Cryptographic Hash Functions Overview

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Questions and Answers

What condition must be met for P(N,k) to exceed 0.5?

k should be approximately $N^{1/4}$

k should be approximately $N^{1/3}$

k should be approximately $N^{1/2}$ (correct)

k should be approximately $N^{1}$

In the context of hash functions, what is the main focus of cryptanalytic attacks?

Identifying vulnerabilities in the hash algorithm (correct)

Maximizing the throughput of data processing

Minimizing the number of collisions

Optimizing the key length for security

Which statement accurately describes the process of using block ciphers as hash functions?

The final hash value is always larger than the input message size

Block ciphers can only create 128-bit hash values

The iterative encryption starts with a constant value and zero-padded final block (correct)

Each block of the message is encrypted independently

What is a significant limitation of using block ciphers as hash functions?

The hash value is too small, typically 64 bits Signup and view all the answers

What was the original purpose of the Secure Hash Algorithm (SHA)?

To be a US standard for the DSA signature scheme Signup and view all the answers

What is an essential property that a cryptographic hash function must possess to ensure that it is not easy to find two different inputs that produce the same output?

Collision resistance Signup and view all the answers

Which of the following statements correctly describes the difference between a document and its fingerprint compared to a message and its digest?

The document and fingerprint are physically linked, while the message and digest can be separated. Signup and view all the answers

Which of the following methods cannot be used as a cryptographic hash function due to lack of preimage resistance?

Lossless compression methods like StuffIt Signup and view all the answers

In the context of the Random Oracle Model, what is the primary function of the oracle when given a message for digest calculation?

To find a digest in its pre-stored table for that message Signup and view all the answers

What is the implication of a hash function not providing second preimage resistance?

An attacker can easily find an alternative input that hashes to the same value as a given input. Signup and view all the answers

What aspect does integrity preservation prioritize in the context of cryptographic systems?

Ensuring that data cannot be altered without detection Signup and view all the answers

Which of the following criteria is NOT a requirement for a secure cryptographic hash function?

Reversible hashing Signup and view all the answers

Why are checksum functions unsuitable for use as cryptographic hash functions?

They are reversible and do not provide strong resistance against collisions. Signup and view all the answers

What happens if a message of length 1024 bits is sent in SHA-512?

Padding is required to meet the block size. Signup and view all the answers

What is the primary limitation of message length in SHA-512?

Messages must be less than $2^{128}$ bits. Signup and view all the answers

How many bits of padding are necessary for an original message of 2590 bits in SHA-512?

353 bits Signup and view all the answers

When the message is 2110 pages long, how many bits does this represent?

2110 * 2048 bits Signup and view all the answers

What is the maximum length of padding that can be utilized in SHA-512?

1023 bits Signup and view all the answers

Which of the following statements about SHA-512 padding is true?

Padding comprises a '1' followed by zeros, with a length field appended. Signup and view all the answers

What characteristic must the length of the original message meet for SHA-512 functionality?

Less than $2^{128}$ bits. Signup and view all the answers

How is W60 created in SHA-512 during the word expansion phase?

Derived from W56, W57, W58, and W59. Signup and view all the answers

What is the length of the original message calculated in this scenario?

897 bits Signup and view all the answers

Which algorithm is considered insecure and has similarities to broken MD5 and SHA-0?

SHA-1 Signup and view all the answers

What structure must SHA-3 preserve from SHA-2?

The online nature and processing of small blocks Signup and view all the answers

In the construction of SHA-1, what is the output hash length produced?

160 bits Signup and view all the answers

Which scheme is NOT mentioned as a method for iterated hash functions?

Chaskey Scheme Signup and view all the answers

What is one of the evaluation criteria for SHA-3?

Operation time & memory usage Signup and view all the answers

What is the initial state of the 5-word buffer in SHA-1?

0x67452301, 0xefcdab89, 0x98badcfe, 0x10325476, 0xc3d2e1f0 Signup and view all the answers

Which statement best describes the SHA-2 family of algorithms?

Shares structural similarities with its predecessors Signup and view all the answers

What is the main security concern raised regarding SHA-1 in 2005?

