General  

First published  1998 (ANS X9.52) 
Derived from  DES 
Cipher detail  
Key sizes  168, 112 or 56 bits (Keying option 1, 2, 3 respectively) 
Block sizes  64 bits 
Structure  Feistel network 
Rounds  48 DESequivalent rounds 
Best public cryptanalysis  
Lucks: 2^{32} known plaintexts, 2^{113} operations including 2^{90} DES encryptions, 2^{88} memory; Biham: find one of 2^{28} target keys with a handful of chosen plaintexts per key and 2^{84} encryptions 
In cryptography, Triple DES is the common name for the Triple Data Encryption Algorithm (TDEA) block cipher defined in each of:
It is so named because it applies the Data Encryption Standard (DES) cipher algorithm three times to each data block.
Triple DES provides a relatively simple method of increasing the key size of DES to protect against brute force attacks, without requiring a completely new block cipher algorithm.
Contents 
The earliest standard that defines the algorithm (ANS X9.52, published in 1998) describes it as the "Triple Data Encryption Algorithm (TDEA)" — i.e. three operations of the Data Encryption Algorithm specified in ANSI X3.92 — and does not use the terms "Triple DES" or "DES" at all. FIPS PUB 463 (1999) defines the "Triple Data Encryption Algorithm (TDEA)", but also uses the terms "DES" and "Triple DES". It uses the terms "Data Encryption Algorithm" and "DES" interchangeably, including starting the specification with:
The Data Encryption Standard (DES) shall consist of the following Data Encryption Algorithm (DES) [sic] and Triple Data Encryption Algorithm (TDEA, as described in ANSI X9.52).
NIST SP 80067(2004, 2008^{[3]}) primarily uses the term TDEA, but also refers to "Triple DES (TDEA)". ISO/IEC 180333 (2005) uses "TDEA", but mentions that:
The TDEA is commonly known as Triple DES (Data Encryption Standard).
Triple DES uses a "key bundle" which comprises three DES keys, K_{1}, K_{2} and K_{3}, each of 56 bits (excluding parity bits). The encryption algorithm is:
I.e., DES encrypt with K_{1}, DES decrypt with K_{2}, then DES encrypt with K_{3}.
Decryption is the reverse:
I.e., decrypt with K_{3}, encrypt with K_{2}, then decrypt with K_{1}.
Each triple encryption encrypts one block of 64 bits of data.
In each case the middle operation is the reverse of the first and last. This improves the strength of the algorithm when using keying option 2, and provides backward compatibility with DES with keying option 3.
The standards define three keying options:
Keying option 1 is the strongest, with 3 x 56 = 168 independent key bits.
Keying option 2 provides less security, with 2 x 56 = 112 key bits. This option is stronger than simply DES encrypting twice, e.g. with K_{1} and K_{2}, because it protects against meetinthemiddle attacks.
Keying option 3 is no better than DES, with only 56 key bits. This option provides backward compatibility with DES, because the first and second DES operations simply cancel out. It is no longer recommended by the National Institute of Standards and Technology (NIST) ^{[4]} and not supported by ISO/IEC 180333.
"Keying option n" is the term used by the standards (X9.52, FIPS PUB 463, SP 80067, ISO/IEC 180333) that define the TDEA, but a variety of other terms have been used elsewhere — in other standards or similarly authoritative documents — to refer to those keying options. These terms include:
As with all block ciphers, encryption and decryption of multiple blocks of data may be performed using a variety of modes of operation, which can generally be defined independently of the block cipher algorithm. However ANS X9.52 specifies directly, and NIST SP 80067 specifies (via SP 80038A^{[7]}), that some modes shall only be used with certain constraints on them that do not necessarily apply to general specifications of those modes. For example, ANS X9.52 specifies that for cipher block chaining, the initialization vector shall be different each time, whereas ISO/IEC 10116^{[8]} does not. FIPS PUB 463 and ISO/IEC 180333 define only the single block algorithm, and do not place any restrictions on the modes of operation for multiple blocks.
In general Triple DES with three independent keys (keying option 1) has a key length of 168 bits (three 56bit DES keys), but due to the meetinthemiddle attack the effective security it provides is only 112 bits. Keying option 2, reduces the key size to 112 bits. However, this option is susceptible to certain chosenplaintext or knownplaintext attacks^{[9]}^{[10]} and thus it is designated by NIST to have only 80 bits of security.^{[5]}
The best attack known on keying option 1 requires around 2^{32} known plaintexts, 2^{113} steps, 2^{90} single DES encryptions, and 2^{88} memory^{[11]} (the paper presents other tradeoffs between time and memory). This is not currently practical and NIST considers keying option 1 to be appropriate through 2030.^{[5]} If the attacker seeks to discover any one of many cryptographic keys, there is a memoryefficient attack which will discover one of 2^{28} keys, given a handful of chosen plaintexts per key and around 2^{84} encryption operations.^{[12]}
The electronic payment industry uses Triple DES and continues to develop and promulgate standards based upon it (e.g. EMV).^{[13]}^{[14]}
Microsoft OneNote uses Triple DES to password protect user content.^{[15]}
, C is the Ciphertext and {k_{1}, k_{2}, k_{3}} is the set of the three encryption keys.]] In cryptography, Triple DES is a block cipher derived from the Data Encryption Standard (DES) cipher by using it three times. Triple DES is also known as TDES or, more standard, TDEA (Triple Data Encryption Algorithm ^{[1]}).
When it was discovered that a 56bit key of DES is not enough to protect from brute force attacks, TDES was chosen as a simple way to enlarge the key space without a need to switch to a new algorithm. The use of three steps is essential to prevent meetinthemiddle attacks that are effective against double DES encryption.
In general TDES with three different keys (3key {k_{1}, k_{2}, k_{3}} TDES) has a key length of 168 bits: three 56bit DES keys (with parity bits 3key TDES has the total storage length of 192 bits), but due to the meetinthemiddle attack the effective security it provides is only 112 bits. A variant, called twokey TDES (2key TDES), uses k_{1} = k_{3}, thus reducing the key size to 112 bits and the storage length to 128 bits. However, this mode is susceptible to certain chosenplaintext or knownplaintext attacks ^{[2]} ^{[3]} and thus it is designated by NIST to have only 80 bits of security ^{[4]}.
By design, DES and therefore TDES, suffer from slow performance in software.^{[5]} TDES is better suited to hardware implementations^{[5]}, and indeed where it is still used is likely to be within a hardware implementation.
TDES is slowly disappearing from use, largely replaced by the Advanced Encryption Standard (AES). One largescale exception is within the electronic payments industry, which still uses 2TDES extensively and continues to develop and spread standards based upon it (e.g. EMV, the standard for interoperation of IC cards; also called "Chip cards", and IC capable POS terminals and ATM's). This guarantees that TDES will remain an active cryptographic standard well into the future.
