Triple DES
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In cryptography, Triple DES (3DES^{[1]}) is the common name for the Triple Data Encryption Algorithm (TDEA) block cipher, which applies the Data Encryption Standard (DES) cipher algorithm three times to each data block. Because of the availability of increasing computational power, the key size of the original DES cipher was becoming subject to brute force attacks; Triple DES was designed to provide a relatively simple method of increasing the key size of DES to protect against such attacks, without designing a completely new block cipher algorithm.
Definitive standardsEdit
The Triple Data Encryption Algorithm (TDEA) is defined in each of:
 ANS^{[2]} X9.521998 Triple Data Encryption Algorithm Modes of Operation^{[3]} (withdrawn)
 FIPS PUB 463 Data Encryption Standard (DES) (PDF) (withdrawn^{[4]})
 Template:PDFlink
 ISO/IEC 180333:2005 Information technology — Security techniques — Encryption algorithms — Part 3: Block ciphers
Name of the algorithmEdit
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: Template:Quotation NIST SP 80067 (2004, 2008^{[5]}) primarily uses the term TDEA, but also refers to "Triple DES (TDEA)". ISO/IEC 180333 (2005) uses "TDEA", but mentions that: Template:Quotation
None of the standards that define the algorithm use the term "3DES".
AlgorithmEdit
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:
 ciphertext = E_{K3}(D_{K2}(E_{K1}(plaintext)))
I.e., DES encrypt with K_{1}, DES decrypt with K_{2}, then DES encrypt with K_{3}.
Decryption is the reverse:
 plaintext = D_{K1}(E_{K2}(D_{K3}(ciphertext)))
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.
Keying options Edit
The standards define three keying options:
 Keying option 1: All three keys are independent.
 Keying option 2: K_{1} and K_{2} are independent, and K_{3} = K_{1}.
 Keying option 3: All three keys are identical, i.e. K_{1} = K_{2} = K_{3}.
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 equivalent to DES, with only 56 key bits. This option provides backward compatibility with DES, because the first and second DES operations cancel out. It is no longer recommended by the National Institute of Standards and Technology (NIST),^{[6]} and is not supported by ISO/IEC 180333.
Other terms used to refer to the keying options Edit
"Keying option n" is the term used by the standards (X9.52, FIPS PUB 463, SP 80067, ISO/IEC 180333) that define the TDEA. However, other terms are used in other standards and related recommendations, and general usage.
 For keying option 1:
 3TDEA, in NIST SP 80057^{[7]} and SP 800782^{[8]}
 Triplelength keys, in general usage^{[9]}^{[10]}
 For keying option 2:
 2TDEA, in NIST SP 80057^{[7]} and SP 800781^{[8]}
 Doublelength keys, in general usage^{[9]}^{[10]}
Encryption of more than one blockEdit
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^{[11]}), 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^{[12]} 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.
SecurityEdit
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^{[13]}^{[14]} and thus it is designated by NIST to have only 80 bits of security.^{[7]}
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^{[15]} (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.^{[7]} 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.^{[16]}
UsageEdit
The electronic payment industry uses Triple DES and continues to develop and promulgate standards based upon it (e.g. EMV).^{[17]}^{[18]}
Microsoft OneNote and Microsoft Outlook 2007 use Triple DES to password protect user content.^{[19]}^{[20]}
See alsoEdit
 Data Encryption Standard (DES)
 DESX
 Advanced Encryption Standard (AES)
 Horst Feistel
 Walter Tuchman
References and notesEdit
 ↑ Google search for "3DES"
 ↑ X9.52 is sometimes erroneously referred to as ANSI X9.52, however the standard itself has the designation ANS X9.52.
 ↑ X9.52 defines TDEA as a compound operation of the Data Encryption Algorithm specified in ANSI X3.921981 Data Encryption Algorithm, and does not include the DEA specification. Thus X9.52 must be read in conjunction with X3.92.
 ↑ Federal Register vol 70, number 96, Announcing Approval of the Withdrawal of Federal Information Processing Standard (FIPS) 46–3, Data Encryption Standard (DES); FIPS 74, Guidelines for Implementing and Using the NBS Data Encryption Standard; and FIPS 81, DES Modes of Operation (PDF)
 ↑ NIST SP 80067 version 1 was published in May 2004. It was revised in May 2008, as version 1.1, making a correction and clarification to the list of weak and semiweak keys. Both versions use the same terminology.
 ↑ NIST SP 80067
 ↑ ^{7.0} ^{7.1} ^{7.2} ^{7.3} NIST Special Publication 80057 Recommendation for Key Management — Part 1: General (Revised), March, 2007 (PDF)
 ↑ ^{8.0} ^{8.1} NIST Special Publication 800782, Cryptographic Algorithms and Key Sizes for Personal Identity Verification, February 2010 (PDF)
 ↑ ^{9.0} ^{9.1} Template:Cite web
 ↑ ^{10.0} ^{10.1} Template:Cite web
 ↑ NIST Special Publication 80038A, Recommendation for Block Cipher Modes of Operation, Methods and Techniques, 2001 Edition (PDF)
 ↑ ISO/IEC 10116:2006 Information technology — Security techniques — Modes of operation for an nbit block cipher
 ↑ Ralph Merkle, Martin Hellman: On the Security of Multiple Encryption (PDF), Communications of the ACM, Vol 24, No 7, pp 465–467, July 1981.
 ↑ Paul van Oorschot, Michael J. Wiener, A knownplaintext attack on twokey triple encryption (PDF), EUROCRYPT'90, LNCS 473, 1990, pp 318–325.
 ↑ Stefan Lucks: Attacking Triple Encryption (PDF), Fast Software Encryption 1998, pp 239–253.
 ↑ Eli Biham: How to Forge DESEncrypted Messages in 2^{28} Steps (PostScript), 1996.
 ↑ EMV 4.2 Specifications, Book 2  Security and Key Management, version 4.2, June 2008
 ↑ VISA
 ↑ Daniel Escapa's OneNote Blog  Encryption for Password Protected Sections, November 2006
 ↑ Microsoft  Encrypt Email Messages, Outlook 2007

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