Triple DES

Use 3 stages of encryption instead of 2, with 2 keys $K_1$ and $K_2$:

• Compatible with standard DES ($K_2=K_1$) if needed.
• There is no known practical attack but a brute-force attack search with $2^{112}$ operations.

Variant of Triple DES exists with $3$ different keys $K_1,K_2,K_3$.

This has an effective key length of $168$ bits. This is also adopted by some internet applications such as PGP and S/MIME.