9 password security

CHAPTER 9
Password Security

Slides adapted from "Foundations of Security: What Every Programmer
Needs To Know" by Neil Daswani, Christoph Kern, and Anita Kesavan
(ISBN 1590597842; http://www.foundationsofsecurity.com). Except as
otherwise noted, the content of this presentation is licensed under the
Creative Commons 3.0 License.

Agenda
 Password systems ubiquitous, vulnerable

 Early password security studies (1979) - Morris,
Thompson: 86% of passwords can be cracked

 Threats: Online & Offline Dictionary Attacks

 Solutions: Hashing & Salting

9.1. A Strawman Proposal
 Basic password system: file w/ username,
password records (colon delimiter)

john:automobile
mary:balloon
joe:wepntkas

 Simple to implement, but risky
 All
users compromised if hacker gets the passwd file
 Done in Java: MiniPasswordManager

9.1. MiniPasswordManager
public class MiniPasswordManager {
/** dUserMap is a Hashtable keyed by username */
private static Hashtable dUserMap;
/** location of the password file on disk */
private static String dPwdFile;

public static void add(String username,
String password) throws Exception {
dUserMap.put(username, password);
}

public static boolean checkPassword(String username,
String password) {
try { String t = (String)dUserMap.get(username);
return (t == null) ? false : t.equals(password);
} catch (Exception e) {}
return false;
}
...
}

9.1. MPM: File Management
public class MiniPasswordManager {
...
/* Password file management operations follow */
public static void init (String pwdFile) throws Exception {
dUserMap = MiniPasswordFile.load(pwdFile);
dPwdFile = pwdFile;
}

public static void flush() throws Exception {
MiniPasswordFile.store (dPwdFile, dUserMap);
}
... // main()
}

9.1. MPM: main()
public static void main(String argv[]) {
String pwdFile = null;
String userName = null;
try {
pwdFile = argv[0];
userName = argv[1];
init(pwdFile);
System.out.print("Enter new password for " + userName + ": ");
BufferedReader br =
new BufferedReader (new InputStreamReader(System.in));
String password = br.readLine();
add(userName, password);
flush();
} catch (Exception e) {
if ((pwdFile != null) && (userName != null)) {
System.err.println("Error: Could not read or write " + pwdFile);
} else { System.err.println("Usage: java MiniPasswordManager" +
" <pwdfile> <username>"); }
}
}

9.1. MPM Analysis
 Two key functions: username, password args
 add()– add entry to dUserMap hashtable
 checkPassword() – lookup in dUserMap

 Read/Write dUserMap from/to disk using
init()/flush()
 MiniPasswordFile helper class exposes
store() and load() methods for these tasks

 More to do to make secure…

9.2. Hashing
 Encrypt passwords, don’t store “in the clear”
 Could decrypt (e.g. DES) to check, key storage?
 Even better: “one-way encryption”, no way to decrypt
 If file stolen, passwords not compromised
 Use one-way hash function, h: preimage resistant
 Ex: SHA-1 hashes stored in file, not plaintext passwd

john:9Mfsk4EQh+XD2lBcCAvputrIuVbWKqbxPgKla7u67oo=
mary:AEd62KRDHUXW6tp+XazwhTLSUlADWXrinUPbxQEfnsI=
joe:J3mhF7Mv4pnfjcnoHZ1ZrUELjSBJFOo1r6D6fx8tfwU=

9.2. Hashing Example

“What is your username & password?”

Does
My name is john. My password is automobile. h(automobile)
=
9Mfsk4EQ…
???

 Hash: “One-way encryption”
 No need to (can’t) decrypt
 Just compare hashes
 Plaintext password not in file, not “in the clear”

9.2. Hashing MPM Modifications
dUserMap.put(username,computeSHA(password));
}

String password) {
try { String t = (String)dUserMap.get(username);
return (t == null) ? false :
t.equals(computeSHA(password));
return false;
}

private static String computeSHA (String preimage) throws Exception {
MessageDigest md = MessageDigest.getInstance("SHA-256");
md.update(preimage.getBytes("UTF-8"));
byte raw[] = md.digest();
return (new sun.misc.BASE64Encoder().encode(raw));
}

9.3. Off-line Dictionary Attacks
Attacker Obtains  Offline: attacker steals file and
Password File: tries combos
joe 9Mfsk4EQ...  Online: try combos against live
mary AEd62KRD...
john J3mhF7Mv... system

Attacker computes possible password hashes
mary has (using words from dictionary)
password h(automobile) = 9Mfsk4EQ...
balloon! h(aardvark) = z5wcuJWE...
Attacker h(balloon) = AEd62KRD...
h(doughnut) = tvj/d6R4

9.4. Salting
 Salting – include additional info in hash

 Add third field to file storing random # (salt)

 Example Entry: john with password automobile
john:ScF5GDhWeHr2q5m7mSDuGPVasV2NHz4kuu5n5eyuMbo=:1515

 Hash of password concatenated with salt:
h(automobile|1515) = ScF5GDhW...