Vulnerability to collision attacks Signup and view all the answers

What is the size of the hash values produced by SHA-1?

160 bits Signup and view all the answers

How many rounds does the SHA-512 algorithm perform in processing a message?

80 Signup and view all the answers

What is the primary output size of the SHA-512 algorithm?

512 bits Signup and view all the answers

How are the round constants used in SHA-512 obtained?

Using the cube root of the first 80 prime numbers Signup and view all the answers

In the SHA-512 algorithm, what is the significance of processing messages in 1024-bit blocks?

It provides higher overall security during processing. Signup and view all the answers

Which functions are utilized within each round of the SHA-512 algorithm?

Conditional and Rotate functions Signup and view all the answers

What differentiates SHA-256, SHA-384, and SHA-512 from SHA-1 in terms of security?

They provide a higher level of security. Signup and view all the answers

What does the one-way property of cryptographic hash functions imply?

It is computationally infeasible to find data that maps to a specific hash. Signup and view all the answers

What security concern is associated with the keyed hash function used in HMAC?

Brute force attack on the key used. Signup and view all the answers

In which scenario would a message authentication code (MAC) typically be used?

When a keyed hash of the message is required. Signup and view all the answers

What is a collision-free property in the context of hash functions?

It guarantees that no two different messages can produce the same hash value. Signup and view all the answers

How does HMAC enhance security in hash functions?

It uses a predefined secret key to ensure message authenticity. Signup and view all the answers

What is a common use case for cryptographic hash functions outside of message integrity?

Generating session keys and nonces. Signup and view all the answers

Which of the following hash functions is typically used for digital signatures?

Keyed hash functions. Signup and view all the answers

What key aspect should be considered when selecting a hash function?

The choice should balance between speed and security constraints. Signup and view all the answers

Study Notes

Cryptography and Network Security - Week 9-11

Message Integrity: Cryptography systems often provide secrecy but not integrity. Integrity is required when secrecy is not necessary, ensuring data hasn't been altered.
Document and Fingerprint: One way to ensure document integrity is through fingerprinting. A digital "fingerprint" (message digest) is comparable to a fingerprint and is linked to the document.
Message and Message Digest: The electronic equivalent of a document/fingerprint is a message/digest pair. A hash function generates the digest (fingerprint) from the message.
Difference: Documents and fingerprints are physically linked. Messages and digests can be separated, and critically, the digest needs protection from tampering.
Checking Integrity: A hash function produces a current digest for a message. Comparing this to an earlier digest (known value) ensures data integrity, and if the digests match the message hasn't changed.

Cryptographic Hash Function Criteria

Preimage Resistance: Given a hash value, it is computationally infeasible to find an input that produces that hash value.
Second Preimage Resistance: Given an input and a hash value, it is computationally infeasible to find another input that produces the same hash value.
Collision Resistance: It is computationally infeasible to find two different inputs that produce the same hash.

Random Oracle Model

The Random Oracle Model was introduced in 1993 by Bellare and Rogaway.
An ideal model for hash functions, assuming the hash function outputs a random result, uncorrelated with the input.
This is a mathematical model for assessing if hash functions are trustworthy.
It simplifies the analysis of cryptographic protocols. (Simplified by ignoring the specific mathematical implementation)
The random values are only used for evaluating, that is, evaluating only.

Attacks on Random Oracle Model

Preimage Attack: Algorithm that tries to find an input that maps to a given hash value.
The difficulty is proportional to 2ⁿ, where n is the number of bits in the hash.
Second Preimage Attack: Given a message and its hash, tries to find another message with the same hash.
The difficulty is proportional to 2ⁿ.
Collision Attack: Seeks to find two different messages that produce the same hash.
The difficulty is proportional to 2^n/2, where n is the number of bits in the hash.

Message Authentication

Modification Detection Code (MDC): A message digest verifying message integrity, not the sender.
Message Authentication Code (MAC): Used to authenticate the originator of a message. It involves a secret key only known to sender and receiver.
MAC Properties: A MAC is a condensed checksum of a variable-length message using a secret key. It should be computationally hard to find different messages that result in the same MAC.
MAC Requirements: Knowing a message and its MAC, it should be infeasible to find another message having the same MAC. MACs should be distributed uniformly. A MAC should depend equally on all bits of the message.

