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  #1  
Old March 3rd, 2012, 11:08 PM
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Hashing 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 one-word 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.

Can anyone help?

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Old March 4th, 2012, 12:24 AM
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There are two mathematical properties of multiplication that make finding a collision trivial, pick one:
http://en.wikipedia.org/wiki/Commutative
http://en.wikipedia.org/wiki/Identity_element
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Old March 4th, 2012, 03:05 AM
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Explain more please AS I'M BEGINNER

Explain more please AS I'M BEGINNER

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Old March 4th, 2012, 10:40 AM
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OK, let me ask two questions as a starting point:

(1) What is the hash value of Bob's message?

(2) Do you understand what a hash collision is?

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Old March 5th, 2012, 09:56 AM
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That's my answer but I can't know exactlt wat's the message
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

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