary and can always find some words with hash value 24.
For example:
"lab": 12*1*2 mod 26 = 24
"bee": 2*5*5 mod 26 = 50 mod 26 = 24
"sad": 19*1*4 mod 26 = 76 mod 26 = 24
"up": 21*16 mod 26 = 24
"letter" 12*5*20*20*5*18 mod 26 = 24
"calculate" 3*1*12*3*21*12*1*20*5 mod 26 = 24
March 5th, 2012, 01:16 PM

All of those are answers to the question. Each of those are a message Alice could have spoofed because they have the same hash as PLANE. Additionally, you already demonstrated that the hash of those words collides with the hash of PLANE.
So at this point you've fully answered the question several times over.
PHP FAQ
Originally Posted by Spad
Ah USB, the only rectangular connector where you have to make 3 attempts before you get it the right way around