How does Triple-DES (3DES) work and what is the EDE scheme?

3DES applies DES three times in an Encrypt-Decrypt-Encrypt (EDE) sequence with two or three different keys, providing approximately 112-bit effective security.

* 3DES runs DES as Encrypt-Decrypt-Encrypt, so equal keys collapse to single DES for compatibility. *

EDE with two keys (K1, K2):

Encryption: C = E(K1, D(K2, E(K1, M)))
Decryption: M = D(K1, E(K2, D(K1, C)))

EDE with three keys (K1, K2, K3):

Encryption: C = E(K3, D(K2, E(K1, M)))
Decryption: M = D(K1, E(K2, D(K3, C)))

Why EDE (not EEE)?

• Backwards compatibility: if K1=K2=K3, 3DES-EDE reduces to single DES (the D in the middle cancels one E)
• This allowed gradual migration from DES to 3DES

Security:

• 2-key 3DES: only ~57–60 bits effective security (not allowed since 2009)
• 3-key 3DES: ~112 bits effective security (meet-in-the-middle reduces it from 168 bits)
• Much slower than AES (3x DES operations)
• 3-key 3DES permitted by NIST until 2030

Go deeper:

• Triple DES (Wikipedia) — EDE, the meet-in-the-middle bound, and deprecation timeline.