2 cissp ® Official Study Guide Eighth Edition

Birthday Attack 
A
birthday attack
focuses on fi nding collisions. Its name comes from a statistical phe-
nomenon known as the birthday paradox. The birthday paradox states that if there are 
23 people in a room, there is a 50 percent chance that any two of them will have the 
same birthday. This is not the same year, but instead the same month and day, such as 
March 30. 
With February 29 in a leap year, there are only 366 possible days in a year. With 367 
people in a room, you have a 100 percent chance of getting at least two people with the 
same birthdays. Reduce this to only 23 people in the room, and you still have a 50 percent 
chance that any two have the same birthday. 
This is similar to fi nding any two passwords with the same hash. If a hashing function 
could only create 366 different hashes, then an attacker with a sample of only 23 hashes 
has a 50 percent chance of discovering two passwords that create the same hash. Hashing 
algorithms can create many more than 366 different hashes, but the point is that the birth-
day attack method doesn’t need all possible hashes to see a match. 

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Chapter 14 
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Controlling and Monitoring Access
From another perspective, imagine that you are one of the people in the room and you 
want to fi nd someone else with the same birthday as you. In this example, you’ll need 
253 people in the room to reach the same 50 percent probability of fi nding someone else 
with the same birthday. 
Similarly, it is possible for some tools to come up with another password that creates 
the same hash of a given hash. For example, if you know that the hash of the administrator 
account password is 1A5C7G, some tools can identify a password that will create the same 
hash of 1A5C7G. It isn’t necessarily the same password, but if it can create the same hash, 
it is just as effective as the original password. 
You can reduce the success of birthday attacks by using hashing algorithms with enough 
bits to make collisions computationally infeasible, and by using salts (discussed in the 
“Rainbow Table Attacks” section next). There was a time when security experts considered 
MD5 (using 128 bits) to be collision free. However, computing power continues to improve, 
and MD5 is not collision free. SHA-3 (short for Secure Hash Algorithm version 3) can use 
as many as 512 bits and is considered safe against birthday attacks and collisions—at least 
for now. Computing power continues to improve, so at some point, SHA-3 will be replaced 
with another hashing algorithm with longer hashes and/or stronger cryptology methods 
used to create the hash.