Passwords
The Good, The Bad and The Ugly
• Authenticate: You are what you know
• Assumption: The password is something only the user
knows
– Proves identity of the user
– Gives access to data/functionality that only the user should
be able to have
The Good, The Bad and The Ugly
• The good
– Rely on memory only, no extra hardware/software
– Easy to change if compromised
– Universally used, requires terminal access only
• The bad
– Weak passwords, easy to guess by strangers
– Personal passwords, easy to guess by friends
– Many passwords, reuse or forget/reset
• The ugly
– Compromised passwords reused at other servers
– Compromised passwords give access to more than
just user data
Ideal Passwords Are
• Strong
– Hard to guess by strangers and friends
• Memorable
– Easily and consistently recalled by the user
• Diverse
– Very different passwords for different sites
Opposite Requirements
• Strong
– Look like random character strings
• Memorable
– Have some connection to user memories/values
• Diverse
– Hard to remember many strong passwords,
which are uncorrelated
Some Other Auth Failure Causes
• Capitalization, plural, punctuation, character
replacement
– Hard to remember
• Misspelling
• Password policies are not public during auth.
• Hard to associate a password with a site
– Username too
• Making choices is easy, remembering them is hard
Password Storage
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Alice inputs her password, computer verifies
this against list of passwords
If computer is broken into, hackers can learn
everybody’s passwords
One-way hashes are easy to compute but hard to
reverse (reversing requires exhaustive search)
– Use one-way functions, store the result for
every valid password
– Perform one-way function on input,
compare result against the list
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Password Storage
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Hackers can compile a list of frequently used
passwords, apply one-way function to each
and store them in a table – dictionary attack
Host adds random salt to password, applies
one-way function to that and stores result and
salt value
– Randomly generated, unique and long enough
How People Use Passwords*
• ½ million users over 3 months
• Instrument browser, observe
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Number of passwords/sites total and per day
Password strength and types
Password reuse (only able to detect substring matches)
Recall via reset events
*“A Large-Scale Study of Web Password Habits”
Dinei Florencio and Cormac Herley, Microsoft Research, Proc. of WWW 2007
Password Usage
• An average user has 25 password accounts
• On average 7 actively used, unique passwords
– 8 passwords typed per day, includes duplicates
• An average password is reused at 6 sites
– Strong passwords (>60 bits, 9 chars) are reused at
4.48 sites
– Weak passwords (<30 bits, 4 chars) are reused at
6 sites
Lots of need for passwords, lots of reuse
Password Strength
Policy drives password strength
Password Composition
Most passwords contain only letters
Password Reset
• 4.28 % of Yahoo users forgot their password
over 3 month period
• 15% of visits were for password resets
• No results for other sites
Password resets are frequent
How People Reuse Passwords*
• Collected publicly available leaked password sets
– Containing email and password
– This way authors can evaluate if one could use
credentials stolen from one site to log into another
– Without email one could mix up the above measure
with measure how many users use popular passwords
• 6,077 unique users with at least two leaked
passwords (97.75% had exactly two)
– So this study is really asking “if you have two passwords
how likely it is that they are similar/same”
*“A Tangled Web of Password Reuse”
Anupam Das et al, UIUC, Proc. of NDSS 2014
Do People Reuse Passwords?
• Password reuse strategies
2-3 chars
Yes! Many passwords are reused with simple strategies
Attacker can try a small number of candidates to
version a compromised password into the correct one
Prominent Transformation Rules
A few trials for the attacker
Attack Success
30% within 10 attempts
85% of non-identical passwords can be guessed
if a password at another site is leaked for the same user
Why People Version Passwords
Ran a survey, 220 respondents
Version to comply with policy, increase security or
increase recall
How Many Different Passwords
Ran a survey, 220 respondents
Most have 2-4
How Are Passwords Saved
Ran a survey, 220 respondents
Most are memorized
How People Create Passwords*
• Lab study with 49 participants
– Created passwords for fictitious banking, email and
news website
– While thinking aloud
– 3 password composition policies, each user assigned to
only one of them
• 1class6: 6 characters
• 2class8: 8 characters, 2 classes
(e.g., upper and lowercase letters)
• 3class12: 12 characters, 3 classes
• Post-study interview to investigate strategies
*”I added ! at the end to make it secure”
Blasé Ur et al, CMU, Proc. of SOUPS 2014
Password Strength
38% are guessable by dictionary attacks
Examples of Passwords
• Guessed
– Tyrone1975
– Gandalf*8
– Triptrip1963
• Not guessed
– 5cupsoftoys
– AfNaHiLoco
– 7301Poplarblvd$
Passwords mostly not on popular password lists
Does Value of Site Matter?
• 43% of users considered all accounts to be of
equal value
– Either reused the same password or different
password but same composition strategy
• 14% thought their news account was of low value
• 22% thought only bank account was of high value
and news and email of low value
• Remaining 20% considered all accounts of
different value (highest bank or email)
Does Value of Site Matter?
