Authentication

10 important questions on Authentication

Name three examples of passwordless authentication.

  1. SSH (Secure Shell): Remotely access a (computer) system with end-to-end encryption.
  2. FIDO Authentication: Developed by FIDO Alliance to get rid of passwords.
  3. Web authentication (WebAuthn): Part of FIDO2 and currently in the making. An interface for authenticating users to web applications and services with public-key cryptography.



Remote login is a process in which user can login into remote computer and use its services as if it were local machine.

What is user authentication?

A server assuring that services are only offered to the rightful user(s).

What is user registration?

When a user interacts with a service for the first time, a record will be created in a user register which includes the username.
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Is SHAKE128 good for password hashing? If not, give a better hash function alternative.

SHAKE128 is extremely fast, which means the attacker is also fast.

We need resource-hungry hash functions, like  scrypt, bcrypt or ARGON2. These slower functions will automatically also slow down the attacker.

Resource-hungry means an application is using tremendous amounts of CPU time slot, memory, storage, and/or network bandwidth.

What is the entropy of a password? And do we want a high or low entropy?


It expresses the amount of uncertainty that an attacker has about a password.

We want a high entropy with respect to any reasonable attacker.

What is the formula of entropy? What unit do we use for the entropy of a password?


The unit is bits.

Assume we have a cube die that is biased in the following way:
  • Faces 1 and 2 have the same probability.
  • Faces 3, 4, 5 and 6 have the same probability.
  • The probability of face 1 is twice that of face 3.

What is the value of the entropy of the outcome of a dice roll?

  1. If we take a bag and put 1, 2, ... 6 in the bag they all have the same probability.
  2. If we place 1 and 2 in there once more, then they (1 and 2) now have twice as much probability of getting draw randomly.
  3. If we count the total elements in the bag we get 6 + 2 = 8.
  4. For 1 we have 2/8 (= 1/4). For 2 we also have 2/8 (= 1/4). For 3 we have 1/8 and the same for 4, 5 and 6.

What is injective encoding?

An algorithms that encodes (= coderen) a sequence of input stings X1, X2, X3... into a single string X in a way that we can unambiguously reconstruct the input strings from X.

When are fixed-length input strings injective? When are variable-length input strings injective?

Fixed-length input strings =  Always injective.

Variable-length input strings = Only injective with Length-Value (LV) encoding. This means each string is encoded as a sequence of two elements
  • L: A byte that species the length of the string
  • V: The string itself
This is limited to 255-byte strings.

What is the value of the entropy of a fair coin toss?

Use 1 and 0 for heads and tails.

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