(Which technique involves spotting variations in encrypted data and plotting how the characters relate to...

Frequency analysis is a classical cryptanalysis technique that exploits predictable statistical patterns in natural language. In English, certain letters (like E, T, A, O, I, N) occur more frequently than others, and common digrams/trigrams (TH, HE, IN, ER) appear with recognizable distribution. When a cipher preserves character boundaries (as in many substitution ciphers), the ciphertext will also show frequency patterns—though mapped to different symbols. The analyst counts ciphertext character occurrences, compares the distribution to expected English letter frequencies, and infers likely plaintext mappings. “Spotting variations” refers to observing differences in how often symbols appear and using that to plot relationships between ciphertext and standard English. Brute force instead tries all keys; known-plaintext attacks rely on having plaintext–ciphertext pairs; chosen-ciphertext attacks involve decrypting attacker-selected ciphertexts. Those are different attack models. Frequency analysis is specifically about statistical correlation between ciphertext symbols and language characteristics, which is why it is effective against monoalphabetic substitution and weak polyalphabetic schemes with short periods.