Lucks: 232 known plaintexts, 2113 operations including 290 DES encryptions, 288 memory; Biham: find one of 228 target keys with a handful of chosen plaintexts per key and 284 encryptions
In cryptography, Triple DES (3DES), officially the Triple Data Encryption Algorithm (TDEA or Triple DEA), is a symmetric-keyblock cipher, which applies the DES cipher algorithm three times to each data block.
While the government and industry standards abbreviate the algorithm's name as TDES (Triple DES) and TDEA (Triple Data Encryption Algorithm),RFC 1851 referred to it as 3DES from the time it first promulgated the idea, and this namesake has since come into wide use by most vendors, users, and cryptographers.
The Triple Data Encryption Algorithm is variously defined in several standards documents:
RFC 1851, The ESP Triple DES Transform (approved in 1995)
ANSI ANS X9.52-1998 Triple Data Encryption Algorithm Modes of Operation (approved in 1998, withdrawn in 2008)
FIPS PUB 46-3 Data Encryption Standard (DES) (approved in 1999, withdrawn in 2005)
NIST Special Publication 800-67 Revision 2 Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher (approved in 2017)
ISO/IEC 18033-3:2010: Part 3: Block ciphers (approved in 2005)
The original DES cipher's key size of 56 bits was generally sufficient when that algorithm was designed, but the availability of increasing computational power made brute-force attacks feasible. Triple DES provides a relatively simple method of increasing the key size of DES to protect against such attacks, without the need to design a completely new block cipher algorithm.
A naive approach to increase strength of a block encryption algorithm with short key length (like DES) would be to use two keys (K1, K2) instead of one, and encrypt each block twice: EK2(EK1(plaintext)). If the original key length is n bits, one would hope this scheme provides security equivalent to using key 2n bits long. Unfortunately, this approach is vulnerable to meet-in-the-middle attack: given a known plaintext pair (x, y), such that y = EK2(EK1(x)), one can recover the key pair (K1, K2) in ~2n steps, instead of ~22n steps one would expect from algorithm with 2n bits of key.
Therefore, Triple DES uses a "key bundle" that comprises three DES keys, K1, K2 and K3, each of 56 bits (excluding parity bits). The encryption algorithm is:
ciphertext = EK3(DK2(EK1(plaintext)))
I.e., DES encrypt with K1, DES decrypt with K2, then DES encrypt with K3.
Decryption is the reverse:
plaintext = DK1(EK2(DK3(ciphertext)))
I.e., decrypt with K3, encrypt with K2, then decrypt with K1.
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
All three keys are independent. Sometimes known as 3TDEA or triple-length keys.
This is the strongest, with 3 × 56 = 168 independent key bits. It is still vulnerable to meet-in-the-middle attack, but the attack requires 22 × 56 steps.
Keying option 2
K1 and K2 are independent, and K3 = K1. Sometimes known as 2TDEA or double-length keys.
This provides a shorter key length of 112 bits and a reasonable compromise between DES and Keying option 1, with the same caveat as above. This is an improvement over "double DES" which only requires 256 steps to attack. NIST has deprecated this option.
Keying option 3
All three keys are identical, i.e. K1 = K2 = K3.
This is backward compatible with DES, since two operations cancel out. ISO/IEC 18033-3 never allowed this option, and NIST no longer allows it.
Each DES key is 8 odd-parity bytes, with 56 bits of key and 8 bits of error-detection. A key bundle requires 24 bytes for option 1, 16 for option 2, or 8 for option 3.
Encryption of more than one block
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 800-67 specifies via SP 800-38A 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 does not. FIPS PUB 46-3 and ISO/IEC 18033-3 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 56-bit DES keys), but due to the meet-in-the-middle attack, the effective security it provides is only 112 bits. Keying option 2 reduces the effective key size to 112 bits (because the third key is the same as the first). However, this option is susceptible to certain chosen-plaintext or known-plaintext attacks, and thus, it is designated by NIST to have only 80 bits of security. This can be considered broken, as the whole 3des keyspace can be searched thoroughly by affordable consumer hardware as of 2017.
Practical Sweet32 attack on 3DES-based cipher-suites in TLS required blocks (785 GB) for a full attack, but researchers were lucky to get a collision just after around blocks which took only 25 minutes.
The security of TDEA is affected by the number of blocks processed with one key bundle. One key bundle shall not be used to apply cryptographic protection (e.g., encrypt) more than 64-bit data blocks.
— Recommendation for Triple Data Encryption Algorithm (TDEA) Block Cipher (SP 800-67 Rev2)
OpenSSL does not include 3DES per default since version 1.1.0 (August 2016), and considers it a "weak cipher".
The electronic payment industry uses Triple DES and continues to develop and promulgate standards based upon it (e.g. EMV).
"Annex B Approved Cryptographic Algorithms – B1.1 Data Encryption Standard (DES)". EMV 4.2: Book 2 - Security and Key Management (4.2 ed.). EMVCo. June 2008. p. 137. The double-length key triple DES encipherment algorithm (see ISO/IEC 18033-3) is the approved cryptographic algorithm to be used in the encipherment and MAC mechanisms specified in Annex A1. The algorithm is based on the (single) DES algorithm standardised in ISO 16609.