SE 231 Assignment 2 PDF

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SE 231 - Assignment 2Usama HashmiReview Questions 3.List and briefly describe the principal threats to the secrecy of passwords.Offline dictionary attack: The attacker obtains the system password file and compares the password hashes against hashes of commonly used passwords. If a match is found, th...

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SE 231 - Assignment 2 Usama Hashmi

Review Questions 3.2 List and briefly describe the principal threats to the secrecy of passwords. Offline dictionary attack: The attacker obtains the system password file and compares the password hashes against hashes of commonly used passwords. If a match is found, the attacker can gain access by that ID/password combination. Specific account attack: The attacker targets a specific account and submits password guesses until the correct password is discovered. Popular password attack: A variation of the preceding attack is to use a popular password and try it against a wide range of user IDs. A user’s tendency is to choose a password that is easily remembered; this unfortunately makes the password easy to guess. Password guessing against single user: The attacker attempts to gain knowledge about the account holder and system password policies and uses that knowledge to guess the password. Workstation hijacking: The attacker waits until a logged-in workstation is unattended. Exploiting user mistakes:    

System assigned passwords are more likely to write it down by the user because it is difficult to remember. This situation creates the potential for an adversary to read the written password. A user may intentionally share a password, to enable a colleague to share files, for example. Attackers are frequently successful in obtaining passwords by using social engineering tactics that trick the user or an account manager into revealing a password. Many computer systems are shipped with preconfigured passwords for system administrators. Unless these preconfigured passwords are changed, they are easily guessed.

Exploiting multiple password use: Attacks can also become much more effective or damaging if different network devices share the same or a similar password for a given user. Electronic monitoring: If a password is communicated across a network to log on to a remote system, it is vulnerable to eavesdropping.

Review Questions 3.3 What are two common techniques used to protect a password file? Common techniques used to protect a password file: Storing password hashes: Storing password hashes involves not just storing the user passwords using the hash function. The hash value of the password is generated and stored as the hashed password.

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User password entries are hashed and matched with the hashed password for verification, using an authentication token. In this process there is no way to obtain the password file because only an authentication token is returned.

Restricting and controlling user access: Limited access right to the password file, along with encryption of the password file ensures that the file cannot be obtained easily.

Review Questions 3.6 List and briefly describe the principal physical characteristics used for biometric identification. Facial characteristics: The computer using the relative location and shape of key facial features such as eyes, nose, lips, chin, and shape etc. for identification. Alternately infrared camera use face thermogram that correlated with the vascular system in the human face for identification. Fingerprints: The fingerprint pattern of ridges and furrows on the surface of fingertips is unique across the entire human population, the fingerprint recognition stores and matches fingerprint features as a numerical surrogate for identification. Hand geometry: Hand geometry systems identify features of the hand, including shape, and lengths and widths of fingers. Retinal pattern: The pattern formed by veins beneath the retinal surface is unique and therefore suitable for identification. Iris: The detailed structure of the iris is unique for each person. Signature: Each individual has a unique style of handwriting and this is reflected especially in the signature, which is typically a frequently written sequence. This uniqueness of handwriting is used for identification. Voice: Voice patterns are tied to the physical and anatomical characteristics of the speaker. This uniqueness is used for identification.

Review Questions 3.9 Describe the general concept of a challenge-response protocol. Challenge-response protocol is a protocol used to authenticate user passwords:    

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A user first transmits his or her identity to the remote host. The host generates a random number “r”, often called a nonce, and returns this nonce to the user. In addition, the host specifies two functions, h() and f(), to be used in the response. This transmission from host to user is the challenge. The user’s response is the quantity f(r’, h(P’)), where r’ = r and P’ is the user’s password. The function h is a hash function, so the response consists of the hash function of the user’s password combined with the random number using the function f. The user input is verified by comparing results from the generated hash function h() and the random number function f().

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Problems 3.2 An early attempt to force users to use less predictable passwords involved computer-supplied passwords. The passwords were eight characters long and were taken from the character set consisting of lowercase letters and digits. They were generated by a pseudorandom number generator with 215 possible starting values. Using the technology of the time, the time required to search through all character strings of length 8 from a 36-character alphabet was 112 years. Unfortunately, this is not a true reflection of the actual security of the system. Explain the problem. Here's an explanation of the problem:   

Pseudo-random number generator can generate 215 possible starting values. A computer-generated password with 8 characters in length using 36-character alphabets will take 368 possible combination, which makes approximately 241 the starting values. In order to find the passwords using an attack of some sort, only 2 15 combinations are required.

Thus, this method does not provide much security.

Problems 3.7 Because of the known risks of the UNIX password system, the SunOS-4.0 documentation recommends that the password file be removed and replaced with a publicly readable file called /etc/publickey. An entry in the file for user A consists of a user’s identifier IDA, the user’s public key, PUa, and the corresponding private key, PRa. This private key is encrypted using DES with a key derived from the user’s login password Pa When A logs in, the system decrypts E(Pa, PRa) to obtain PRa. a. The system then verifies that was correctly supplied. How? The user private key and public key are inverse of each other. Thus, in order to verify the value of PUa, the value of PRa can be used.   

First the public key of the user is encrypted using an arbitrary block X. Then, we can decrypt the obtained encrypted value using the private key of the user, which is PRa. That is: o X = D(PRa, E[PUa, X]) o Where D = decrypt, E = encrypt

b. How can an opponent attack this system? Similar to the system verification method, an opponent can guess the login password P. Let the guessed password be P*.  

Opponent can computer the private key PRa decrypting the guessed login password P*. Then check the obtained private key using an arbitrary block X*, as follows” o First the public key of the user is encrypted using an arbitrary block X*.

Then, we can decrypt the obtained encrypted value using the private key of the user, which is PRa. o That is:  X* = D(PRa, E[PUa, X*])  Where D = decrypt, E = encrypt Additionally, other blocks can be verified to validation whether the guessed password P* is the same as the correct password P. o

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Problems 3.9 Assuming you have successfully answered the preceding problem and understand the significance of the salt, here is another question. Wouldn’t it be possible to thwart completely all password crackers by dramatically increasing the salt size to, say, 24 or 48 bits? System security in terms of the salt method depends on the user pool. Using the hash function of the cipher text, passwords are generated with the original user password and the randomly generated salt and both are stored in password file. Increasing the salt size lead to resolution of the same salt problem, and different salts for each user just means, that the attacker needs to do encryption for each password. This does not increase the security of the system, though it will thwart some password crackers but not all....