Authentication (and Unix Password Security)

Authentication(and Unix Password Security)

Authentication means • to establish the proof of identity. • Authentication techniques may vary depending on the kind of resource being accessed. • The various kinds of access can be classified into • user-to-host • host-to-host • user(or process) –to – user (process)

Authentication is done by • by something you are (SYA) • by something you know (SYK) • by something you have (SYH) • SYA is more reliable and accurate compared to SYH.

Authentication • SYA authentication applies to humans. • shape of your face, color of the hair), height, weight, your signature, finger prints, etc. • The last two falls in the category of biometrics – techniques that measure biological characteristics or physical phenomena (e.g. finger print and hand print analysis, retinal scans, voice, handwriting recognition, etc). • Signature is not a good SYA, however with the association of time taken for signature is a good SYA!

Authentication • SYK is the most commonly used end-user authentication (user to systems). • e.g: user name and password. • Can also be applied to programs that exchange the data over the network without human intervention. • The strength of SYK authentication depends on whether what is known is a secret, and can be kept as a secret.

Authentication • In and of itself, SYH is the least way to authenticate. • The mere possession of an object that can be borrowed, stolen, or duplicated is a poor way to identify its holder. • Key to the computer room • Strength of SYH in greatly improved when combined with SYK.

SYK • with respect to accessing computer systems. In other words authenticating users for accessing computer systems.

User-to-Host authentication • Typical methods are • static passwords • challenge and response • one-time passwords • trusted third parties

Static passwords • Most ubiquitous authentication scheme employed on the computer systems (and internet today) • A user chosen or assigned password (or PIN)–something that only the user should know. • It is an example of SYK • An example: /etc/passwd where the derivative of the password of the users is stored. • refer to the Unix encrypted password system

Unix passwords • derivative of the password is stored in an encrypted (scrambled) form and NOT the plain password itself. • the method of scrambling is known (crypt is the program that is normally used and the source code of the program is freely available – written in c). • translating from the encrypted to the plain text form is very, very difficult.

Static passwords - problems • A password guesser (also known as cracker) can be used to guess some of the passwords even the passwords (or its derivatives) are stored in encrypted form. • The passwords can be guessed because of their poor choice • such as password is same as the user name or the actual user name, or the popular words in the dictionary, etc. • How the cracker programs work?

Cracker principle • /* single password cracker which checks whether the password is same as user name * For example, user name is srini and password is srini * scan the password file for the same user name and password*/#include <stdio.h>#include <pwd.h>int main(int argc, char **argv) { struct passwd *pw while (pw=getpwent() ) { • char *crypt(); char *result; result = crypt(pw->pw_name, pw->pw_passwd); if (!strcmp(result, pw->pw_passwd) ) { printf (“%s has the same password \n”, pw->pw_name); } } exit(0);}The principle here is to guess the password, pass though crypt program and get the encrypted version of the guessed password and compare it with the encrypted version of the password stored in the system. If they match, you know the plain text of the password.

Password Crackers – dictionary attack • Crackers are known as dictionary attack because • create a dictionary of possible passwords • generate the corresponding passwords for the words in the above dictionary (assuming no salt) • Match the entries in the generated passwords against the actual passwords on the system for a possible match. • All the above operations can be done off-line!!

Password-1 encry pw-1 Password-2 encry pw-2 Password-3 encry pw-3 Password-4 encry pw-4 … … … … … … … … … … Password-n encry pw-n Dictionary Attack – How it works? crypt() Password file which contains users encrypted passwords Dictionary Of passwords

Educating users • Never use a portion or variation of your account name or another account name. • Never use a portion of variation of your real name, office or home address, or phone number. • Never use words or variations of words found in any dictionary, especially /usr/dict words. • Never use pairings of short words found in any dictionary (dogcat) • Never use dictionary words or names spelled backwards (like terces) • Never use syllables or words from a foreign language. • Never use repeated character string (like AAAABBBB, LLOOVVEE) • Never use passwords containing only numeric digits. • Always use passwords at least seven characters long (how many words with 7 characters are possible?). Many Unix versions use only 8 characters while some new ones may permit 16 or more characters. • Always use a mixture of upper- and lowercase characters. This is especially valuable rule. • Always use at least one or two non-alphanumeric characters, like numeric digits, punctuation marks, dollar sign, carat, etc.All these leads to a large number of combinations which may take the cracker program long enough to crack.