• Users tended to create weak passwords for
low-value sites and heavily reuse them
– They were more careful about high-value sites
– Most didn’t understand why reuse is bad if
password is strong
• But they didn’t understand how to create a
strong password
– Common misconception: changing or adding a few
characters to common words makes strong
password
Users assumed a human guesser
Password Creation Strategies
• Usually same approach for all accounts
– Link to site or use user-specific words
– Then just version
– Improves memorability
• Capital letters in the beginning, numbers at
the end
• 35% of users relied on memory only to store
passwords so memorability was of high
concern
Very predictable
Password Creation Strategies
Password Creation Strategies
Password Creation Strategies
• Words chosen based on personal significance
– Names of people, pets, locations, dates
• Many passwords site-specific
• Simple transformation rules that are supposed to
make password more secure
– Attackers don’t know my pet’s name
– Attackers expect me to use my birthday, I will use my
friend’s birthday
– I don’t have this info on Facebook
– Hard-to-spell words, passphrases from books
– Don’t work against automated attacks
Users don’t expect automated attacks
Possible Improvements
• Help users choose strong passwords
(Telepathwords, password strength meters)
• Educate users about good choosing strategies
(start with passphrase, replace some characters)
• Come up with a different way to do passwords
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Draw them
Pick favorite picture
Security questions
Use memories of personal events
Mnemonic Passphrase (MNPass)
STEEL Group
Simon Woo and Jelena Mirkovic
Spring 2016
About Passphrases
• USC ITS: About Your USC NetID Password
(https://itservices.usc.edu/accounts/password/)
Have to be memorable
Not necessarily True (Rao et al. )
Have to be Secure
MUST
Clear tension between memorability and security
Mnemonic Passphrase (MNPass)
• System-Generated Passphrases (SysPass): Secure but less
memorable. Also, does not scale as number of accounts increase
• User-Chosen Passphrases (Upass): Memorable but less secure
• Our Goal (MNPass): explore Memorable, Secure, Diverse, and
Usable Passphrase generation
– We hypothesis people are creative enough but just not trained to come up
with good passphrases
– Increase Memorability: By letting users choose their own instead system
assigns and also provide the first letter mnemonic
– Increase Security: By carefully generating mnemonic chars (MNChars)
constraints, where each word user chooses should contain a letter from
MNchars in order
MNPass Example
Study Link
Examples
Preliminary Result
• Memorability
• Security (Entropy)
Try it out
• http://leps.isi.edu/wpass/comb/mnpassspass-USC/step0.php
Life-Experience Passwords (LEPs)
Problem with passwords
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Strong passwords are easily forgotten
Weak passwords are easily broken
Users reuse passwords at different sites
This holds for non-textual passwords too, plus they
are more difficult to use
memorability
guessability
Life-experience Passwords
• Use memories from a user’s past
• Collect facts – time, locations, people, activities,
conversations
– No preferences, no opinions
• Turn this into Q & A pairs
– Questions become prompts
– Answers become LEP
Life-experience
Passwords
CREATION
AUTHENTICATION
user narrative user title
user answers
title
hash
question
Factoid extraction
factoid
question
answer
hash
store
title
match?
Challenges
• How to collect memories, needs to be user-friendly
– “Tell me a story” vs Q & A
• How to mine for useful data
– Using natural language processing, hard in general
• How to detect weak facts
– E.g. relationships vs names, empty stories
• How to avoid use of sensitive info in LEPs
• How to deal with synonyms, misspellings, etc.
• How to store these passwords using one-way hashes
User Studies
• Ask a user to create
– 3 LEPs
– 3 LEPs and 3 ordinary passwords (OPs)
– 10 LEPs and 10 OPs
• User returns after 1 week to authenticate
• Measure strength, memorability and guessability
LEPs Are Strong
• Average strength 82-106 bits
– Equivalent to a strength of a 3class12 password
– This is a strength against dictionary attack
– Strength against brute-force attacks is 132-164 bits (3class25)
• Ordinary (3class8) passwords have 56-bit strength
• LEPs consist of 5-7 facts
LEPs Have Good Recall
• 36-58% recalled after a week
– Versus 27% of ordinary passwords
– Recall climbs to 65-79% if we allow for 4-out-of-5 matches
• Recall similar to recall of security questions as
measured by Google and Microsoft
– But security questions have lower security against
dictionary attacks (8.4-13% are guessable)
LEPs Have Good Diversity
• Less than 0.5% are the same and up to 34% are
similar
– Attacker must guess 34-48 bits to authenticate
• Around 2% of OPs are the same and up to 43% are
similar
– Attacker must guess 37 bits to authenticate
Issues
• LEPs took 5x longer to create and authenticate
• How do we store LEPs?
– Hash per answer
• Easy to break by guessing most likely answers first
– Hash per LEP
• User must recall all facts
– Several hashes of strong combinations of facts
• No feedback to user what they missed
Participate in Our User Study
• Not a part of class
– No penalty if you don’t participate
– No gain wrt grade if you do
• Need participants by Feb 1st
• We’ll pay 10$ per participant and you get to
contribute to knowledge of how to build better
passwords 
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Need groups of friends to take the study together
We’ll work around your schedule
Study takes 30 minutes max
E-mail me or Simon Woo (simonwoo@usc.edu)