Lesson test: What logging in actually is

Sign in

Libre University uses your GitHub account. Signing in is only needed to sit a final test, so the score is kept on your profile.

What logging in actually is

1.[3p]
A signed-in user requests `GET /invoices/4192`, an invoice belonging to somebody else. Which check is missing?

Authorization, since the session proves who they are and says nothing about what they may reach

Correct

Authentication, since the session could have been forged

The answer is: Authorization, since the session proves who they are and says nothing about what they may reach

Identification, since the email address was never confirmed

The answer is: Authorization, since the session proves who they are and says nothing about what they may reach

None, since a signed-in user is entitled to read the site

The answer is: Authorization, since the session proves who they are and says nothing about what they may reach

2.[3p]
Match each operation to what it actually provides.

Hash
MAC
Encryption
Base64

concealment of the content from anyone without the key
a way of writing bytes down, reversible by anyone
proof that a holder of the key produced this exact message
a one way digest, with no key and no way back

Show the answer

Hash: a one way digest, with no key and no way back MAC: proof that a holder of the key produced this exact message Encryption: concealment of the content from anyone without the key Base64: a way of writing bytes down, reversible by anyone

3.[3p]
A token is six characters drawn from a 32-character alphabet. How many bits of entropy does it carry?

CorrectNot quite: 30

4.[2p]
How many random bytes must `crypto.getRandomValues` fill to produce a token with 128 bits of entropy?

CorrectNot quite: 16

5.[3p]
Why is `Math.random` unacceptable for generating a session identifier?

Its output is a deterministic function of a small state that can be solved for from a few observed values

Correct

It returns floats rather than bytes

The answer is: Its output is a deterministic function of a small state that can be solved for from a few observed values

It is slower than crypto.getRandomValues

The answer is: Its output is a deterministic function of a small state that can be solved for from a few observed values

It repeats exactly every 2^32 calls

The answer is: Its output is a deterministic function of a small state that can be solved for from a few observed values

6.[2p]
Base64 encoding the contents of a cookie stops the user from reading and altering them.

True

The answer is: False

False

Correct

7.[3p]
Comparing a secret with `===` leaks information. What exactly leaks, and to whom?

The length of the correct prefix, to anyone who can time the response, because comparison stops at the first difference

Correct

The secret itself, to anyone reading the source code

The answer is: The length of the correct prefix, to anyone who can time the response, because comparison stops at the first difference

The hash of the secret, to anyone who can trigger an error page

The answer is: The length of the correct prefix, to anyone who can time the response, because comparison stops at the first difference

Nothing, unless the secret is stored in the clear

The answer is: The length of the correct prefix, to anyone who can time the response, because comparison stops at the first difference

8.[3p]
An attacker steals your users table, with emails, salts, password hashes and session rows. Which defences still do useful work?

Select all that apply

Slow password hashing, which bounds how fast the stolen hashes can be guessed offline

Correct

Storing a hash of each session token rather than the token itself

Correct

A second factor, which the stolen table does not supply

Correct

The answer is: Slow password hashing, which bounds how fast the stolen hashes can be guessed offline, Storing a hash of each session token rather than the token itself, A second factor, which the stolen table does not supply

9.[2p]
Put the three questions in the order a request must answer them.

Is the claim true?
May they do this particular thing?
Who is this claiming to be?

Show the answer

b, a, c

1.[3p] A signed-in user requests GET /invoices/4192, an invoice belonging to somebody else. Which check is missing?

2.[3p] Match each operation to what it actually provides.

3.[3p] A token is six characters drawn from a 32-character alphabet. How many bits of entropy does it carry?

4.[2p] How many random bytes must crypto.getRandomValues fill to produce a token with 128 bits of entropy?

5.[3p] Why is Math.random unacceptable for generating a session identifier?

6.[2p] Base64 encoding the contents of a cookie stops the user from reading and altering them.

7.[3p] Comparing a secret with === leaks information. What exactly leaks, and to whom?

8.[3p] An attacker steals your users table, with emails, salts, password hashes and session rows. Which defences still do useful work?

9.[2p] Put the three questions in the order a request must answer them.

1.[3p]
A signed-in user requests `GET /invoices/4192`, an invoice belonging to somebody else. Which check is missing?

2.[3p]
Match each operation to what it actually provides.

3.[3p]
A token is six characters drawn from a 32-character alphabet. How many bits of entropy does it carry?

4.[2p]
How many random bytes must `crypto.getRandomValues` fill to produce a token with 128 bits of entropy?

5.[3p]
Why is `Math.random` unacceptable for generating a session identifier?

6.[2p]
Base64 encoding the contents of a cookie stops the user from reading and altering them.

7.[3p]
Comparing a secret with `===` leaks information. What exactly leaks, and to whom?

8.[3p]
An attacker steals your users table, with emails, salts, password hashes and session rows. Which defences still do useful work?

9.[2p]
Put the three questions in the order a request must answer them.