Solved

Probability of different files with same checksum

Posted on 2011-03-15
5
888 Views
Last Modified: 2013-12-01
What is the probability that two files with the same extension, file size and checksum are actually different?
0
Comment
Question by:hankknight
5 Comments
 
LVL 73

Assisted Solution

by:sdstuber
sdstuber earned 125 total points
ID: 35137667
depends on the data and the checksum algorithm

for example

sum all bytes mod 2

half of all files will report the same check sum, the other half will report the other check sum
0
 
LVL 8

Accepted Solution

by:
point_pleasant earned 125 total points
ID: 35137711
for MD5 checksums

MD5 checksums are 128 bits wide (typically expressed as a sequence of 32 hexadecimal characters). So there are 2 to the 128th = 340282366920938463463374607431768211456 possible checksums.

in your situation the probability is for all purposes zero.
0
 
LVL 32

Assisted Solution

by:phoffric
phoffric earned 125 total points
ID: 35137766
   http://en.wikipedia.org/wiki/Checksum#Parity_byte_or_parity_word

Parity byte or parity word

"The simplest checksum algorithm is the so-called longitudinal parity check, which breaks the data into "words" with a fixed number n of bits, and then computes the exclusive or of all those words. The result is appended to the message as an extra word. To check the integrity of a message, the receiver computes the exclusive or of all its words, including the checksum; if the result is not a word with n zeros, the receiver knows that a transmission error occurred."

"With this checksum, any transmission error that flips a single bit of the message, or an odd number of bits, will be detected as an incorrect checksum. However, an error that affects two bits will not be detected if those bits lie at the same position in two distinct words. If the affected bits are independently chosen at random, the probability of a two-bit error being undetected is 1/n."

 -- fast check, but not reliable.
0
 
LVL 27

Assisted Solution

by:d-glitch
d-glitch earned 125 total points
ID: 35137792
I agree with sdstuber that the answer depends on the checksum algorithm,
but the sum of the bytes mod2 is not a likely candidate.

I agree with point pleasant that for practical purposes, a 128 byte checksum
will give approx zero chance of random collision.  But not all checksums are
128 bits wide.

You may also have to consider the chance that the files are not random.  It is
possible to construct a file that does have the same size and checksum of any
target file.  The larger the checksum, the more difficult it is.  Some cryptographic
attacks rely on this technique.

What are you relying on the checksum for?  Is there any incentive for malice?
0
 
LVL 73

Expert Comment

by:sdstuber
ID: 35137859
>>>  sum of the bytes mod2 is not a likely candidate.


agreed,  extreme example was chosen simply for illustration
0

Featured Post

Do You Know the 4 Main Threat Actor Types?

Do you know the main threat actor types? Most attackers fall into one of four categories, each with their own favored tactics, techniques, and procedures.

Join & Write a Comment

By default, Carbonite Server Backup manages your encryption key for you using Advanced Encryption Standard (AES) 128-bit encryption. If you choose to manage your private encryption key, your backups will be encrypted using AES 256-bit encryption.
The Delta outage: 650 cancelled flights, more than 1200 delayed flights, thousands of frustrated customers, tens of millions of dollars in damages – plus untold reputational damage to one of the world’s most trusted airlines. All due to a catastroph…
This tutorial will walk an individual through the process of installing the necessary services and then configuring a Windows Server 2012 system as an iSCSI target. To install the necessary roles, go to Server Manager, and select Add Roles and Featu…
This tutorial will walk an individual through setting the global and backup job media overwrite and protection periods in Backup Exec 2012. Log onto the Backup Exec Central Administration Server. Examine the services. If all or most of them are stop…

744 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

11 Experts available now in Live!

Get 1:1 Help Now