DES (Data Encryption Standard) uses a 56-bit key — too short for modern security. Triple DES (3DES) applies DES three times (112-bit effective security) — still deprecated (slow, vulnerable to Sweet32).
DES: cracked in 22 hours in 1999 (EFF DES Cracker). Modern GPUs crack it instantly. 3DES: still in some legacy systems but deprecated by NIST in 2017. Replace with AES-256 everywhere. Both are exam topics — know why they're deprecated.