How do brute force decryption attacks know when they've found the right solution? [closed]

Question

How do brute-force attacks on encrypted data know when they've found the right key to decrypt the data? Is there a way to know that data's been decrypted, other than having a human looking at it? What if it's not human-friendly data?

Answer 1

Cryptanalysts hope to have known ciphertext and plaintext. A key which decrypts that ciphertext to that plaintext is certainly the right key.

Without known plaintext, the data's format must be known. For example, plaintext HTML contains tags. A phone directory plaintext contains phone numbers. And so on.