Keyed Hash Functions

Cryptographic hash functions can also be used for keyed-MACs
Generally faster than block ciphers
Improved by using HMAC (Hash-based message authentication code)
Addresses some weaknesses in earlier keyed hash designs with a more rigorous method.

HMAC

A keyed hash function, standardized in RFC2104.
Designed to use a hash function without modification.
Has added overhead for key-use, often just 3 hash block calculations.
Provides the security of the underlying hash algorithm and addresses some vulnerabilities in earlier designs.

Hash Function Requirements

Variable Input Size: Hash functions can accept messages of any length.
Fixed Output Size: Hash functions produce a hash of fixed length.
Efficiency: Hash computation should be relatively quick, for practical use.
Preimage Resistance: Finding an input that produces a given output should be computationally difficult.
Second Preimage Resistance: Finding a second input that has the same output as a given input should be computationally difficult.
Collision Resistance: Finding two different inputs that produce the same output should be computationally difficult.
Pseudorandomness: The output of the hash function should satisfy standards for pseudorandomness tests.

Birthday Attacks

A collision attack technique that exploits the properties of hash functions.
It attempts to find two different inputs that produce the same hash.
Its difficulty is proportional to the square root of the number of possible hash outputs.

Hash Function Cryptanalysis

Hash functions use iterative structures, processing messages in blocks.
Cryptanalysis attacks exploit structural weaknesses in hash functions.

Block Ciphers as Hash Functions

Some simple methods use block ciphers as hash functions.
These functions often result in small/insufficient hash values (e.g., 64 bits).

Secure Hash Algorithm (SHA-1 and SHA-512)

SHA-1: A secure hash algorithm, producing a 160-bit hash value.
SHA-512: A revised version, producing 512-bit hash values. More secure and robust.
SHA algorithms use a compression function, iterative in nature.
Both use a modular-style structure for calculating the hash.
The algorithms involve various cryptographic transformations like (e.g., rotations, additions, non-linear functions).

MD5

A 128-bit hash algorithm.
Designed by Ronald Rivest, it was used extensively before its vulnerabilities were discovered.
Multiple security concerns and attacks have been found.
Now considered insecure for most purposes.
An example of a hash function often used in the past but now considered less secure against new attacks.

Modified Secure Hash Algorithms (SHA-256, SHA-384, SHA-512)

SHA-2 algorithms are stronger and more resistant to attacks.

Additional Security Services

Confidentiality: Applies a layer of encryption to prevent unauthorized access to the message.
Nonrepudiation: Ensures that the sender cannot deny sending the message.
A digital signature can typically handle the latter three (integrity, authenticity and nonrepudiation) by itself, while confidentiality typically requires additional encryption layers.

Attack Types and Forgery Types

Key-Only Attacks: Attacks focusing on obtaining the secret key.
Known-Message Attacks: Attacks making use of known pairs of messages, keys, and MAC values
Chosen-Message Attacks: Attacks where an attacker can choose messages to be signed.
Existential Forgery: Attack where an attacker can create a valid signature for a message without knowing the private key.
Selective Forgery: Attack where an attacker has some control over the messages used in the forgery.
Total Break: An attacker is able to derive the private key from the algorithm.

Digital Signature Schemes

RSA: A public-key-based digital signature scheme.
ElGamal: Another public-key-based digital signature scheme.
Schnorr: A digital signature scheme.
DSS (Digital Signature Standard): A standard for digital signatures.
ECDSA (Elliptic Curve Digital Signature Algorithm): A digital signature scheme that is based on elliptic curve cryptography.

Conclusion:

Modern hashing and keying mechanisms are becoming more sophisticated, to provide better security, and more efficient processes.

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Description

This quiz explores fundamental concepts related to cryptographic hash functions, including their properties, limitations, and the Secure Hash Algorithm (SHA). It also examines the methodologies behind using block ciphers as hash functions and the implications of various security features. Test your knowledge and understanding of cryptographic principles.