Hash Functions Overview

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Questions and Answers

What is a primary reason hash functions are sometimes referred to as compression functions?

They require more computational resources to process.

They convert larger data into smaller fixed-size outputs. (correct)

They eliminate the need for input data.

They increase the size of the input data.

What does Pre-Image Resistance protect against?

Reversing a hash function to discover the original input. (correct)

Substituting a legitimate input with a different one.

Finding two different inputs with the same hash.

Generating a hash collision.

Which property of hash functions makes it hard to find two different inputs that produce the same hash?

Second Pre-Image Resistance

Digest Compression

Pre-Image Resistance

Collision Resistance (correct)

How are popular hash functions classified in terms of output length?

They generate hash values between 160 and 512 bits. Signup and view all the answers

What does Second Pre-Image Resistance specifically safeguard against?

Identifying a new input that hashes to the same value as a known input. Signup and view all the answers

What is a notable characteristic of hash functions regarding computational efficiency?

They are generally fast to compute compared to symmetric encryption. Signup and view all the answers

Which statement best describes collision resistance in hash functions?

It confirms that finding collisions is computationally hard. Signup and view all the answers

What output size do hash functions typically produce?

160 to 512 bits Signup and view all the answers

What value can Alice compute using her private key and Bob's public key?

g^ab mod p Signup and view all the answers

What is the main challenge that Eve faces when trying to determine the shared secret?

Calculating the value of b from g^b mod p Signup and view all the answers

In the context of public key exchange, what does static-static mode imply?

Each party retains the same keys for multiple communications Signup and view all the answers

What mathematical operation do Alice and Bob perform to compute their public keys?

Exponentiation with modulus Signup and view all the answers

What is one disadvantage of public key encryption?

It is generally slower than private key encryption Signup and view all the answers

If Bob selects a private key of 3 and Alice selects a private key of 4, what is the value of Bob's public key if g = 9 and p = 23?

16 Signup and view all the answers

What allows Alice and Bob to compute a shared secret after exchanging public keys?

They retain their private keys Signup and view all the answers

What is the result of Alice's symmetric key calculation using y = 16, a = 4, and p = 23?

9 Signup and view all the answers

What is the primary purpose of public and private keys in a public-key encryption scheme?

To allow secure data sharing by encrypting with one key and decrypting with the other. Signup and view all the answers

Which of the following describes the ciphertext?

The scrambled message that results from applying the encryption algorithm to plaintext. Signup and view all the answers

In the context of Diffie-Hellman key exchange, what is the role of the large prime number p?

It acts as the basis for generating public keys. Signup and view all the answers

What does Alice do after she computes her public key using her secret integer a in the Diffie-Hellman algorithm?

She sends the public key directly to Bob. Signup and view all the answers

Why is it important that public keys can be distributed openly?

Anyone can encrypt messages, but only the recipient can decrypt them. Signup and view all the answers

What is the main advantage of using the Diffie-Hellman key exchange method?

It enables secure key exchange without directly sharing the keys. Signup and view all the answers

Which of the following statements about the encryption algorithm in public-key encryption is accurate?

It transforms plaintext into ciphertext based on the given key. Signup and view all the answers

What would likely happen if two different keys are applied to the same plaintext in a public-key encryption scheme?

The ciphertext will differ based on the key used. Signup and view all the answers

What is a man-in-the-middle (MITM) attack?

A type of cyberattack where a malicious actor relays communications between two parties. Signup and view all the answers

Which of the following describes a common approach used in man-in-the-middle attacks?

Intercepting the communication via a rogue connection. Signup and view all the answers

What is the main goal of a man-in-the-middle attack?

To steal sensitive information by impersonating both parties. Signup and view all the answers

What is an example of session hijacking that is similar to man-in-the-middle attacks?

Sidejacking, where session cookies are stolen. Signup and view all the answers

Which method is NOT typically associated with man-in-the-middle attacks?

Directly attacking the server of either party. Signup and view all the answers

What is an 'Evil Twin' in the context of man-in-the-middle attacks?

A rogue Wi-Fi network that mimics a legitimate one. Signup and view all the answers

How does public key encryption relate to man-in-the-middle attacks?

It does not prevent unauthorized physical access to data once received. Signup and view all the answers

Which scenario best exemplifies a man-in-the-middle attack?

A user unknowingly connects to a fake Wi-Fi network created by the attacker. Signup and view all the answers

What is the main purpose of a message authentication code (MAC)?

To confirm the authenticity and integrity of a message. Signup and view all the answers

In the MAC formula, what does the variable 'K' represent?

