3DES, also known as Triple-DES, uses the standard DES algorithm three times. To encrypt, the first operation invokes DES in encrypt mode, the second uses DES in decryption mode, and the last operation uses encrypt mode again (also known as EDE mode). Decryption is performed in DED mode. As it is a reuse of DES, it shares similar attributes; it is still a 64-bit block cypher, each operation still uses initial and final permutation steps, and it still is based on the underlying Feistel network design.
DES was originally specified as a hardware device, with software implementations not being certified for use to protect secrets. Once the original DES key size of 56 bits was recognized as too cryptographically weak to continue to be used safely, cryptographers needed a safe algorithm while they awaited a new FIPS standard for encryption. Instead of taking the risk on an unproven algorithm, DES was easily reused simply by chaining three encoders together, connecting the output of each to the input of the next.
Keying options for 3DES include three independent keys; two independent keys where the key for the first and third encryptions are identical; and three identical keys. The three identical key system is backwards compatible with ordinary DES as the first and second encryptions cancel each other out, and as a result is not secure.
3DES is still considered secure, but not necessarily efficient when compared to modern algorithms such as AES.
3DES remains in common use today, particularly in the financial industry.