How Hard Is It To Crack A Password?


With modern computer hardware, you can guess 100,000,000 passwords/sec, using readily available graphics processing units (that number is probably conservative).  These computers have specialized co-processors that are designed to do math-intensive calculations for rendering computer graphics.  They’re equally good at doing math-intensive calculations for cracking passwords. If you don’t own such a system, you can rent one on-line for a little over $2 per hour from Amazon.

 Let’s take the worst case:

 A password made up of upper and lower case letters, numerals and punctuation, gives 96 possible characters.  A 14 character length password (NIST recommended) gives you 4.8 x 10^27 combinations.  Even at 100,000,000 per second, that would take you more than a million years to guess if you had to try every possible combination.  Since, on average, you’ll guess the password after trying only half of the possible combinations, we’re down to 500,000 years.

 If you can’t wait that long, here are some strategies to make things go faster:

 Those numbers presume a real random password.  Unfortunately, we humans (as Randy Munroe points out) don’t do well with random characters.  We can’t reliably remember them.  So we tend to come up with non-random passwords that we can remember. 

 It’s well known that most people pick easy to guess passwords, even if they’re trying not to.  And it’s also true that the more often you have to change your password the worse your choice will become.  If you have to change your password every few weeks or months, you’re more likely to choose an easily guessable one. 

 Depending on whom you ask, there are about 250,000 words in the English language.  Most people base their passwords on one of them. If you’re forced to add a number or punctuation character, you most likely add it to the end of the word.  You might use “leet speak” (‘4’ for ‘A’; ‘1’for ‘L’; ‘3’ for ‘E’; etc.) character substitutions too.

Two digits in front or after the word increases the search space by 10000 times.    Adding leet substitutions increases it 16 times.

 With those additions to standard words we get 2.5 x 10^5 x 16 x 10^4 = 40 x 10^9 possible passwords.

So, using my rented graphics-rendering computer, searching every English word, with one or two digits in front or after it, using any combination of “leet” substitutions would take less than 10 minutes to crack (40 x 10^9 / 10^8 = 400 seconds).  And I still haven’t used up my $2.

 Let’s make it just a little more complicated:  I’ve seen many passwords that vary the letter case, usually adding one or two capital letters.  Assuming an average word length of 6 letters, that increases the time by a factor of 36.  That ups the cracking time to around four hours.

So for a little less than $10, I can find every password that’s based on an English word, that adds one or two digits, one punctuation character and up to two mixed case letters.  In my experience visiting lots of client systems, I’ve covered 90% of the passwords in use.  And it’s only lunchtime.

 There are many additional strategies to search for more complicated passwords.  A simple one is based on the idea that we tend to make up passwords that can be pronounced – that is, ones that use standard English consonant blends and diphthongs.  For example, if I want to invent a word, I’m much more likely to come up with “grooz” than “zmloqk.” The first uses standard blends (“gr”) and standard diphthongs (“oo”).  “Zmloqk” uses nonstandard combinations.  We’re much less likely to choose those, and a password cracker can take advantage of that fact.

 Another oft-touted strategy is to use a phrase or combination of words.  This is a good idea if you do it right.  With my same rented computer, I can guess 3 word combinations (“blue pickled trucks”) of the 50,000 most common words in about a day and a half.  Four words, however, increases the cracking time to almost 200 years.

 In the end, we’re stuck with this dilemma of human nature:  we can create complex passwords that are impossible to guess, but we can’t remember them.  On the other hand, most passwords we do come up with can be cracked with a few hours’ time and beer money.  Keep that in mind the next time you choose a password.

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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.