How does a brute-force attack search through possible encryption keys?

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A Vigenere cipher table displays the alphabet in both the top row and left column, with rows shifted to create a grid for encryption and decryption. Highlighted letters illustrate how to use the table. An explanation and example using the words cipher, key, and plaintext appears to the right. — cryptology In encrypting plaintext, the cipher letter is found at the intersection of the column headed by the plaintext letter and the row indexed by the key letter. To decrypt ciphertext, the plaintext letter is found at the head of the column determined by the intersection of the diagonal containing the cipher letter and the row containing the key letter.

A brute-force attack is a frontal assault on a cryptosystem that relies on the exhaustive search of the entire key space to uncover the correct encryption key.1

The process functions as follows:

Systematic Decryption: The cryptanalyst takes a block of ciphertext and systematically attempts to decrypt it using every possible key in the system's key space, one after another.1
Verification: After each decryption attempt, the analyst checks the output to see if it results in meaningful text. If a block of meaningful text is produced, the analyst then tests that specific key against subsequent blocks of ciphertext to verify its validity.1
Computational Effort: The security of a system against this attack is defined by its work function, which is the total computational effort required to test all possible keys. If an analyst possesses sufficient time and computing resources, they will eventually identify the correct key.1

Modern systems mitigate this risk by using larger key sizes, such as 128-bit or 2048-bit keys, which make the total number of possible combinations computationally infeasible to search within a reasonable timeframe.1