Triple DES
In cryptography, Triple DES (3DES or TDES), officially the Triple Data Encryption Algorithm (TDEA or Triple DEA), is a symmetric-key block cipher, which applies the DES cipher algorithm three times to each data block. The Data Encryption Standard's (DES) 56-bit key is no longer considered adequate in the face of modern cryptanalytic techniques and supercomputing power. A CVE released in 2016, "CVE-2016-2183" disclosed a major security vulnerability in DES and 3DES encryption algorithms. This CVE, combined with the inadequate key size of DES and 3DES, NIST has deprecated DES and 3DES for "new" applications  in 2017, and for "all" applications by 2023. It has been replaced with the more secure, more robust AES. 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. History. In 1978, a triple encryption method using DES with two 56-bit keys was proposed by Walter Tuchman; in 1981 Merkle and Hellman proposed a more secure triple key  version of 3DES with 112 bits of security. Standards. The Triple Data Encryption Algorithm is variously defined in several standards documents: Algorithm. 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 formula_1 instead of one, and encrypt each block twice: formula_2. If the original key length is formula_3 bits, one would hope this scheme provides security equivalent to using key formula_4 bits long. Unfortunately, this approach is vulnerable to meet-in-the-middle attack: given a known plaintext pair formula_5, such that formula_6, one can recover the key pair formula_1 in formula_8 steps, instead of the formula_9 steps one would expect from an ideally secure algorithm with formula_4 bits of key. Therefore, Triple DES uses a "key bundle" that comprises three DES keys, formula_11, formula_12 and  formula_13, each of 56 bits (excluding parity bits). The encryption algorithm is: That is, DES encrypt with formula_11, DES "decrypt" with formula_12, then DES encrypt with formula_13. Decryption is the reverse: That is, decrypt with formula_13, "encrypt" with formula_12, then decrypt with formula_11. 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. Keying options. The standards define three keying options: 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. NIST (and the current TCG specifications version 2.0 of approved algorithms for Trusted Platform Module) also disallows using any one of the 64 following 64-bit values in any keys (note that 32 of them are the binary complement of the 32 others; and that 32 of these keys are also the reverse permutation of bytes of the 32 others), listed here in hexadecimal (in  each byte, the least significant bit is an odd-parity generated bit, it is discarded when forming the effective 56-bit keys): With these restrictions on allowed keys, Triple DES has been reapproved with keying options 1 and 2 only. Generally the three keys are generated by taking 24 bytes from a strong random generator and only keying option 1 should be used (option 2 needs only 16 random bytes, but strong random generators are hard to assert and it's considered best practice to use only option 1). 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. Security. 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 insecure, and, as consequence Triple DES has been deprecated by NIST in 2017. The short block size of 64 bits makes 3DES vulnerable to block collision attacks if it is used to encrypt large amounts of data with the same key. The Sweet32 attack shows how this can be exploited in TLS and OpenVPN. Practical Sweet32 attack on 3DES-based cipher-suites in TLS required formula_22 blocks (785 GB) for a full attack, but researchers were lucky to get a collision just after around formula_23 blocks , which took only 25 minutes. OpenSSL does not include 3DES by default since version 1.1.0 (August 2016) and considers it a "weak cipher". Usage. The electronic payment industry uses Triple DES and continues to develop and promulgate standards based upon it, such as EMV. Earlier versions of Microsoft OneNote, Microsoft Outlook 2007 and Microsoft System Center Configuration Manager 2012 use Triple DES to password-protect user content and system data. However, in December 2018, Microsoft announced the retirement of 3DES throughout their Office 365 service. Firefox and Mozilla Thunderbird use Triple DES in CBC mode to encrypt website authentication login  credentials when using a master password. Implementations. Below is a list of cryptography libraries that support Triple DES: Some implementations above may not include 3DES in the default build, in later or more recent versions.