Symmetric ciphersLegacy - avoid
Triple DES (TDEA)
Symmetric block cipher (legacy)
How it works
3DES applies DES three times to each block, normally as encrypt-decrypt-encrypt (EDE) with three independent keys. It was a way to extend DES's life after its 56-bit key became too short. A meet-in-the-middle attack means three 56-bit keys give roughly 112 bits of security, not 168.
Sizes: 64-bit block; 168-bit key with three keys (about 112 bits of effective security)
Status: NIST disallowed 3DES for encryption after 2023; decryption of old data only.
Strengths
- Much stronger than single DES
- Still found in payment and legacy systems, so you need to recognise it
Weaknesses and known attacks
- 64-bit block size: Sweet32 (2016) showed that long sessions encrypting large amounts of data produce block collisions that leak plaintext.
- Slow, because it runs the DES algorithm three times.
- NIST withdrew SP 800-67 Rev. 2 on 1 January 2024 and no longer allows it for applying new encryption.
Use it for
- Decrypting existing legacy data during a migration
Don't use it for
- Any new encryption
- TLS cipher suites (disable them)
Exam tip
3DES = 64-bit block, three DES passes, effective 112-bit security. If a question mentions Sweet32 or a 64-bit block cipher birthday attack, the fix is to move to AES.