9.4. Salting Functions
public static int chooseNewSalt() throws NoSuchAlgorithmException {
return getSecureRandom((int)Math.pow(2,12));
}

/* Returns a cryptographically random number in the range [0,max) */
private static int getSecureRandom(int max) throws
NoSuchAlgorithmException {
SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
return Math.abs(sr.nextInt()) % max;
}

public static String getSaltedHash(String pwd,
int salt) throws Exception {
return computeSHA(pwd + "|" + salt);
}

9.4. Salting in MPM (1)
/* Chooses a salt for the user, computes the salted hash of the
user's password, and adds a new entry into the userMap hashtable for
the user. */
int salt = chooseNewSalt();
HashedPasswordTuple ur = new
HashedPasswordTuple(getSaltedHash(password,salt),salt);
dUserMap.put(username,ur);
}

String password) {
try { HashedPasswordTuple t =
(HashedPasswordTuple)dUserMap.get(username);
return (t == null) ? false :
t.getHashedPassword().equals
(getSaltedHash(password,t.getSalt()));
return false;
}

9.4. Salting in MPM (2)
 dUserMap stores HashedPasswordTuple,
hashed password and salt
 To add(), we chooseNewSalt() to
getSecureRandom() number in [0, 4096)

 getSaltedHash() to compute h(passwd|salt)
 checkPassword() by comparing hash on file
w/ salted hash of input password and salt on file

9.4. Salting: Good News
 Dictionary attack against arbitrary user is harder
 Before Salts: hash word & compare with password file
 After Salts: hash combos of word & possible salts

 n-word dictionary, k-bit salts, v distinct salts:
 Attacker must hash n*min(v, 2k) strings vs. n (no salt)
 If many users (>> 2k, all salts used), 2k harder attack!
 Approx. same amount of work for password system

9.4. Off-line Dictionary Attack
Foiled!
h(automobile2975) = KNVXKOHBDEBKOURX
h(automobile1487) = ZNBXLPOEWNVDEJOG
h(automobile2764) = ZMCXOSJNFKOFJHKDF
h(automobile4012) = DJKOINSLOKDKOLJUS
h(automobile3912) = CNVIUDONSOUIEPQN
…Etc…
h(aardvark2975) = DKOUOXKOUDJWOIQ
h(aardvark1487) = PODNJUIHDJSHYEJNU
…Etc…
Too many
/etc/passwd: combinations!!!
john LPINSFRABXJYWONF 2975 Attack is
mary DOIIDBQBZIDRWNKG 1487 Foiled!
joe LDHNSUNELDUALKDY 2764

9.4. Salting: Bad News
 Ineffective against chosen-victim attack
 Attackerwants to compromise particular account
 Just hash dictionary words with victim’s salt

 Attacker’s job harder, not impossible
 Easy for attacker to compute 2kn hashes?
 Then offline dictionary attack still a threat.

9.4. BasicAuthWebServer
(BAWS)
 Adapt (and rename) SimpleWebServer from
Ch. 2 to use MiniPasswordManager
 Used to implement HTTP authorization
 Only authenticated clients can access documents
from our server
 First, create a password file:
$ java MiniPasswordManager pwdfile hector
Warning: Could not load password file. #pwdfile doesn't exist yet
Enter new password for hector: lotsadiserts
$ java com.learnsecurity.MiniPasswordManager pwdfile dan
Enter new password for dan: cryptguru
$ cat pwdfile #now it exists (after hector is added)
dan:O70FKijze89PDJtQHM8muKC+aXbUJIM/j8T4viT62rM=:3831
hector:laX1pk2KoZy1ze64gUD6rc/pqMuAVmWcKbgdQLL0d7w=:1466

9.4. HTTP Authorization
 Client makes request: GET /index.html HTTP/1.1

 Server requests authentication:
HTTP/1.0 401 Unauthorized
WWW-Authenticate: Basic realm="BasicAuthWebServer"
 Client replies with base-64 encoded
username/password combo:
GET /index.html HTTP/1.1
Authorization: Basic aGVjdG9yOmxvdHNhZGlzZXJ0cw==

 Only encoded, not encrypted in basic HTTP auth
 Eavesdropper can sniff: HTTP digest authorization
and/or SSL can help

9.4. BAWS Explanation
 BAWS = BasicAuthWebServer

 During processRequest(), use
getAuthorization() to check
Credentials object (stores username and
password)

 Then checkPassword() to determine whether
to serveFile()

 main() method modified to accept password
filename from command line

9.4. BAWS: processRequest()
public void processRequest(Socket s) throws Exception {
//... some code excluded...
if (command.equals("GET")) { // handle GET request
Credentials c = getAuthorization(br);
if ((c != null) &&
(MiniPasswordManager.checkPassword(c.getUsername(),
c.getPassword()))) {
serveFile(osw, pathname);
} else { osw.write ("HTTP/1.0 401 Unauthorized");
osw.write ("WWW-Authenticate: Basic"+
"realm=BasicAuthWebServer");
}
} else { //... some code excluded ...}
}

