Final answer:
An attack on a block cipher seeks to distinguish it from an ideal cipher by exploiting specific vulnerabilities. An ideal cipher is a theoretical concept of a perfectly secure cipher that practical ciphers try to approximate. Understanding these attacks helps in evaluating and improving the security of block ciphers.
Step-by-step explanation:
An attack on a block cipher is a non-generic method of distinguishing the block cipher from an ideal cipher. The purpose of this type of attack is to exploit specific characteristics of the block cipher that would not appear in an ideal cipher, which is a theoretical cipher that is perfectly secure against any attack. In contrast to an ideal cipher real-world block ciphers can have certain vulnerabilities or patterns that make them distinguishable and potentially susceptible to various types of attacks such as differential cryptanalysis or linear cryptanalysis.
An ideal cipher is a useful conceptual tool in cryptography because it represents a cipher with maximum security, and any practical cipher strives to approximate the ideal. However, no practical block cipher can be truly ideal; thus the security of block ciphers is evaluated based on their resistance to known attacks. Distinguishing a block cipher from an ideal cipher through non-generic attacks can reveal the cipher's weaknesses aiding cryptographers in improving the cipher's design or developing more secure alternatives.