3DES

Triple DES, commonly called 3DES, is a symmetric-key block cipher that applies the DES algorithm three times to each data block. It is designed to increase the effective key length of DES by performing DES in a sequence: encrypt with a first key, decrypt with a second key, then encrypt with a third key (an EDE sequence). The standard notation is DES_K1-DES_K2-DES_K3, and the usual block size remains 64 bits, as with DES.

Keying options include three independent keys (K1, K2, K3) for a nominal 168-bit key, or two-key configurations where K3 equals K1, yielding about 112 bits of practical security. A degenerate option, where all keys are identical, reduces 3DES to single DES. In practice, the two- or three-key forms were used to achieve greater security than DES alone while maintaining compatibility with systems that supported DES.

3DES can operate in the same modes as DES, such as CBC, CFB, OFB, and ECB, but the most common implementation in protocols is 3DES in CBC mode. Because it applies DES three times, 3DES is considerably slower than AES and benefits less from hardware acceleration available for AES.

Security and status: 3DES increases resistance to brute-force attacks relative to DES, but cryptanalytic results imply a practical security level around 112 bits, not the full 168-bit nominal key length. With advances in cryptography and the adoption of AES, 3DES is considered deprecated for new designs. It remains in use in some legacy systems and protocols but is generally being phased out in favor of AES.