INFORMATION SECURITY IN COMPUTING

QUESTION

Q1 (Marks 3 + 3 + 3 + 3 = 12)

(i)

Why is it a bad idea to use the same RSA key pair for both signing and decryption?

(ii)

Suppose that Alice signs the message M = “I love security” and then encrypts it with

Bob’s public key before sending it to Bob. As well known, Bob can decrypt this to

obtain the signed message and then encrypt the signed message with, say, Charlie’s

public key and forward the resulting ciphertext to Charlie. Could Alice prevent this

“attack” by using symmetric key cryptography?

(iii)

Alice receives a single ciphertextC from Bob, which was encrypted using Alice’s

RSA public key. Let Mbe the corresponding plaintext. Alice challenges Trudy to

recoverM under the following rules. Alice sends C to Trudy, and Alice agrees to

decrypt one ciphertext that was encrypted with Alice’s public key, provided that it is

notC, and give the resulting plaintext to Trudy. Is it possible for Trudy to recover M?

(iv)

This problem deals with digital signatures. How and why does a digital signature

provide integrity? How and why does a digital signature provide non-repudiation?

Q2 (Marks 3 + 3 + 2 + 3 = 11)

(i)

Alice forgets her password. She goes to the system administrator’s office, and the

admin resets her password and gives Alice the new password.

a. Why does the SA reset the password instead of giving Alice her previous

(forgotten) password?

b. Why should Alice re-reset her password immediately after the SA has reset it?

c. Suppose that after the SA resets Alice’s password, she remembers her previous

password. Alice likes her old password, so she resets it to its previous value. Would it

be possible for the SA to determine that Alice has chosen the same password as

before? Why or why not?

(ii)

Suppose you have six accounts, each of which requires a password, and you choose

distinct passwords for each account.

a. If the probability that any given password is in Trudy’s password dictionary is 1/4,

what is the probability that at least one of your passwords is in Trudy’s dictionary?

b. If the probability that any one of your passwords is in Trudy’s dictionary is reduced

to 1/10, what is the probability that at least one of your passwords is in Trudy’s

dictionary?

(iii)

Suppose Alice requires passwords for eight different accounts. She could choose the

same password for all of these accounts. With just a single password to remember,

Alice might be more likely to choose a strong password. On the other hand, Alice

could choose different passwords for each account. With distinct passwords, she

might be tempted to choose weaker passwords since this might make it easier for her

to remember all of her passwords.

a. What are the trade-offs between one well-chosen password versus several weaker

passwords?

b. Is there a third approach that is more secure than either of these options?

(iv)

Suppose that when a fingerprint is compared with one other (non- matching)

fingerprint, the chance of a false match is 1 in 1010. Suppose that the FBI fingerprint

database contains 107 fingerprints.

a. How many false matches will occur when 100,000 suspect finger- prints are each

compared with the entire database?

b. For any individual suspect, what is the chance of a false match?

Q3 (Marks 3 + 3 + 6 = 12)

(i)

In the text, we argued that it’s easy to delegate using capabilities.

a. It is also possible to delegate using ACLs. Explain how this would work.

b. Suppose Alice delegates to Bill who then delegates to Charlie who, in turn,

delegates to Dave. How would this be accomplished using capabilities? How would

this be accomplished using ACLs? Which is easier and why?

c. Which is better for delegation, ACLs or capabilities? Why?

(ii)

Suppose that packets sent between Alice and Bob are encrypted and integrity

protected by Alice and Bob with a symmetric key known only to Alice and Bob.

a. Which fields of the IP header can be encrypted and which cannot?

b. Which fields of the IP header can be integrity protected and which cannot?

c. Which of the firewalls—packet filter, stateful packet filter, application proxy—will

work in this case, assuming all IP header fields that can be integrity protected are

integrity protected, and all IP header fields that can be encrypted are encrypted?

Justify your answer.

(iii)

The anomaly-based intrusion detection example presented in the lecture (L#8) is

based on file-use statistics.

a. Many other statistics could be used as part of an anomaly-based IDS. For example,

network usage would be a sensible statistic to consider. List five other statistics that

could reasonably be used in an anomaly-based IDS.

b. Why might it be a good idea to combine several statistics rather than relying on just

a few?

c. Why might it not be a good idea to combine several statistics rather than relying on

just a few?

