3DES is one of the implementations of #Data Encryption Standard (DES). It applies DES algorithm three times, which is also 48 rounds, to each block using 2 or 3 keys, each 56 bits, with a total size of 112 or 168 bits, on 64-bit data in order to encrypt# the messages. However, this comes as a cost of performance. It is standardised in ANSI X9.17 and ISO 8732.
The scheme is summarised with the following formula:
$$ \begin{aligned} C &= E_{K1}[D_{K2}[E_{K1}[P]]]\\ P &= D_{K1}[E_{K2}[D_{K1}[C]]] \end{aligned} $$
Where:
- \(C\) is the 64-bit ciphertext
- \(P\) is the 64-bit plaintext
- \(K\) is the key used with size of 56 bits
- \(E\) is the encryption algorithm and $D4 is the decryption algorithm
Note: If \(K1 = K2\), then it works like #Data Encryption Standard (DES).
Note: However, due to the nature of Feistel Cipher Structure#, there is no significance differences between encryption and decryption other than order of the subkeys. Decryption algorithm is using the reverse order of the subkeys in encryption algorithm.
3DES is typically used for financial key management, and other asymmetric cryptographic scheme such as Pretty Good Privacy (PGP) and Secure/Multipurpose Internet Mail Extensions (S/MIME).