Lesson test: Second factors: TOTP and recovery codes

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Second factors: TOTP and recovery codes

1.[3p]
What does TOTP stop completely, and what does it leave untouched?

It stops credential stuffing outright and leaves real-time phishing untouched

Correct

It stops phishing outright and leaves credential stuffing untouched

The answer is: It stops credential stuffing outright and leaves real-time phishing untouched

It stops both, provided the window is one step either side

The answer is: It stops credential stuffing outright and leaves real-time phishing untouched

It stops offline cracking only, and nothing that happens over the network

The answer is: It stops credential stuffing outright and leaves real-time phishing untouched

2.[3p]
Put the steps of RFC 4226 dynamic truncation in order.

Read four bytes at the offset and clear the top bit of the first
Take the low four bits of the last byte as an offset
Compute HMAC-SHA-1 over the eight-byte counter
Reduce the 31-bit value modulo ten to the power of the digit count

Show the answer

c, a, d, b

3.[2p]
An HMAC output ends in the byte `0x5a`. What offset does dynamic truncation use?

CorrectNot quite: 10

4.[3p]
With a 30-second step and a window of one step either side, how many seconds can a single code be accepted for?

CorrectNot quite: 90

5.[3p]
Why must a TOTP secret be encrypted at rest rather than hashed like a password?

The server has to recompute codes from it, so it needs the original value back

Correct

Hashing would break the base32 encoding used in the QR code

The answer is: The server has to recompute codes from it, so it needs the original value back

HMAC keys cannot be passed through SHA-256

The answer is: The server has to recompute codes from it, so it needs the original value back

It is public anyway, since it appears in the provisioning URI

The answer is: The server has to recompute codes from it, so it needs the original value back

6.[3p]
Which of these are real weaknesses of SMS as a second factor?

Select all that apply

A phone number is recoverable from the carrier by anyone who can persuade a support agent

Correct

The SS7 routing network has demonstrated interception weaknesses

Correct

The user has no way to tell which site requested the code

Correct

The code is shorter than a TOTP code

The answer is: A phone number is recoverable from the carrier by anyone who can persuade a support agent, The SS7 routing network has demonstrated interception weaknesses, The user has no way to tell which site requested the code

7.[2p]
Allowing SMS to reset a hardware key leaves the account with SMS-level security.

True

Correct

False

The answer is: True

8.[3p]
Eight recovery codes of eight base32 characters are stored as SHA-256. Why is that broken?

Each carries 40 bits and any of the eight matches, so a stolen table falls in seconds on one card

Correct

SHA-256 is reversible for inputs under ten characters

The answer is: Each carries 40 bits and any of the eight matches, so a stolen table falls in seconds on one card

Base32 codes collide too often to be unique

The answer is: Each carries 40 bits and any of the eight matches, so a stolen table falls in seconds on one card

The codes cannot be marked as single use

The answer is: Each carries 40 bits and any of the eight matches, so a stolen table falls in seconds on one card

9.[2p]
Which hash should short recovery codes be stored with instead of SHA-256? One word.

CorrectNot quite: Argon2id

1.[3p] What does TOTP stop completely, and what does it leave untouched?

2.[3p] Put the steps of RFC 4226 dynamic truncation in order.

3.[2p] An HMAC output ends in the byte 0x5a. What offset does dynamic truncation use?

4.[3p] With a 30-second step and a window of one step either side, how many seconds can a single code be accepted for?

5.[3p] Why must a TOTP secret be encrypted at rest rather than hashed like a password?

6.[3p] Which of these are real weaknesses of SMS as a second factor?

7.[2p] Allowing SMS to reset a hardware key leaves the account with SMS-level security.

8.[3p] Eight recovery codes of eight base32 characters are stored as SHA-256. Why is that broken?

9.[2p] Which hash should short recovery codes be stored with instead of SHA-256? One word.

1.[3p]
What does TOTP stop completely, and what does it leave untouched?

2.[3p]
Put the steps of RFC 4226 dynamic truncation in order.

3.[2p]
An HMAC output ends in the byte `0x5a`. What offset does dynamic truncation use?

4.[3p]
With a 30-second step and a window of one step either side, how many seconds can a single code be accepted for?

5.[3p]
Why must a TOTP secret be encrypted at rest rather than hashed like a password?

6.[3p]
Which of these are real weaknesses of SMS as a second factor?

7.[2p]
Allowing SMS to reset a hardware key leaves the account with SMS-level security.

8.[3p]
Eight recovery codes of eight base32 characters are stored as SHA-256. Why is that broken?

9.[2p]
Which hash should short recovery codes be stored with instead of SHA-256? One word.