Differential cryptanalysis

Differential cryptanalysis is a powerful attack technique against block ciphers that analyzes how specific differences in plaintext pairs propagate through the cipher to produce predictable differences in ciphertext. Developed by Eli Biham and Adi Shamir in the late 1980s, it was later revealed that IBM researchers had discovered the technique in the 1970s and used it to design the S-boxes of the Data Encryption Standard — knowledge that remained classified until the academic rediscovery made continued secrecy impossible.

The attack is a statistical method, not a brute-force search. By choosing plaintext pairs with carefully controlled differences and observing the resulting ciphertext differences, an attacker can deduce information about the secret key. The effectiveness of differential cryptanalysis depends on the cipher's round function and S-box design: well-designed ciphers resist it, while poorly designed ones can fall to it using relatively few chosen plaintexts.