The shared secret key. Signup and view all the answers

If an attacker alters a message without changing the MAC, what will happen when the receiver calculates the MAC?

The calculated MAC will differ from the received MAC. Signup and view all the answers

What does the use of a sequence number in a message accomplish?

It guarantees the proper order of message delivery. Signup and view all the answers

What happens if the received MAC matches the computed MAC?

The message is confirmed to be from the sender and unchanged. Signup and view all the answers

What is a limitation of MAC due to its symmetric nature?

It requires the establishment of a shared secret key. Signup and view all the answers

How does a MAC function compare to traditional encryption?

It need not be reversible as it does not aim to decrypt. Signup and view all the answers

Who must know the secret key in order to create a valid MAC?

Both the sender and the receiver of the message. Signup and view all the answers

Study Notes

Hash Functions

Hash function is a mathematical function that maps data of arbitrary size to a fixed-size output.
Hash functions are deterministic, meaning the same input always produces the same output.
Hash functions are also known as compression functions, digests
Hash functions with an n-bit output are called n-bit hash functions. Popular hash functions generate values between 160 and 512 bits.
Hash function computation h(x) is typically a fast operation.
Hash functions are significantly faster than symmetric encryption.

Properties of Hash Functions

Pre-Image Resistance: It should be computationally difficult to reverse a hash function. Given a hash value z, it should be challenging to find an input value x that hashes to z.
Second Pre-Image Resistance: Given an input x and its hash h(x), it should be hard to find another input y with the same hash value. This protects against attackers who want to substitute a different input for the original value.
Collision Resistance: It should be computationally difficult to find two different inputs that result in the same hash. This property is also known as collision-free hash function.

Public-key Cryptosystems

Consists of six components: plaintext, encryption algorithm, public and private keys, ciphertext, and decryption algorithm.
Public key encryption scheme works by generating a pair of keys for each user: one public and one private.
Public keys are shared with others, while private keys are kept secret.
To send a confidential message, Bob encrypts the message using Alice's public key. Only Alice can decrypt it using her private key.

Diffie-Hellman Key Exchange Algorithm

A method for securely exchanging cryptographic keys over a public communication channel.
The keys are not actually exchanged, they are jointly derived.
The process involves agreeing on a large prime number p and a generator g, then each party generates a private key and calculates a public key based on p, g, and the private key.
Both parties exchange their public keys and compute a shared secret using their own private key and the other party's public key.
Example steps: Alice and Bob agree on p=23 and g=9. Alice chooses a =4 and Bob chooses b=3 as their private keys. They then calculate their public keys (g^a mod p) and (g^b mod p) and exchange them.
Alice calculates (g^b)^a mod p and Bob calculates (g^a)^b mod p, resulting in the same shared secret (g^ab mod p).

Problems with Public Key Exchange

Speed: Public-key encryption is secure but computationally intensive, compared to symmetric encryption.
Security: Public-key encryption alone does not protect against attacks on the private key itself. If an unauthorized party gains access to the server containing the private key, they can access all the data encrypted with that key.

Man-in-the-Middle Attack

A malicious actor intercepts communication between two parties, impersonating both parties and gaining access to confidential information.
The attacker intercepts the exchange of public keys during the Diffie-Hellman key exchange. Each party thinks they are communicating with the other, but are actually communicating with the attacker.
The attacker can then decrypt the messages, and send altered or fake messages back to the parties.
This can occur in any form of online communication, including email, social media, and web browsing.

Man-in-the-Middle Attack Variants

Sidejacking: The attacker sniffs data packets to steal session cookies, which may contain login information.
Evil Twin: A rogue Wi-Fi network that appears legitimate. When users connect to this network, the attacker can launch a Man-in-the-Middle attack.

Message Authentication Code (MAC)

A technique for verifying the integrity and authenticity of a message.
A secret key is used to generate a fixed-size block of data (MAC) based on the message.
The message and MAC are transmitted to the intended recipient, who uses the same secret key to calculate a new MAC.
The received MAC is compared to the calculated MAC to verify the authenticity and integrity of the message.

MAC Limitations

Establishment of Shared Secret: MAC relies on a shared secret key, requiring a secure channel to establish the key initially.
Key Management: Maintaining and distributing secret keys securely is a complex process.

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Description

This quiz provides an overview of hash functions, their properties, and characteristics. Learn about pre-image resistance and the significance of hash function speed compared to symmetric encryption. Test your understanding of these essential cryptographic concepts.