SOLUTION

Security in Computing

 (i)

Why is it a bad idea to use the same RSA key pair for both signing and decryption?

Ans:

 I think it’s a really bad idea to use asymmetric keys in both encryption and signature scenarios. It’s too easy to abuse the signature part as a “decryption oracle” for attackers that would like to decrypt messages.

It is a bad idea to use the same RSA Key pair for signing and decryption because there will be a chance of getting attack by outer sources, which means that the signing provided by the RSA Key and the Decryption provided by the same key is not such secured than by using other RSA Key for both the processes.

(ii)

Suppose that Alice signs the message M = “I love security” and then encrypts it with

Bob’s public key before sending it to Bob. As well known, Bob can decrypt this to

obtain the signed message and then encrypt the signed message with, say, Charlie’s

public key and forward the resulting cipher text to Charlie. Could Alice prevent this

“attack” by using symmetric key cryptography?

Ans:

Why would Alice want to prevent this? Presumably, Alice sent the information to Bob such that he could decode it specifically so that Bob could do whatever he wanted with the information.

Is your issue that Bob leaked the data to Charlie? There isn’t anything you can really do about this. If Bob can read the data, he can always share it with someone else. If symmetric key cryptography was used, Bob could just send the key itself to Charlie as well as the encrypted message. Better yet, he can still just the decrypted message. There is no way to prevent Bob from doing things if he’s the intended recipient of the data.

(iii)

Alice receives a single ciphertextC from Bob, which was encrypted using Alice’s

RSA public key. Let Mbe the corresponding plaintext. Alice challenges Trudy to

recoverM under the following rules. Alice sends C to Trudy, and Alice agrees to

decrypt one ciphertext that was encrypted with Alice’s public key, provided that it is

notC, and give the resulting plaintext to Trudy. Is it possible for Trudy to recover M?

(iv)

This problem deals with digital signatures. How and why does a digital signature

provide integrity? How and why does a digital signature provide non-repudiation?

Ans:

A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit. Digital signatures are commonly used for software distribution, financial transactions, and in other cases where it is important to detect forgery or tampering.

A digital signature scheme typically consists of three algorithms:

• A key generation algorithm that selects a private key uniformly at random from a set of possible private keys. The algorithm outputs the private key and a corresponding public key.
• A signing algorithm that, given a message and a private key, produces a signature.
• A signature verifying algorithm that, given a message, public key and a signature, either accepts or rejects the message’s claim to authenticity.

I’m not sure how data in the DPLA will be made available, but I’d like to start the discussion that is overlooked and needs some serious attention – data integrity. It is not enough to categorize and make data available and just let people have access to it. It is not enough to just secure a server where the data resides and ensure the important files and processes haven’t been tampered with. It is of the utmost importance that the data can be trusted to be original and there is management of the data from the time it is created to the time it is destroyed – data provenance. What value is well organized data when the data has been altered and there is no way to determine what changed or even which files were changed? Who would trust the well organized data? How long would it take to determine which of the thousands or millions of files have changed? Data integrity should be one of the foundations of the DPLA. Whether the DPLA simple provide the links to the content archives or the DPLA stores the content on its own servers, integrity should be a foundation of how the material is made available from beginning to end. The integrity of the data should be managed from the moment it is being digitized.

Alice forgets her password. She goes to the system administrator’s office, and the

admin resets her password and gives Alice the new password.

a. Why does the SA reset the password instead of giving Alice her previous

(forgotten) password?

Ans. System Administrator resets the password instead of giving her previous password because the database is storing that password which can be destroyed by not inputting the incorrect password. Therefore the reset algorithm works.

b. Why should Alice re-reset her password immediately after the SA has reset it?

Ans. Because he knows the procedure how to reset the password.

c. Suppose that after the SA resets Alice’s password, she remembers her previous

password. Alice likes her old password, so she resets it to its previous value. Would it

be possible for the SA to determine that Alice has chosen the same password as

before? Why or why not?