Preventing unsecured passwords • Accounts without passwords • Managing dormant accounts • Not allowing passwords that is similar to the user names, derivatives or words in /usr/dict file or insisting that the password to contain at least one non-alphabet character (npasswd program on Linux forces this feature). • Maintaing the shadow password and/or enforcing password aging.

Shadow password • Shadow passwords are hidden in shadow, a file that is readable only by the super (administrator) user. • typically it is in /etc/shadow, readable by root. • When the shadow password is implemented, then a “*” or “x” is placed in the password field of the /etc/passwd file. • In Unix, password is used to encrypt a string of 64 bit zeros using the crypt() function, typically 25 times. The final encrypted 64 bits are unpacked into a string of 11 printable characters that are stored in the /etc/passwd or /etc/shadow file.

Adding salt to the password • Although the source code of crypt() is readily available, no technique has been discovered to translate the encrypted password back into the original password. • Only possible attack is via a brute-force attack or by a dictionary attack. • The previous method can allow the attacker to store the pre-encrypted version of the dictionary words and matching it against the passwords stored in the /etc/passwd file. • To over come this problem add a salt to the password.

How adding salt works? • When you change the password, the /bin/passwd program selects a salt based on the time of the day. • The salt is converted into a two-character string (12 bits in fact) and is stored in the /etc/passwd file along with the encrypted “password” • The encryption of the string of 64 bits of zeros is done by the string which is the concatenation of the salt and the user supplied password string! • Having salt means that the same password can encrypt in 4096 different ways! and this makes it much harder for the attacker to build a reverse dictionary for translated encrypted passwords.

Password aging • Forces the user to change the password regularly. • when the allotted lifetime of a password expires, at the next login the user must change it, or denied access to the system. • smit is the tool that allows the configuration of the password aging in Linux. • Another strategy of aging is password history. • User is prevented from using one of the earlier used passwords (thereby repetition of the password is not possible).

Static passwords with one-way hash • When clear text password is passed though a communication channel, it can be snooped. • Using a challenge-response scheme, it is possible for a host to verify a user who knows the password without requiring the user sending the password through the communication channel. • The challenge (or the answer) string is concatenated with the password and a one-way hash of this string is sent to the server for validation.

Challenge-and-response protocol K K I am Alice. Alice Bob A random x (challenge) y=E(K,x) y (response) z=E(K,x) Accept if y=z.

EASY ! Input (such as a password) F Output (say of 100 bits) 1-way function HARD ! ? F Output (say of 100 bits)

Examples of 1-way functions • a 1-way hash function is also a 1-way function • a secret key cipher is a 1-way function (from key space to ciphertext space, with a fixed plaintext) • RSA encryption algorithm • many more ...

One-time passwords • S/key due to Leslie Lamport and implemented by Phil Karn in Unix. • Handheld authenticators. • Smart cards

One-time passwords • As the name implies a password is used only once. Typically password is generated by applying repeatedly MD5 algorithm on a secret password. • Let p the password and f is the one-way MD5 function. Initially let n=9, then the first time password transmitted for verification will be f9(p) and next time it will be f 8(p) and so on.

Handheld Authenticators • They are handheld password generators or token and belongs to the category of SYH authentication. • Similar to challenge-response scheme, where the host issues a challenge string that the user keys into the authenticator. The response appears on the authenticator’s display, which the user then sends it to the host.

Authenticating with multiple computers/applications • You need one password for each machine for each user. • The number of password can increase if you include machines, host-to-host authentication, program-to-program authentication. • 10 users – 1 machine  10 passwords • 10 users, 10 machines  100 passwords • 10 users, 10 machines, 10 applications running on each machines  assume that each user need to authenticate to a machine and to each application, then the number of passwords = ?

Trusted third parties HOST HOST User KDC (key distribution Centre) User User HOST HOST User

Advanced authentication • Kerberos (by MIT) • KriptoKnight (by IBM) • SPX (by DEC) • Lotus Notes • DCE • Microsoft • ......

Host-to-host authentication • IP address/name authentication – can be considered as no authentication • Digital signatures (such as MD5) and encryption can be used to authenticate the identity of the sender.

