(also 3DES, Triple Data Encryption Algorithm, TDES, TDEA)
A Feistel network-based, symmetric-key encryption algorithm derived from the original Data Encryption Standard. It uses the DES cipher but runs through it three times with three separate encryption keys. 3DES is largely out of use in favor of more secure algorithms like AES-256 and XChaCha20.
Triple DES uses 168-bit keys (three 56-bit DES keys), but because of its vulnerability to the meet-in-the-middle attack, the effective security it provides is 112 bits. During the meet-in-the-middle attack, the hacker tries to use brute force to simultaneously encrypt the plaintext and decrypt the intermediate ciphertext (plaintext that was only encrypted by one key) to find a match. If they succeed, it is highly likely that the keys used to encrypt and decrypt are the same as the ones used by the cipher.
The structure of the cipher allows the hacker to attack the encryption from both sides at the same time, therefore basically “meeting in the middle.” That’s why the actual security of 3DES is less than the keys the cipher uses.