Ans. Yes System Admin can recognize that she is using the previous password because it notifies that the password is already used before.

(ii)

Suppose you have six accounts, each of which requires a password, and you choose

distinct passwords for each account.

a. If the probability that any given password is in Trudy’s password dictionary is 1/4,

what is the probability that at least one of your passwords is in Trudy’s dictionary?

b. If the probability that any one of your passwords is in Trudy’s dictionary is reduced

to 1/10, what is the probability that at least one of your passwords is in Trudy’s

dictionary?

Ans.  An ideal password is something that you know, something that a computer can verify that you know, and something nobody else can guess—even with access to unlimited computing resources. We’ll see that in practice it’s difficult to even come close to this ideal. Undoubtedlyyouarefamiliarwithpasswords.It’svirtuallyimpossibletouseacom-putertodaywithoutaccumulatingasignificantnumberofpasswords. An Oneimportantfactregarding password is that many things act as passwords. For example, the PIN numberfor an ATM card is equivalent to a password. And if you forget your “real” password,a friendly website might authenticate you based on your social security number, yourmother’smaidenname, oryourdateofbirth, inwhichcase, these“thingsthatyouknow”are acting as passwords. An obvious problem is that these things are not secret. We’ll see that, when users select passwords, they tend to select bad passwords, which makes password “cracking” surprisingly easy. In fact, we’ll provide some basic mathematical arguments to show that it’s inherently difficult to achieve security via passwords.

(iii)

Suppose Alice requires passwords for eight different accounts. She could choose the

same password for all of these accounts. With just a single password to remember,

Alice might be more likely to choose a strong password. On the other hand, Alice

could choose different passwords for each account. With distinct passwords, she

might be tempted to choose weaker passwords since this might make it easier for her

to remember all of her passwords.

a. What are the trade-offs between one well-chosen password versus several weaker

passwords?

b. Is there a third approach that is more secure than either of these options?

Ans. An ideal password is something that you know, something that a computer can verify that you know, and something nobody else can guess—even with access to unlimited computing resources. We’ll see that in practice it’s difficult to even come close to this ideal. Undoubtedlyyouarefamiliarwithpasswords.It’svirtuallyimpossibletouseacom-putertodaywithoutaccumulatingasignificantnumberofpasswords. An Oneimportantfactregarding password is that many things act as passwords. For example, the PIN numberfor an ATM card is equivalent to a password. And if you forget your “real” password,a friendly website might authenticate you based on your social security number, yourmother’smaidenname, oryourdateofbirth, inwhichcase, these“thingsthatyouknow”are acting as passwords. An obvious problem is that these things are not secret. We’ll see that, when users select passwords, they tend to select bad passwords, which makes password “cracking” surprisingly easy. In fact, we’ll provide some basic mathematical arguments to show that it’s inherently difficult to achieve security via passwords.

(iv)

Suppose that when a fingerprint is compared with one other (non- matching)

fingerprint, the chance of a false match is 1 in 1010. Suppose that the FBI fingerprint

database contains 107 fingerprints.

a. How many false matches will occur when 100,000 suspect finger- prints are each

compared with the entire database?

b. For any individual suspect, what is the chance of a false match?

Ans. An ideal password is something that you know, something that a computer can verify that you know, and something nobody else can guess—even with access to unlimited computing resources. We’ll see that in practice it’s difficult to even come close to this ideal. Undoubtedlyyouarefamiliarwithpasswords.It’svirtuallyimpossibletouseacom-putertodaywithoutaccumulatingasignificantnumberofpasswords. An Oneimportantfactregarding password is that many things act as passwords. For example, the PIN numberfor an ATM card is equivalent to a password. And if you forget your “real” password,a friendly website might authenticate you based on your social security number, yourmother’smaidenname, oryourdateofbirth, inwhichcase, these“thingsthatyouknow”are acting as passwords. An obvious problem is that these things are not secret. We’ll see that, when users select passwords, they tend to select bad passwords, which makes password “cracking” surprisingly easy. In fact, we’ll provide some basic mathematical arguments to show that it’s inherently difficult to achieve security via passwords.

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