Authentication Methods in Network & Internet Security • Something you know • Passwords • PINs • Mother’s maiden name • Something you have • ATM card • Smart card • Digital certificate • Something you are • Biometrics • Positive identification • Never lost or stolen

SKA methods

Biometric Techniques • Biometrics identify people by measuring some aspect of individual anatomy or physiology (such as your hand geometry or fingerprint), some deeply ingrained skill, or other behavioral characteristic (such as your handwritten signature) or something that is a combination of the two, (such as your voice) Anderson P 261 • Handwritten signatures • Face Recognition • Fingerprints • Iris Codes • Devices

Enrollment: Add a biometric identifier to a database Fingerprint, Voice, Facial or Iris Store Present biometric Capture Process IDENTIFIED Compare Match Verification: Match against an enrolled record No Match DENIED Present biometric Capture Process How do Biometrics Work?

Handwritten signatures(1) • Basis of western commerce • The electronic replacement, digital signatures, is a later topic • Will a forged signature be accepted ? • Apparently this depends on the care and skill of the person examining the signature • In Australia, the banks do not automatically check signatures on cheques and etc • There are many different conventions regarding signatures, and in different countries, these are quite different

Handwritten signatures(2) • Signatures can be readily forged • There is significant variability between genuine signatures from the same individual • So here is a weak mechanism that works very well in practice • There appears to be a consensus that the dynamics of a signature are difficult to forge • Using a pressure pad to record the time and pressure as a signature is formed

Face Recognition(1) • The oldest way ! • There is widespread acceptance (and requirement !) for photo ID • The issuing of other authentication devices (like passwords, key cards, digital signatures) usually depends on facial recognition by the agents of the issuing authority • Anderson points out (p264) that photo-ID is not particularly reliable, • But has a very significant deterrent effect • Basis of the Australian Customs “SmartGate”

Face Recognition(2) • To identify faces in a crowd: • Need to capture a full-face image • Use imaging software to extract the face proper • Need to locate key features of the face to provide orientation and scaling (as with all biometric applications) • Either use global pattern recognition, • Or extract critical dimensions • NOT as reliable as advertised (false positive)

Accuracy • False Negative (rejection) rate • Measures how often an authorized user, who should be recognized by the system, is not recognized. • I am not recognised as me! • False Positive (acceptance) rate • Measures how often a non-authorized user, who should not be recognized by the system, is falsely recognized. • You are pretending to be me!

d Non-matching prints Matching Threshold Matching prints False non-matches False matches Matching vs. Non-Matching Prints

Fingerprints(1) • Accounts for the majority of sales of biometric equipment • The ridges that cover the fingertips make patterns, that were classified in the 1800’s • These patterns have loops of several distinct types, branches, and endpoints. The details are collectively called minutiae • Fingerprints are the mainstay of criminal systems identification • Because of this association with criminals, commercial users are very reluctant to impose fingerprinting systems upon their clients • after 911 – it is becoming a de-facto standard

Fingerprint Image Identification

Iris Codes(1) • Iris patterns believed to be unique • Apparently these patterns are randomly generated, and so cannot be predicted for any individual • The patterns are easy enough to detect • They do not wear out • They are protected by the eyelids and cornea • Iris images are much easier to capture and process than fingerprints • A processing technique is used to generate a 256 byte iris code • In tests to date, there have been almost zero false positives rates (although these are under laboratory conditions)

Iris Codes(2) • There are some practical difficulties: • Capturing the iris image is intrusive • The subject has to be co-operative, although for entry control, this is not an issue

Devices • PDA’s and smart cards are being equipped with ‘readers’ to detect a single fingerprint • This combines all three authentication methods: • What is carried – the card or PDA • What is known – the PIN number to open the card or PDA • What is a characteristic – the fingerprint template

Iris Face Finger Signature Voice Accuracy Very High Medium High High Medium Ease of Use Medium Medium High High High Barrier to Very High Medium High Medium Medium Attack User Medium Medium Medium Very High High Acceptability Long Term High Medium High Medium Medium Stability Interference Coloured Lighting Dryness Changing Noise, Contacts Aging, Dirt, Signatures Colds, Glasses, Age, Weather Hair Race Technology Comparison

Accuracy v. Affordability v. Acceptability 0 1 Affordability >> 2 3 4 Accuracy >> Courtesy, Veridicom Corp.

Biometrics ? The last word • Anderson p 264 • In general, biometric mechanisms tend to be much more robust in attended operations, where they assist a guard rather that replace him (sic). The false alarm rate may then be actually help by keeping the guard alert.

Selecting a Biometric Solution Who can help?

Authentication (and Unix Password Security)