9.4. BAWS: getAuthorization()
private Credentials getAuthorization (BufferedReader br) {
try { String header = null;
while (!(header = br.readLine()).equals("")) {
if (header.startsWith("Authorization:")) {
StringTokenizer st =
new StringTokenizer(header, " ");
st.nextToken(); // skip "Authorization"
st.nextToken(); // skip "Basic"
return new Credentials(st.nextToken());
}
}
return null;
}

9.4. BAWS: main()
public static void main (String argv[]) throws Exception {
if (argv.length == 1) {// Initialize MiniPasswordManager
MiniPasswordManager.init(argv[0]);
/* Create a BasicAuthWebServer object, and run it */
BasicAuthWebServer baws = new BasicAuthWebServer();
baws.run();
} else {
System.err.println ("Usage: java BasicAuthWebServer"+

"<pwdfile>");
}
}

9.5. Online Dictionary Attacks
 Attacker actively tries combos on live system

 Can monitor attacks
 Watch for lots of failed attempts
 Mark or block suspicious IPs

 Avoid server verification: sees password in clear
 Vulnerableto phishing: impersonator steals password
 Password-authenticated key exchange (PAKE), zero-
knowledge proofs avoid sending password
 PAKE & zero-knowledge not yet efficient, commercial

9.6. Additional Password
Security Techniques
 Several other techniques to help securely
manage passwords: Mix and match ones that
make sense for particular app

 Strong Passwords  Limiting Logins
 “Honeypots”  Artificial Delays
 Filtering
 Last Login
 Aging
 Image Authentication
 One-Time Passwords
 Pronounceable

9.6.1. Strong Passwords
 Not concatenation of 1 or more dictionary words
 Long as possible: letters, numbers, special
chars
 Can create from long phrases:
 Ex: “Nothing is really work unless you would rather be
doing something else” -> n!rWuUwrbds3
 Use 1st letter of each word, transform some chars into
visually or phonetically similar ones
 Protect password file, limit access to admin
 UNIX used to store in /etc/passwd (readable by all)


9.6.2. “Honeypot” Passwords
 Simple username/password (guest/guest)
combos as “honey” to attract attackers
 Bait attackers into trying simple combos

 Alert admin when “booby-trap” triggered
 Could be indication of attack
 ID the IP and track to see what they’re up to

9.6.3. Password Filtering
 Let user choose password
 Within certain restrictions to guarantee stronger
password
 Ex: if in the dictionary or easy to guess

 May require mixed case, numbers, special chars
 Can specify set of secure passwords through regular
expressions
 Also set a particular min length

9.6.4. Aging Passwords
 Encourage/require users to change passwords
every so often
 Every time user enters password, potential for
attacker to eavesdrop
 Changing frequently makes any compromised
password of limited-time use to attacker
 Could “age” passwords by only accepting it a
certain number of times

 But if require change too often, then users will
workaround, more insecure

9.6.5. Pronounceable
Passwords
 Users want to choose dictionary words because
they’re easy to remember

 Pronounceable Passwords
 Non-dictionary words, but also easy to recall
 Syllables & vowels connected together
 Gpw package generates examples
 e.g. ahrosios, chireckl, harciefy

9.6.6. Limited Login Attempts
 Allow just 3-4 logins, then disable or lock
account
 Attacker only gets fixed number of guesses
 Inconvenient to users if they’re forgetful
 Legitimate user would have to ask sys admin to
unlock or reset their password
 Potential for DoS attacks if usernames compromised
and attacker guesses randomly for all, locking up
large percentage of users of system

9.6.7 Artificial Delays
 Artificial delay when user tries login over network
 Wait 2n seconds after nth failure from particular
IP address
 Only minor inconvenience to users (it should only take
them a couple of tries, 10 seconds delay at most)
 But makes attacker’s guesses more costly, decreases
number of guesses they can try in fixed time interval

 HTTP Proxies can be problematic
 One user mistyping password may delay another user
 Need more sophisticated way to delay

9.6.8. Last Login
 Notify user of last login date, time, location each
time they login
 Educate them to pay attention
 Tell user to report any inconsistencies

 Discrepancies = indications of attacks

 Catch attacks that may not have been noticed
 Ex: Alice usually logs in monthly from CA
 Last login was 2 weeks ago in Russia
 Alice knows something’s wrong, reports it

9.6.9. Image Authentication
 Combat phishing: images as second-factor
 Ask users to pick image during account creation
 Display at login after username is entered
 Phisher can’t spoof the image
 Educate user to not enter password
if he doesn’t see the image he picked

 Recently deployed by PassMark, used on
www.bofa.com and other financial institutions

9.6.10. One-Time Passwords
 Multiple uses of password gives attacker
multiple opportunities to steal it
 OTP: login in with different password each time

 Devices generate passwords to be used each
time user logs in
 Device uses seed to generate stream of passwords
 Server knows seed, current time, can verify password

 OTP devices integrated into PDAs, cell-phones

Summary
 Hashing passwords: don’t store in clear
 Dictionary Attacks: try hashes of common words

 Salting: add a random #, then hash
 Dictionaryattack harder against arbitrary user
 But doesn’t help attack against particular victim

 Other Approaches:
 Image Authentication
 One-time Passwords
 ...

9 password security

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