(Which encryption algorithm encrypts with one key, decrypts with another key, and then encrypts with...

3DES (Triple DES) commonly uses an Encrypt–Decrypt–Encrypt (EDE) sequence. In the two-key form, it encrypts with key K1, decrypts with key K2, then encrypts again with K1. In the three-key form, it encrypts with K1, decrypts with K2, then encrypts with K3. The EDE construction was chosen partly for backward compatibility: if K1=K2=K3, the scheme reduces to single DES, allowing older systems to interoperate in constrained ways. AES and IDEA do not use an EDE triple-stage process as their defining structure; they are single-pass block ciphers with internal rounds. DES is a single-pass algorithm (one key) rather than a triple application with multiple keys. Therefore, the algorithm described—encrypt with one key, decrypt with another, encrypt with the first—is 3DES. Although now considered legacy, it remains a classic example of increasing effective security by applying a block cipher multiple times with independent keys.