A message digest is generated by a cryptographic hash function that processes the entire input file or message, not just a specific portion. The statement that the digest is calculated using "at least 128 bytes of the file" is incorrect. This confuses the size of the input data with the size of the output digest. A fundamental property of a hash function is to take a variable-length input and produce a fixed-length output. The entire input must be processed to ensure that any change to the original file, no matter how small, results in a different message digest, which is essential for integrity verification.

A. The original file cannot be created from the message digest.

This is a correct statement describing the one-way (pre-image resistance) property, a fundamental security requirement for any cryptographic hash function.

B. Two different files should not have the same message digest.

This is a correct statement describing the collision resistance property. It should be computationally infeasible to find two different inputs that produce the same hash output.

D. Messages digests are usually of fixed size.

This is a correct statement. A defining characteristic of a hash function is that it produces a fixed-length output (e.g., 256 bits for SHA-256) regardless of the input's size.

1. National Institute of Standards and Technology (NIST). (2015). FIPS PUB 180-4

Secure Hash Standard (SHS).

Section 5.1

"Padding the Message": This section details the process of taking the message of length l bits and padding it so that it can be processed. This confirms that the entire message is used as input

directly refuting option C.

Section 4

"PROPERTIES OF SECURE HASH ALGORITHMS": This section states

"For a given algorithm

the hash function is a one-way function; it is computationally infeasible to find a message that corresponds to a given message digest." This supports the correctness of option A. It also describes the goal of collision resistance

supporting option B.

Section 1

"INTRODUCTION": This section states that the secure hash algorithms "produce a condensed representation of a message called a message digest" and that this digest is a "fixed-size bit string." This supports the correctness of option D.

2. Katz

J.

& Lindell

Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press.

Chapter 5

Section 5.1

"Definitions": A hash function is formally defined as a function H that maps arbitrary-length inputs to a fixed-length output. This supports option D and refutes option C

which implies a partial or minimum input. The chapter further defines pre-image resistance (supporting A) and collision resistance (supporting B) as essential security properties.

3. Rivest

R. (1992). RFC 1321: The MD5 Message-Digest Algorithm. IETF.

Section 3

"MD5 Algorithm Description": The overview describes the algorithm operating on an "arbitrary-length message" to produce a "128-bit 'fingerprint' or 'message digest'." Step 1

"Append Padding Bits

" explicitly details how the original message is padded to a specific length for processing

confirming the entire message is the input. This supports options D and A

while refuting C.